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STUDY OF THE MECHANISMS OF SOME REACTIONS OF
1-BUTYL BENZENETHIOLSULFINATE
by
TZU-LI JU, B.S.
A THESIS
IN
CHEMISTRY
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
IN -
CHEMISTRY
May 1979
ACC'3312
ACKNOWLEDGEMENTS
I am deeply indebted to Professor John L. Kice for his suggestion
of this problem and direction of the investigation described herein.
My appreciation is also extended to Professor Richard A. Bartsch for his
generous assistance with the gas liquid chromatography, and also to Dr.
Gary L. Blackmer for his helpful criticism.
n
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ii
LIST OF TABLES iv
LIST OF FIGURES v
I. INTRODUCTION 1
II. RESULTS 14
A. The Acid- and Sulfide-Catalyzed Decomposition of t--Butyl Benzenethiolsulfinate 14
B. Reactions of 2-Methyl-2-propanethiolate Ion and Hydroxide Ion with t.-Butyl Benzeneth io l su l f i na te 23
III. DISCUSSION 30
A. The Acid- and Sulfide-Catalyzed Decomposition of t.-Butyl Benzenethiolsulfinate 30
B. Reaction of 2-Methyl-2-propanethiolate Ion and Hydroxide Ion with t-Butyl Benzenethiolsulfinate 40
IV. EXPERIMENTAL SECTION 47
LIST OF REFERENCES 56
APPENDIX 60
m
LIST OF TABLES
1. Acid-and Sul fide-Catalyzed Decomposition of t_-Butyl Benzenethiolsulf inate: Fractions from Chromatography on S i l i ca Gel 16
2. Products and Yields of Acid- and Sulfide-Catalyzed Decomposition of L-Butyl Benzenethiolsulf inate 21
3. Kinetics of Reaction of 2-Methyl-2-propanethiolate Ion with JL-Butyl Benzenethiolsulf inate in 60% Dioxane at 25°C 24
4. Kinetics of Reaction of 2-Methyl-2-propanethiolate Ion wi th t.-Butyl Benzenethiolsulfinate in Buffer (pH = 10) Solution 25
5. Kinetics of the Alkal ine Hydrolysis of L-Butyl Benzenethiolsulf inate in 60% Dioxane at 25°C 27
I V
LIST OF FIGURES
Dependence of k ^ ^ on Hydroxide Ion Concentration for SulfiMdce ii.yo X lu • Ml 29
Fixed Concentration of L-Butyl Benzenethiol-inate (1.96 x 10"^ M)
INTRODUCTION
Thiolsulfinates (I) were first prepared 60 years ago by Zincke by
the hydrolysis of aromatic sulfenyl chlorides.^"^ The structure was
thought by some early workers to be the anhydride of sulfenic acids,
II. Proof that thiolsulfinates actually have structure I was provided
subsequently by Backer and Kloosterziel^ who showed that if two dif-
RSSR RS-O-SR
0
I II
ferent groups are attached to the sulfur atoms, two isomers are obtain
able, for example. III and IV. If the actual structure were the anhy
dride, V, only one compound would be found. Additional evidence from
iSSAr II 0
III
RSSAr
II 0
IV
RS-O-SAr
V
7-9 infrared studies by Ghersetti and Modena has shown that the thiol-
sulfinates possess a sulfoxide group by comparison of their spectra
with those of sulfoxides, sulfinates and sulfinyl chlorides.
Several methods for the preparation of thiolsulfinates have been re
ported. Since Zincke's original observations, a number of other workers
have prepared thiolsulfinates by his method. ' Backer prepared
14 15 thiolsulfinates by the hydrolysis of sulfenyl piperidides. ' A third
method involves the controlled oxidation of a disulfide. ' Since the
sulfinyl group of a thiolsulfinate is a chiral center, thiolsulfinates
are capable of optical activity, and optically active thiolsulfinates
have been prepared by asymmetric oxidation of the corresponding disul
fide with optically active percamphoric acid.^^'^°
The most general method for preparation of thiolsulfinates, and
perhaps the best, since it allows the preparation of unsymmetrical
thiolsulfinates, is the method first successfully accomplished by
Backer and Kloosterizel. In this method, a mercaptan is coupled with
a sulfinyl chloride in the presence of pyridine in ethyl ether solvent. 35
Barnard has used this method to prepare specifically labeled phenyl( S)
35 21
benzenethiolsulfinate and phenyl benzenethiol( S)sulfinate.
The chemistry of thiolsulfinates received little attention until
the characterization of the bacteriostatic principle of the common gar
lic (Allium Sativuum) as allyl 2-propene-l-thiolsulfinate (VI, allicin) 22 was reported by Cavallito, Back and Suter in 1944. This discovery has
CH2CHCH2S(0)SCH2CH=CH2
VI
led to a great deal of work on the biological activity of thiolsulfin
ates. There have been further reports of their bacteriostatic action.
^^'•^^ Also, activity against tumors ' has been reported. Certain
cyclic thiolsulfinates have been found to occur naturally and to pos-
sess biological activity. Calvin has suggested that thioctic acid
monosulfoxide, which contains a five-membered cyclic thiolsulfinate,
may be an intermediate in the primary quantum conversion act in photo-
synthesis. A number of thiolsulfinates have been shown to inhibit the
autoxidation of polyolefins and to have utility as stabilizers for
synthetic rubber.^^"^^
The sulfur-sulfur bond in a thiolsulfinate is much more labile
than the S-S bond of a disulfide, and thiolsulfinates have been obser
ved to undergo cleavage of their S-S bond readily by a variety of mech
anisms. The following types of mechanisms have so far been observed:
(a) Thiolsulfinates can be attacked by good nucleophiles wery
readily with a resultant cleavage of the S-S bond. Depending
on the nucleophile, attack mav occur orimarily at the sul-0 II
fenyl (-S-) or sulfinyl (-S-) sulfur as shown in Equations 1 44 and 2, respectively.
ArS-SAr' + Nu II 0
•» ArSO" + Ar'SNu (1)
0 II
-> ArS-Nu + Ar'S (2)
(b) Thiolsulfinates can participate in radical reactions i n i t i a
ted by the hemolytic scission of the S-S bond (Equation 3)
50 at moderate temperatures, a process faci l i tated by the unusually weak S-S bond (bond strength ~ 35 Kcal).
0
ArSSAr > ArSO- + ArS- (3)
(c) Alkyl thiolsulfinates where the carbon a to the sulfenyl sul
fur bears one or more hydrogens can generate unstable sulfenic
acids by a cycloelimination reaction (Equation 4); the sulfe-
51 nic acids can be trapped in good yields with acetylenes,
(Equation 5).
II d ^ ^ R-S^-S-C<^ > RSOH + S = C < ^ (4)
R' R "
RSOH + R'C = CR' ' > ^ ^ C = C C ^ (5)
RS/" H
0
(d) The cleavage of the S-S bond in a thiolsulfinate can be
catalyzed by the cooperative efforts of acid and a nucleo-45- 49 phile in the fashion outlined in Equation 6.
+ +
ArS-SAr' + H > ArS-SAr' (6a) II I 0 OH
+ + Nu + Ar'S-SAr > Ar'S-Mu + ArSOH (6b)
OH
The present thesis is primarily concerned with the study of fur
ther aspects of the acid- and nucleophile-catalyzed cleavage, although
a portion of it also deals with the reactions of two strong nucleo
philes (RS" and OH') with a highly hindered thiolsulfinate.
The mechanisms of the acid- and nucleophile-catalyzed reactions
45-49 of the thiolsulfinates have been explored by Kice, et al. One
quite striking example is the sulfide-catalyzed reaction of phenyl
49
benzenethiolsulfinate with sulfinic acids (Equation 7), where ex
tremely small (-10 M) concentrations of n -alkyl sulfides produced
large rate effects. This reaction is first-order in both thiol
sulfinate and alkyl sulfide. Even though sulfinic acid is
involved in the stoichiometry of the react ion, the rate is independent
2 ArSO^H + PhSSPh ^—> 2 PhSO^SPh + H«0 (7) ! R«S ^ ^
0 '^
of sulf inic acid concentration. This means that sulfinic acid does not
intervene chemically until after the rate-determining step. The reac
tion is strongly acid catalyzed, exhibits a solvent isotope effect in
acetic acid of • HOAC' ' DOAC ^ ^•^^' ^"^ ^^°^^ ^ marked rate dependence
on the structure of the catalyzing sulfide (see k in Appendix 1 ) . Elec-
46 49 tron-withdrawing R groups in R«S retard the rate. ' This last
observation is consistent with a mechanism in which the sulfide acts
as a nucleophile, while the solvent isotope effect and the exact depen
dence of rate on acidity suggest that the acid catalysis involved is of
the specific lyonium ion rather than the general acid variety. A mech
anism (Schemel) which involves rate-determining nucleophilic attack of
the sulfide on the sulfinyl-protonated thiolsulfinate (Equation 9) was
49 therefore suggested.
Scheme 1
Mechanism of the Sul fide-Catalyzed Reaction of
Thiolsulfinates with Sulfinic Acids
PhSSPh + H\ ^ ' PhS-SPh (8)
0 OH
R„S + Ph -SPh <;--.!-> Rj-SPh + PhSOH (9) I k OH '
0
R2SSPh + ArS02H f i i t - " ArSSPh + R2S + H^ (10)
PhSOH + R2S + H "
PhSOH + ArS02H
's" Ji»
0
R2S-SPh + H2O
0 II
ArSSPh + H2O
(11)
(12)
0
Further evidence for this mechanism is provided by the fact that
4fi alkyl sulfides have also been found by Kice and Large to catalyze the
reaction of phenyl benzenethiolsulfinate with mercaptans (Equation 13).
PhSSPh + 2 R'SH II 0
H R2S
2 PhSSR' + H2O (13)
This sul f ide-catalyzed thiolsulf inate-mercaptan reaction has exactly
the same formal kinetics and the same rate constant under a given set
of conditions as Equation 7. I t also shows the same dependence of the
48
rate constants on the sulf ide structure. The two processes must there
fore have the same rate-determining step (Equation 9 ) . In the case of
the mercaptan react ion, this rate-determining step is then followed by
R2?SPh + R'SH -^^-^^ > R'SSPh + H" + R2S
and
PhSOH + R'SH -5> R'SSPh + H2O
(14)
(15)
rather than by Equations 10 and 12.
In acidic aqueous dioxane in the absence of added nucleophiles,
optically active phenyl benzenethiolsulfinate racemizes only wery
slowly. However, addition of small amounts of alkyl sulfides, halide
ions, or thiocyanate ions leads to quite rapid racemization,^^ (Equa
tion 16). The racemization reaction is f irst order in both nucleo-
(+)-PhSSPh ^—> (+)-PhSSPh (16) II II 0 0
phile and hydrogen ion; its solvent isotope effect indicates that it is
specifically oxonium ion catalyzed. Although only the acid- and nucleo
phile-catalyzed racemization of C")-PhSSPh occurs in their absence,
addition of sulfinic acid or mercaptan to such aqueous dioxane solutions
leads to the disappearance of PhS(0)SPh via the acid- and nucleophile-
catalyzed reactions with ArSOpH (Equation 7) and R'SH (Equation 13)
already described. Under a given set of conditions, the rate constants
for the nucleophile- and acid-catalyzed racemization and for the reac
tions of PhS(0)SPh with ArS02H and R'SH are the same, indicating that all
three reactions may involve the same rate-determining step. Racemization
of PhS(0)SPh accordingly involves the following mechanism.
(+)-PhSSPh + H" > (+)-PhSSPh ;i I 0 OH
Nu" + (+)-PhSSPh > PhSNu + PhSOH >
OH (+)-PhSSPh + Nu' + H" (17)
II 0
8
The disproportionation of phenyl benzenethiolsulfinate (in acetic
acid-1% water containing some sulfuric acid) to phenyl disulfide and
phenyl benzenethiolsulfonate (Equation 18) is also markedly catalyzed
0 M+ II
2 PhSSPh ~^—> PhSSPh + PhSSPh (18) I! ^ 2 ^ II
0 0
by added alkyl or aryl sulfides.^^ Although the formal kinetics of
this sulfide catalyzed disproportionation are exactly the same as those
of the previously described sulfide-catalyzed PhS(0)SPh-sulfinic acid
and PhS(0)SPh-mercaptan reactions.(i.e., the reaction is first-order
in both PhS(0)SPh and sulfide and subject to specific acid catalysis)
the dependence of its rate on sulfide structure (see k. in Appendix 1)
is entirely different from that observed for the other two sulfide-
catalyzed reactions. Experiments using esr offer no indication that
free radicals are intermediates in the reaction. For this reason, the
only mechanism for the sulfide-catalyzed disproportionation which ap
pears to be compatible with both the kinetics and the dependence of
rate on sulfide structure is the one outlined in Scheme 2. I t was
shown that, provided k, [PhS(0)SPh] > k_5[H20], as does not seem un
reasonable for an acetic acid-1% water solvent, this mechanism will
lead to a first-order dependence of the rate on sulfide concentration,
acidity of the medium, and thiolsulfinate concentration. I t can also
account for the observed dependence of k on sulfide structure, given
the fashion in which the ratio:
would be expected to change markedly with variation in sulfide structure
Scheme 2
Mechanism for the Sulfide-Catalyzed Disproportionation
Of Phenyl Benzenethiolsulfinate
+ K + PhSSPh + H ^ PhS-SPh (8)
II I 0 OH
+ ll + R2S + PhS-SPh ^ 1 ^ R2S-SPh + PhSOH (9)
OH Ph
+ k. I R«S-SPh + PhSSPh > PhS-S-SPh + R«S (19) ^ II II ^
0 0
Ph
PhS-S-SPh + H2O ^ ^ > PhSSPh + PhS02H + H"*" (20)
0
0
PhS02H + R2S-SPh -^-^^—> PhSSPh + R2S + H (21)
+ ^5 PhSOH + R2S + H >s 1 " R2S-SPh + H2O (22)
-5
The mechanism shown in Scheme 2 wil l lead to a first-order
dependence of the kinetics on thiolsulfinate concentration for reactive
10
sulfides as catalysts only for reaction conditions where k4[PhS(0)SPh]
> k_5[H20]. When k^[PhS(0)SPh] < k_5[H20],a 3/2 order dependence on
thiolsulfinate concentration is expected. Thus a change from acetic
acid-1% water to a much more aqueous medium, such as 60% dioxane, should
lead to a change from first to 3/2 order in the kinetic dependence of
the rate of the acid- and sulfide-catalyzed disproportionation on thiol-
45
sulfinate concentration. That is exactly what Kice and Cleveland ob
served. This constituted important support for the correctness of the
mechanism in Scheme 2.
Phenyl benzenethiolsulfinate is readily hydrolyzed by alkali with
40 44 the stoichiometry shown in Equation 23. Kice and Rogers reported that the reaction takes place in two distinct stages. In the first rapid
3 PhSSPh + 2 OH' ^ 2 PhSSPh + 2 PhSO"« + H^O (23) II '^ ^ 0
stage, there is attack by OH' on the thiolsulfinate at approximately
the same rate (k^Q/k^ = 1.2) at both the sulfenyl sulfur (Equation 24a),
and sulfinyl sulfur (Equation 24b). Then the PhS" (formed from 24b) re
acts at a rapid rate with the remaining thiolsulfinate to give the disul-
--5—> PhSOH + PhSO' -2tL.>2 PhSO" (24a)
OH + PhSSPh II 0
- 5 ^ PhS' + PhS02H - ^ ^ ^ P h S " + PhSO'2 (24b)
PhS" + PhSSPh ^ ^ -> PhSSPh + PhSO"
(5
11
fide (Equation 25). The slower second stage of the reaction is presumed
to be the disappearance of the sulfinate ion (PhSO') through a series of
reactions of as yet unknown mechanism but having the overall stoichiometry:
3 PhSO' + 2 H2O 5> PhSSPh + PhS02' + 2 OH'
52 Fava and Ilceto showed that the rate of nucleophilic substitution
at the dicoordinate sulfur in trBuS-SO " was 10^ slower than the rate at
the same sulfur in CH-CHpSSOg'. This was presumably due to the same kind
of steric effect that causes displacement of the halogen in t^-BuCH^Br to be 5
10 slower than that for CH3CH2Br. The thiolsulfinate, PhS(0)S-t^-Bu, is a
known compound, and, from Fava's results, one would expect that nucleo
philic attack on the sulfur attached to the t;-butyl group would be orders
of magnitude slower than the corresponding reaction involving phenyl ben-
zenethiolsulfi nate.
Since the mechanisms proposed for both the acid- and sulfide-cat
alyzed disproportionation of phenyl benzenethiolsulfinate (and for its
alkaline hydrolysis) involve, in one or more of their stages, nucleophilic
attack on the PhS- sulfur, we were interested in seeing in what way and to
what extent do both the acid- and sul fide-catalyzed disappearance of t-
butyl benzenethiolsulfinate (and its alkaline hydrolysis) would be altered
from the behavior exhibited by phenyl benzenethiolsulfinate.
57
A preliminary study, by Kice and Venier, of the acid- and sulfide-
catalyzed disappearance of jt-butyl benzenethiolsulfinate in acetic
acid-1% water as solvent, showed that although it exhibits the same for
mal kinetics and the same dependence on acidity as found for the acid-
12
and sulfide-catalyzed disproportionation of phenyl benzenethiolsulfinate,
the reaction both occurs much more slowly and exhibits a quite di f ferent
rate dependence on sulf ide structure than that found for the acid-and su l -
fide-catalyzed decomposition of phenyl benzenethiolsulfinate (see Appen
dix 1). In the sulfide-catalyzed disproportionation of phenyl benzene
th io lsu l f ina te , 11-BU2S and (PhCH2)2S are of almost exactly equal react i
v i ty as catalysts. In the decomposition of t^-butyl benzenethiolsulfinate,
n-BugS is 7.3 times more reactive as a catalyst than (PhCH2)2S. This
type of difference in the react iv i ty of ivbutyl and benzyl sulfides is
v i r tua l l y the same as that observed for the acid- and sulfide-catalyzed
reaction of phenyl benzenethiolsulfinate with sul f in ic acids or thiols
(in which n -Bu2S is almost 8 times better as a catalyst than is (PhCH2)2S.
The greater the electron density on the sulfur of R^S, the more reactive
the sulf ide is as a catalyst.
In the acid- and sulfide-catalyzed reactions of phenyl benzenethiol
sulf inate with su l f in ic acids or th io l s , the rate-determining step (k, in
Scheme 1) is nucleophilic attack of the sulfide on protonated-PhS(0)SPh.
While in the acid- and sulfide-catalyzed disproportionation of phenyl
benzenethiolsulfinate, the rate-determining step is the reaction of
R2S-$Ph with PhS(0)SPh (step k of Scheme 2). The kinetic behavior of
the sulfide-catalyzed decomposition of t-butyl benzenethiolsulfinate
suggests that in this reaction, in contrast to the situation in the sul
fide-catalyzed disproportionation of phenyl benzenethiolsulfinate, the
rate-determining step is presumably the attack of the sulf ide on the
protonated th io lsu l f inate . I f one were to determine the products of
13
the sulfide-catalyzed decomposition of t -butyl benzenethiolsulfinate,
one could distinguish whether such a reaction of R^S with protonated-
PhS(0)S-t^-Bu involves attack at the dicoordinate sulfur, or whether,
because of the great steric hindrance to attack at this position pro
vided by the t;-butyl group, attack occurs preferentially at the other
sulfur. It was for this reason that the study of the products of the acid-
and sulfide-catalyzed decomposition of t^butyl benzenethiolsulfinate de
scribed in this thesis was undertaken.
We have seen that in the hydrolysis of phenyl benzenethiolsulfinate,
attack of hydroxide ion apparently occurs at both sulfurs at about the
44 same rate. In the alkaline hydrolysis of t^butyl benzenethiolsulfinate,
the severe steric hindrance to attack at the sulfenyl sulfur might be
expected to cause attack of hydroxide ion on this thiolsulfinate to oc
cur essentially exclusively at the sulfenyl group (i.e., 0H~ + PhS(0)S-t^-Bu
> PhSOpH + t^BuS'). One might also expect that the rate of reaction
of the t-BuS', produced by this reaction, with t^butyl benzenethiolsul
finate to give t-butyl disulfide, would be very slow due to steric hin
drance. For these reasons it appeared that a brief study of both the
products and kinetics of the alkaline hydrolysis of t -butyl benzenethiol
sulfinate, as well as a study of the reaction of the thiolate anion,
t^-BuS", with t -butyl benzenethiolsulfinate might be informative and use
ful, particularly in terms of comparison with the behavior of phenyl
benzenethiolsulfinate under the same conditions.
RESULTS
Section A
The Acid- and Sulfide-Catalyzed Decomposition of
t-Butyl Benzenethiolsulfinate
Synthesis of t-Butvl Benzenethiolsulfinate
tjButyl benzenethiolsulfinate was prepared by the general method CO
outlined by Chau and Kice for the preparation of p.-fluorophenyl ben
zenethiolsulfinate from 2.-flLiorothiophenol and benzenesulfinyl chloride.
In this synthesis,benzenesulfinyl chloride is f irst prepared by reacting
thionyl chloride with sodium benzenesulfinate (Equation 26). The sul
finyl chloride is then reacted with 2-methyl-2-propanethiol (t.-BuSH)
in the presence of pyridine in ether solution to form the desired pro
duct (Equation 27). The thiolsulfinate was purified by recrystalliza-
tion from chloroform-hexane and stored in the freezer until used.
S0C1« PhS-ONa ^-» PhS-Cl (26)
II II 0 0
PhS-Cl + t-BuSH ^Jl^^—> P h S - S - t - B u II - (o) II 0 ^N"^ 0 ( 2 7 )
Product Studies of the Decomposition of t-Butyl Benzenethiolsulfinate
The decomposition of t_-butyl benzenethiolsulfinate (0.1 M.) was
carried out in acetic acid-1% water containing 0.1 M. sulfuric acid and
0.01 M n.-butyl sulfide at 40°C for 8 hours. The sulfuric acid and
14
15
[i-butyl sulfide acted as catalysts. At the end of the reaction time,
the solution was poured into a large volume of water and extracted with
ether. The ether extracts were washed with water, 5% sodium bicarbonate,
and then dried over anhydrous magnesium sulfate. The residue was then
chromatographed on silica gel (70-230 mesh) in order to separate the
different reaction products in the residue. Three fractions were eluted
from the silica gel (see Table I).
The first fraction eluted consisted of a mixture of three components
as judged from its gas-liquid chromatographic behavior. The first of
these was shown to be rnbutyl sulfide. Comparison of spectral proper
ties and glc retention times with known samples indicated the other two
compounds to be di-t^-butyl tri sul fide, t^-RuSSS-t^-Bu, and phenyl t -butyl
disulfide, PhSSBu-t^. To confirm unequivocally the presence of di-t^-butyl
trisulfide in Fraction I, a sample of the fraction was subjected to pre
parative gas chromatography and the part believed to be di-t^-butyl tri
sulfide, on the basis of its retention time, was shown by comparison of
its nmr spectrum and mass spectrum with that of a known sample of di-t_-
butyl trisulfide to definitely be this compound.
In the glc of the first fraction there were no peaks at the reten
tion times for either t -butyl disulfide or phenyl disulfide, showing
these substances to be absent.
The nmr spectrum of the first fraction had an aromatic multiplet at
5 7.2-7.5, and two sharp singlets at 6 1.39 and 1.32, due to the t -butyl
trisulfide and t-butyl phenyl disulfide, respectively. The ratio of the
integrated intensity of the t -butyl peak at 5 1.32 to that of the aromatic
TABLE I
Acid- and Sul fide-Catalyzed Decomposition of t.-Butyl
Benzenethiolsulfinate: Fractions from
Chromatography on Silica Gel^
16
Run #
1
2
3
4
5
6
7
PhS(0)S-t^-Bu
mg
1076
2140
2140
2140
2140
2140
2140
n-Bu2S
mg
84
159
164
742
157
160
159
lb
mg
375
860
860
1347
946
978
908
I ic
mg
66
115
150
210
170
151
154
I l l d
mg
305
584
620
700
758
697
748
Reaction conditions: 0.1 M PhS(0)SBu-t^ and 0.01 M n Bu2S in acetic acid-1% water containing O.IM sulfuric acid at 40°C for 8 hours (Run #4 contained 0.05 M 11-BU2S).
Fraction I is a mixture of 11-BU2S, PhSS-t -Bu and t -BuSSS-;t -Bu.
^Fraction I I is a pure PhS02SS-t_-Bu.
^Fraction I I I contains PhS02SPh and a small amount of PhS02SS-t -Bu The relative amount of PhS02SS-t -Bu is 10-15% mole fraction in the Fraction I I I .
17
multiplet was 9:5, as required for t -butyl phenyl disulfide. The ratio of
the integrated intensity of the two t -butyl peaks at 5 1.39 and 1.32 was
approximately 1.0 showing that the relative amount of trisulfide was one-
half that of t -butyl phenyl disulfide. From the weight of Fraction I and
relative amounts of t.-butyl phenyl disulfide and di-l-butyl trisulfide
as indicated by the nmr, the yields of t.-butyl phenyl disulfide and di-r
t.-butyl trisulfide were estimated as 0.26 + 0.02 and 0.13 t 0.01 mmole
per mmole of t.-butyl benzenethiolsulfinate, respectively.
The second fraction eluted was an oil which crystallized upon
cooling in the freezer. This compound could be recrystallized from 4:1
hexane-ethanol at -50°C. Even after recrystallization, it melted below
room temperature. The infrared spectrum of the compound showed two
strong absorptions at 1320 and 1135 cm' indicating the presence of an
/SOp group. The nmr spectrum consisted of an aromatic multiplet (5H) at
7.6-8.2 and a sharp singlet (9H) at 6 1.39 due to a t-butyl group.
The mass spectrum indicated a molecular ion of mass 262, and this, plus
the elemental analysis and other features of the mass spectrum,estab
lished that the compound had the structure PhS02SSBu-t^, rather than
being the simple thiolsulfonate, PhS02SBu-t^, as had first been thought.
The yield of this product, as estimated from the weight of the second
fraction plus the small amount found by nmr to tail over into fraction
III is 0.09 - 0.01 mmole per mmole of t-butyl benzenethiolsulfinate.
The third eluted fraction consisted of the known compound phenyl
benzenethiolsulfonate contaminated with a small amount of 2-methylpro-
pane-2-sulfenic benzenesulfonic thioanhydride (PhSn2SS-t^-Bu). A pure
18
sample of this thiolsulfonate was obtained by recrystallization of frac
tion III from ethanol. It was identical (m.p., ir) to a known sample of
phenyl benzenethiolsulfonate (PhS02SPh), The amount of phenyl benzene
thiolsulfonate formed in the decomposition was estimated from the weight
of fraction III, after correction for the small amount of 2-methyl-pro-
pane-2-sulfenic benzenesulfonic thioanhydride present. The amount of
the latter was estimated from the small nmr peak for its t-butyl group
seen in the nmr spectrum of the fraction. The yield of phenyl benzene
thiolsulfonate was 0.27 ± 0.02 mmole per mmole of t -butyl benzenethiol
sulfinate.
In a separate experiment, isobutylene was identified as another
decomposition product. A sample of t -butyl benzenethiolsulfinate (1
mmole) was heated at 50°C for 5 hours in acetic-1% water containing
0.1 M sulfuric acid and 0.01 M rv-butyl sulfide. A stream of nitrogen
was passed through the solution during this period to sweep olefins out
of the reaction solution. The olefin was collected in a trap cooled
in liquid nitrogen. The amount of olefin present in the trap was then
determined by a bromination procedure. This procedure indicated that
0.15 mmole olefin per mmole t -butyl benzenethiolsulfinate had been
formed and trapped. This yield (15%) of isobutylene is probably sig
nificantly less than the total amount of isobutylene actually produced
in the decomposition of t -butyl benzenethiolsulfinate. For example,
in acid solution isobutylene can undergo dimerization to diisobutylene
rather readily.^^ For this reason the yield of 15% is definitely a
lower limit.
19
I t was also possible that isobutylene might be formed by the so l -
volysis of t^-butyl benzenethiolsulfinate i t s e l f under the present
reaction conditions. This was shown not to be the case by the following
experiment. 1-Butyl Benzenethiolsulfinate (1.07 g,0.1M) was heated at
40°C for 8 hours in acetic acid-1% water containing 0.1 M sulfur ic acid
but iTO a-butyl su l f ide, and the solution was then worked up by the same
procedure used for the product studies. The residue,after evaporation
of the ether, weighed 0.999 g, and had the same infrared spectrum as
the start ing material. The residue was then chromatographed on s i l i ca
gel , and 0.94 g of material having m.p. 49-52°C and with an infrared
spectrum identical with ^-butyl benzenethiolsulfinate was eluted. The
th io lsu l f inate can thus be recovered in essentially quantitative y ie ld
when heated at 40°C in acetic acid-1% water-0.1 M sulfuric acid in the
absence of £-butyl sul f ide.
Since t_-butyl benzenethiolsulfinate fa i l s to decompose to any
extent in acid solution without the added £-butyl sul f ide, the iso
butylene being detected is not a product of a solvolytic side reaction.
I t is formed direct ly in the sulfide-catalyzed decomposition of t_-butyl
benzenethiolsulfinate.
An additional experiment was performed to see i f any acetone was
formed in the decomposition. This was done by the decomposition of t -
butyl benzenethiolsulfinate (5 mmol) at 40°C in acetic acid-1% water in
the presence of rnbutyl sulf ide and sul fur ic acid in exactly the same way
as in the product study previously described. At the end of the reaction,
about half of the solution was d i s t i l l ed of f under reduced pressure and
20
co l lected in a receiver cooled in dry ice. The d i s t i l l a t e was then
melted and twice i t s volume of water was added. This aqueous solut ion
was then treated with a standard 2,4-dinitrophenylhydrazine test ing
so lu t ion . No detectable prec ip i ta te of acetone 2,4-dini trophenylhy-
drazone formed. In a related experiment, acetone (1.23 mmol) was
dissolved in acetic acid and th is solut ion was d i s t i l l e d under reduced
pressure in the same way as in the preceding experiment. Addit ion of
water and the 2,4-dinitrophenylhydrazine solut ion gave acetone 2 ,4 -d i -
nitrophenylhydrazone in 62% y i e l d . These results indicated the absence
of acetone as a product of the acid- and sulf ide-catalyzed decomposition
of t.-butyl benzenethiolsulf inate.
To confirm that the unsymmetrical d i su l f i de , PhSS-t_-Bu, found in
f rac t ion I , was formed d i rec t l y in the decomposition of t -bu ty l benzene
t h i o l s u l f i n a t e and not as a resul t of the disproport ionation of the two
symmetrical d i su l f i des , PhSSPh and t.-BuSS-t^-Bu, a supplemental experiment
was carr ied out. A mixture of 1-butyl d isu l f ide (0.1 M) and phenyl d i
su l f ide (0.1 M) in acetic acid-1% water containing su l fu r i c acid (0.1 M)
and a-butyl su l f ide (0.01 M) was heated at 40°C for 8 hours, (the same
conditions used fo r the decomposition of the t^-butyl benzenethiolsul
f i n a t e ) . Upon work-up, the residue was subjected to gas l i qu id chromato
graphy. No 1-butyl phenyl d isu l f ide could be detected. The only sub
stances in the residue were the three reactants: a-butyl su l f i de , t-
butyl d i su l f i de and phenyl d i su l f i de . So, i t is obvious that the t.-
butyl phenyl d i su l f i de isolated from the decomposition of t.-butyl ben
zeneth io lsu l f inate is a d i rec t product of the decomposition and is not
21
formed by disproportionation of an in i t ia l ly- formed mixture of jt-butyl
d isul f ide and phenyl d isul f ide.
The various products formed by the acid- and sulfide-catalyzed
decomposition of t.-butyl benzenethiolsulfinate are given in Table I I .
Changing the concentration of a-butyl sulfide from 0.01 to 0.05 M gave
no change in products or products yields (see Table I I ) .
TABLE I I
Products and Yields of Acid- and Sulfide-Catalyzed
Decomposition of jt-Butyl Benzenethiolsulfinate^
Product
PhSS-t.-Bu ( I )
t_-BuSSS-t -Bu ( I )
PhS02SS-t -Bu ( I I )
PhS02SPh ( I I I )
CH2=C(CH3)2
Yield (mmole/mmole PhS(O)S-t^-Bu)
0.26 ± 0.02
0.13 + 0.01
0.09 ± 0.01
0.27 t 0.02
1 .0.15^
^Reaction condi t ions: 0.1 M j t -buty l benzenethiolsulf inate and 0.01 M n-butyl su l f ide in acetic acid-1% water containing 0.1 M su l fu r i c acid at 40°C fo r 8 hours.
^The f ract ions eluted from the chromatography are given in parentheses.
^Minimum y ie l d fo r th i s product. Actual y i e ld could be considerably larger due to dimerizat ion.
Section B
Reactions of 2-Methyl-2-propanethiolate Ion and Hydroxide Ion
with trButyl Benzenethiolsulfinate
Kinetic Studies of the Reaction of 2-Methyl-2-propanethiolate
Ion with t.-Butyl Benzenethiolsulfinate 60
Based on the behavior of thiolate ions with other thiolsulfinates
the reaction of 2-methyl-2-propanethiolate ion with 1-butyl benzenethiol
sulfinate should proceed as follows:
0 11
t;BuS + t_-BuS-SPh -> i-BuSS-t_-Bu + PhSO
PhSO" y > PhSOH t"^"^ > L-BuSSPh
_k§^§—-.^ t -BuSS-t_-Bu + PhS'
The kinetics of the reaction of 2-methyl-2-propanethiolate ion with
tjbutyl benzenethiolsulfinate were studied at 25°C in 60% dioxane. The
2-methyl-2-propanethiolate ion was generated by the addition of a mea
sured amount of standard sodium hydroxide to a solution containing 2-
methyl-2-propanethiol:
trBuSH + OH" > i-BuS' + H2O
The concentrations of 2-methyl-2-propanethiolate ion used (0.5-2 x 10 M)
were such that the thiolate ion was present in considerable stoichiometric
excess over the thiolsulfinate (1.4 x 10"^ M). The course of reaction was
22
23
followed by monitoring the change in the optical density of the solution
at 264 nm and followed first-order kinetics. The experimental f i rst -
order rate constants, k . , are collected in Table I I I . As indicated
by the constancy of kQt5s/[t.~BuS-], the reaction is first-order in 2-
methyl-s-propanethiolate ion.
The kinetics of the reaction were also investigated using solutions
of the thiol (1.42-2.99 x 10"^ M) in a phosphate buffer solution (pH = 10)
of [KH2PO4] : [K2HPO4] = 1:6 in 60% dioxane. Under these conditions the
experimental first-order rate constant, k . , for the disappearance of the
thiolsulfinate will be given by:
k.Kc = k,_,.„.(K|--^"5"/[H^])[t-BuSH] obs t.-BuS
From the value for k._„ _ determined from the runs in Table I I I , the known
pH of the phosphate buffer, and the measured values of k . , one can de
termine what is the pK of t-BuSH in 60% dioxane. The results are shown a
in Table IV. All three runs give an estimated pK for t-BuSH in 60% di
oxane of close to 13.7. This value is in the range that would have been fif)
expected given the pK of thiophenol in 60% dioxane, 9.48, and the ex-fii
pected difference of about 4.5 pK units between the pK^ of thiophenol
and t;-butyl mercaptan based on the assumption that A pK^ for the two
thiols in 60% dioxane should be about the same as in water.
24
t—t 1—1 1—H
LU _ l CQ <" I—
C o
1—t
(U •M fO
t—
o •r— - C
+-> (U c m a. o s. Q . 1
CVJ
,— >»
x : • • - >
0) s: i CVI
M-O
c o
• f—
+J o ta (U a: 4 -o to o
•^ + j <u £=
O o t n CVJ
4-> fO
(U c rt3 X o •^ Q
^ O V£>
c •»-(U
•M fO c=
•r— M-— 3 CO
— o x: + J (U £= <U M C
CQ
'>) + j 3
CQ 1 ,
•Ml
II
.
J \ 1
-—n 1
CO 1— 13 1
CQ CO 1 1—
4-^ 1 ^ ^ 1 1—« s :
i to to
. Q CVi O 1
^ o • -"
X
1—1 S ! 1 LO CO =3 1
CQ CD 1 , I—
•Ml 1—1 X
1—1 2 1 3Z OO CO :3 1
0 0 o ' 1 • "
•Ml 1—1 X
3 CQ
1 ^^ 4 J | —
CO « * ^—** 1
o o — ^~ CO J = X QL. 1 1
=*•=
c zs on
5.8
00 LO
r-»
o •
r_
Lf)
,— 1 —
-!*•
•~
6.0
o VO
r—
o .
r__
LO «X9
00
• *
C\J
5.5
•"" CO
o •
CVJ
CO
1 ^
r—
«d-
CO
5.5
• -"
CO
o •
Csi
00 «!*•
00
«>J-
^
3.2
vo vo o
LO •
o
LD
00
^
LD
25
> t—1
LU _ l CQ <s: h-
(U 4-> rt3 — O
• r -SZ - M <U c rtJ Q . O &-Q . 1
OJ 1 — >> x: 4 J <U
^" 1
OO
M-O
c o •r— +J O n3 <u Qfl
4 -O
to o
't— •M <V
&-(U
i * -M -3
CO
c • 1 —
cu •»->
fO c •r—
M-p —
Z5 to
r—
o •r -^ -M (U C (U N C (U
CQ
— >^
4 J 3
CQ
-Ml sz -M •n-S
c
03 C o •r—
-(-> 3
r—
o CO
^—.* o r—
II
^ Q .
>.^
fC ^ a. 3:
OO T3 3 (U QQ
• ^ ' 1 ta + j | E
• 1 - M-•M O to
LU
1 1
to to
J3 Ln O 1
^ o r—
X
1—1 s 3 1 C/0 CM 3 1
CQ O ' 1 '—
• M I 1—1 X
3 CQ
1
•Ml S 1 CO ^ ^"^"^ 1
o o ^-m^0^ r —
</) JZ X Q-l _ l
=*»=
c 3
Od
o r • CO —
I D •
^
CM ^
• n—
^ •
n—
r—
^-P»*
• CO ^^
r>H CM
• LD
CM r«.
• ^^
• * •
1 —
CM
LD r • CO ^~
^ «!3-
• 0 0
cr> <y»
• CM
^ •
r—
CO
o LO CM
+J «3
<U
ns X
o
5-S
o
to 3
< :
03
26
Product and Kinetic Studies of Alkaline Hydrolysis
of t-Butyl Benzenethiolsulfinate
Product Studies The products of the alkaline hydrolysis of t_-butyl
benzenethiolsulfinate were investigated by adding a solution of t.-butyl
benzenethiolsulfinate (0.5 mmol, 0.01 M) in 40% dioxane dropwise with
good stirring to a 1 M solution of sodium hydroxide (50 mmol ) in the
same solvent under nitrogen. After the addition was complete, the so
lution was stirred for two more hours and then neutralized with acetic
acid. The solution was then poured into a large volume of water and
extracted with methylene chloride. The methylene chloride extracts were
washed with water, and dried over magnesium sulfate. The methylene
chloride was removed by fractional distillation. The residue was sub
jected to gas liquid chromatography. Besides the solvent, dioxane, the
only component evident in the residue was t;-butyl disulfide. This was
identified by comparison of its retention time with a known sample.
There was no peak corresponding to the retention time of t;-butyl phenyl
disulfide. The amount of t -butyl disulfide (0.18 mmol) was determined
from the glc experiment by using a known amount of a-butyl sulfide as an in
ternal reference. The amount of t -butyl disulfide found accounts for 72%
of t -butyl groups present in the original t;-butyl benzenethiolsulfinate.
In a separate experiment, after the reaction was neutralized with
acetic acid, it was then titrated by the cupric alkyl phthalate method
to determine if there were any t -butyl mercaptan formed. The amount
of mercaptan found was 3.93 mg (0.044 mmol). This is equivalent to 8.8%
of t-butyl groups in the original t-butyl benzenethiolsulfinate.
27
JiLnetic, Studies, The kinetics of the alkaline hydrolysis of t -butyl
benzenethiolsulfinate were studied at 25°C in 60% dioxane containing
0.01-0.08 M hydroxide ion. Since the initial concentration of -butyl
benzenethiolsulfinate ranged from 0.98 x 10"^ to 3.92 x 10"^ M, hydroxide
ion was always present in large stoichiometric excess over t -butyl ben
zenethiolsulfinate and the concentration of hydroxide ion remained ef
fectively constant during the course of a run. The kinetics were fol
lowed by monitoring the change in optical density of the solution at 268
nm. The relevant data are given in Table V.
TABLE V
Kinetics of the Alkaline Hydrolysis of t;-Butyl
Benzenethiolsulfinate in 60% Dioxane at 25° C
Run #
1
2
3
4
5
6
7
[PhS(0)S-i^-Bu]Q
X 10"^ M
1.96
0.98
1.96
3.92
1.96
1.96
3.92
[0H-]
M
0.08
0.04
0.04
0.04
0.02
0.01
0.01
• obs X lO'^s"^
7.2
3.54
3.71
4.3
2.13
1.25
1.44
^OH- = ^bs/LOH-J
0.09
0.089
0.093
0.108
0.107
0.125
0.144
28
The pseudo first-order rate constants (see Table V, k . ) were ob
tained for runs using various concentrations of t-butyl benzenethiolsul
finate and hydroxide ion. For a fixed initial concentration (1.96 x
10""* M) of t -butyl benzenethiolsulfinate, a linear plot of kg^s vs. [OH"]
was obtained and is presented in Figure 1. It should be noted that the
line does not go through the origin. Formally, kobs "" given by an equa
tion of the type:
' obs ^ 'o ••" '^OH" f^^'^ •
The slope (kgn-) for Figure 1 is 0.085 sec-lM-l and the Y-intercept (ko)
is 0.39 X 10-3 s-1.
Another peculiarity of the behavior of kobs shown in Table V is
that for a given [0H-], ^Q^. seems to increase with larger initial concen
tration of thiolsulfinate.
Some possible reasons for these two slightly unusual behaviors of
' obs ^^ " T st that kjjjj seems to depend somewhat on [PhS(0)S-t_-Bu]Q and
secondly, kobs shows a dependence on hydroxide ion concentration but it
is not of the form kobs = koH-COH"]. This will be considered in the
Discussion section later.
29
[OH"] x lO", M
Figure 1. Dependence of k i ^ on Hydroxide Ion Concentra
tion for Fixed Concentration of t-Butyl
-4 Benzenethiolsulfinate (1.96 x 10' M)
DISCUSSION
Section A
The Acid- and Sulfide-Catalyzed Decomposition of
l-Butyl Benzenethiolsulfinate
The kinetic study by Kice and Venier^^ of the acid- and sulfide-
catalyzed decomposition of t.-butyl benzenethiolsulfinate showed that
the reaction is first-order in thiolsulfinate and that the experimental
first-order rate constant depends linearly on [R2S] and on the Hammett
acidity function, h^. The dependence of the rate constant on sulfide
structure has the same form as in the reaction of phenyl benzenethiol
sulfinate with either a-butyl mercaptan or aromatic sulfinic acids. It
is quite different than the dependence of rate on sulfide structure found
for the acid- and sulfide-catalyzed disproportionation of phenyl benzene
thiolsulfinate under the same reaction conditions. The kinetic results
indicate that the rate determining step of the decomposition of t -butyl
benzenethiolsulfinate is attack of the alkyl sulfide on protonated t-butyl
benzenethiolsulfinate. There are two centers, however, on the protonated
thiolsulfinate which could be attacked by the sulfide, the sulfenyl sulfur
and the sulfinyl sulfur. Nucleophilic displacement at the sulfinyl center
would produce PhS(0)*R2 (VII) and mercaptan (Equation 28); nucleophilic
displacement at the sulfenyl center, would produce benzenesulfenic acid
and t-BuS-SR2 (VIII) (Equation 29).
OH 0
I II + Phj -S- t^ -Bu + R2S ^ PhS-SR2 + L-BuSH ( 2 8 )
VII 30
31
OH J k' +
PhS-S-t-Bu + R S — ^ PhSOH + t-BuS-SR^ (29) VIII
Kice and Morkved have shown that under the present reaction condi
tions intermediate VII should rapidly hydrolyze to R2S and PhS02H. The
mercaptan,tBuSH, also formed in Equation 28, would be expected to react
further with another protonated t-butyl benzenethiolsulfinate to form
t -butyl disulfide, as in the reaction of phenyl benzenethiolsulfinate
with mercaptan to form the unsymmetrical disulfide^^ (Equation 13). How
ever, in the product studies in the present work, no significant amount
of t -butyl disulfide was found in the products of the acid- and sulfide-
catalyzed decomposition of t -butyl benzenethiolsulfinate. This seems to
rule out the possibility of Equation 28 as the rate-determining step. The
actual products formed, however, do seem to be consistent with attack in
the fashion shown in Equation 29. The rate-determining step is attack of
the sulfide on the sulfenyl sulfur, even though a bulky t -butyl group is
attached to it.
46 49 The previous work ' on the sulfide-catalyzed disproportionation
of phenyl benzenethiolsulfinate indicated that benzenesulfenic acid once
formed by the rate-determining step would be readily transformed into
R„^-SPh (IX) (Equation 11). This intermediate can then suffer nucleo-
PhSOH + H"*" + R2S > PhS-?R2 + H2O (11)
IX
32
philic attack by another molecule of ;^-butyl benzenethiolsulfinate on its
coordinate sulfur, in a reaction analogous to Equation 19 in the dispro
portionation of phenyl benzenethiolsulfinate (see Scheme II), to form the
ion X (Equation 30). Hydrolysis of X leads to t -butyl phenyl disulfide
(Equation 31), which is one major product in the first fraction eluted
from silica gel. The benzenesulfinic acid also formed by Equation 31
would be rapidly converted^^'^^ to the thiolsulfonate, PhS02SPh, by reac
tion with another ion IX (Equation 21).
0 0
R2S-SPh + t_-BuSSPh > ^2^ + Ph-S-S-SPh (30)
IX
t_-Bu
X
0
PhS-S-SPh + H2O > PhS02H + PhSS-t-Bu + H" ( 31)
t-Bu
PhS02H + PhS-SR2 > PhS02S Ph + R2S + H " (21)
IX
The intermediate V I I I , R2S-S-t-Bu, formed in the rate-determining
step is of the san« structural type as IX, R2S-SPh. However, due
the steric hindrance of the dicoordinate sulfur provided by the
t -buty l group, nucleophilic attack on the dicoordinate sulfur of V I I I
is much more d i f f i c u l t than on the sulfenyl sulfur of IX. So the reac
33
tions of intermediate V I I I with ei ther t^-butyl benzenethiolsulfinate or
with benzenesulfinic acid are so slow as to be k inet ica l ly unimportant
and these reactions can be neglected. This is an important difference
between the sulf ide catalyzed decompositions of phenyl benzenethiolsul
f ina te and t^-butyl benzenethiolsulfinate.
Although the ster ic e f fect of t -butyl group in the intermediate V I I I
severely hinders nucleophilic attack on the sulfenyl sulfur of R25-SBu-t_,
V I I I , due to the s t a b i l i t y of a t -butyl cat ion, could readily decom
pose to the th io lsul fox ide, R2S= S, and isobutylene as shown in Equation
32.
R25-S-S-t.-Bu > R2S=S + (CH3)3C+ 7
VIII
CH2 = C
-H^
(32)
CH3
CH3
55 According to a recent report by Beachler and Daley, thiosulfoxides
are very unstable compounds and decompose very rapidly into a monosulfide
and elemental sul fur (Equation 33) . Any reaction of R2S=S with other
R2S=S > R2S + S (33)
species present in the solution would therefore have to be very rapid if
it is to be able to compete with Equation 33. This eliminates, or ren
ders unlikely, a number of otherwise reasonable possibilities such as:
34
R2S=S +^t-BuS-SR2 ^ R2^-S-S-t-Bu + R2S (34)
V I I I XI
In Equation 34, the s te r ic hindrance to attack on the dicoordinate su l
fu r of V I I I provided by the t -bu ty l group makes i t seem unl ike ly that
Equation 34 would have a rate fast enough to compete with Equation 33.
55 The reported very rapid rate for Equation 33 suggests that the
su l fu r may well be l iberated i n i t i a l l y in a highly reactive s ta te , rather
than as the stable Sg. Reaction of the th io l su l f i na te with such reac
t i ve s u l f u r , e i ther atomic su l fur or some other wery reactive form,
could lead to inser t ion of su l fu r in to the S-S bond of the th io l su l f i na te
to form PhS(0)SS-t_-Bu (Equation 35). This compound, unlike PhS(0)S-t^-Bu
in i t s protonated form PhS(OH)SS-t^-Bu has an unhindered sulfenyl su l fur +
next to the -S-(OH) group and can readi ly be attacked by the alkyl su l -
PhSS-t-Bu + S > PhS-S-S-t-Bu (35) II " II 0 0
fide on this unhindered dicoordinated sulfenyl sulfur adjacent to the
protonated sulfinyl group (Equation 37), leading to the intermediate XI
and benzenesulfenic acid.
35
PhS-S-S-t-Bu + H+ > Ph5-S-S-t-Bu (36)
0 in
+ + PhS-S-S-t^-Bu + R2S > PhSOH + R^S-S-S-i-Bu (37)
OH XI
Once the intermediate XI is formed, the formation of L-BuSSSBu-Land
PhS02SSBu-t_ is easily explained. Di-t_-butyl tr isulf ide can be produced
by the attack of t-butyl benzenethiolsulfinate on intermediate XI to
give X I I , followed by hydrolysis of XII (Equation 39). The final product,
PhS02SSBu-t , would result from the reaction of XI with the benzenesulfinic
acid produced in Equation 39,
t-Bu
PhSS-t-Bu R2S-S-S-t_-Bu > PhS-j-S-S-t.-Bu + R2S (38) II 0 0
XI X I I
t -Bu
PhS-S-S-S-t-Bu + H O > PhS-OH + t.-BuSSS-t-Bu + H" (39) II + - 2 11 0 °
X I I
0
PhS-OH + RoS-S-S-t-Bu > PhS-S-S-^-Bu + R S + H (40) II ^ " II 0 0
XI
36
From the mechanism suggested above, the overall reaction is given
in Scheme 3. The stoichiometry shown at the end of the scheme is based
on the assumption that PhSSR2 and t-BuSSSR2 behave similarly in terms
of their reactivity toward t-butyl benzenethiolsulfinate vs. benzenesul
f in ic acid,so that in each case essentially half of the reactive inter
mediate reacts with t-butyl benzenethiolsulfinate and the remainder is
consumed in a subsequent faster reaction with benzenesulfinic acid.
Scheme 3
Overall Mechanism for the Decomposition of t^-Butyl
Benzenethiolsulfinate by Acid- and
Sulfide-Catalyzed Reaction
PhSS-t-Bu + H ^ = ^ PhS-S-t-Bu II - 1 " 0 OH
PhS S - S - t - B u + RoS ^ -^ > PhSOH + R . S - S - t - B u ( 2 9 ) ^2" k 1
OH
PhSOH + R2S + H"" ^ = = ^ PhS-SR2 + H20 ( H )
t-Bu
PhSS-t-Bu 4-PhS-SR. > PhS-5-SPh + R2S (30)
r I
t-Bu
PhS-S-SPh + H2O ^^^^ > PhS02H + t-BuSSPh + H^
0
37
(31)
0
PhS02H + PhS-SR, fast •> PhS-SPh + R2S + H"*"
0
R2S-S-i-Bu -^ R2S=S + (CH3)3C'^
- H^ CH2= C
CH.
CH.
( 2 1 )
( 3 2 )
R2S=S -^^^ > R2S + S ( 3 3 )
PhSS- t -Bu + S -^^—>
0
PhSSS-t -Bu
0
( 3 5 )
PhSSS-L-Bu + H" II 0
•» P h ? - S - S - t - Bu
OH
(36)
PhS-S-S-t-Bu+ R2S > PhSOH + R25sS-t^-Bu
t-Bu OH
( 3 7 )
PhSS- t -Bu +R2SSS-t^-Bu > P h S - j - S - S - ^ - B u + R2S
0 0
( 3 8 )
38
t-Bu r , ,
PhS-5-S-SBu-t + H2O - ^ i l U PhSOoH + t-BuSSS-t-Bu+ H (39)
0
(40)
0
0
PhS02H + RJsS-t-Bu -JMl^ PhSSS-t-Bu+ R„S + H+ II " ^ 0
Overall equation is
3J5 PhSS-t_-Bu >PhSS-i-Bu+PhSOpSPh + h t-BuSSS-t-Bu I "
+ h PhS02SS-t-Bu+CH2=C(CH3)2 + H2O
I . e . ,
PhSS-i-Bu > 0.29 PhSS-t -Bu+ 0.29 PhS02SPh
I 0 + 0 . 1 4 t-BuSSS-l-Bu+0.14 PhS02SS-i-Bu
+ 0.29 CH2=C(CH3)2 + 0.14 H2O
From Scheme 3, the rate constant for the sulfide-catalyzed decom
position of t-butyl benzenethiolsulfinate, k , , is equal to:
' d ~ ^^ ' l H''"
where kJ is the rate constant for the rate-determining step (Equation
29), K' is the equilibrium constant for the protonation of t.-butyl ben
zenethiolsulfinate, and a is the total number of molecules of jt-butyl
benzenthiolsulfinate consumed for each occurrence of Equation 29. Under
39
product study conditions a. is 3.5, but under kinetic conditions i t could
be as low as 1.5 i f t-butyl benzenethiolsulfinate is unable at this much
lower concentration to trap a l l of the atomic sulfur liberated by the
decomposition of ion V I I I .
The rate constant for the sulfide-catalyzed reaction of phenyl ben
zenethiolsulfinate with either thiols or sulf inic acids, k , is given
by (see Scheme I ) :
k = Kk aj + .
The ratio (k|j/k ) is therefore given by:
k ' k ' d _ K' * 1 — a k - K k, •
Experimentally one finds (see Appendix 1) that k ' /k = 6 x 10"^. Due
to the steric hindrance surrounding the dicoordinate sulfur of protonated
t -butyl benzenethiolsulfinate, attack at that center should be retarded
and, therefore, k]'should be much smaller than k-j. Since t_-BuS is induc
tively a somewhat weaker electron-withdrawing group than PhS, K' should
be somewhat larger than K. This, plus the fact that a^may be as large as
3.5, means that k-j'/k-. is undoubtedly considerably smaller than the 6 x
10'^ value of k^j'/kg. This is not surprising since a rate ratio for
W(0)S-t-Bu/^PhS(O)SPh °^ 5 X 10-^ has been found in other work in this
thesis for nucleophilic attack of t^-BuS" on these two thiolsulfinates.
Section B
Reaction of 2-Methyl-2-propanethiolate Ion and Hydroxide
Ion wi th t -Buty l Benzenethiolsulf inate
Kinet ic Studies of the Reaction of 2-Methyl-2-propanethiolate
Ion wi th t -Buty l Benzenethiolsulf inate
Table I I I shows that the reaction of 2-methyl-2-propanethiolate ion
w i th t - bu t y l benzenethiolsul f inate is f i r s t - o r d e r in th io la te ion and
f i r s t - o r d e r in t h i o l s u l f i n a t e . The rate constant i s about 16 M' sec'
which is approximately 2 x 10 slower than the reaction of 1-butanethio-
la te ion wi th phenyl benzenethiolsulf inate^^ (k = 2.9 x 10 M" sec' ).
52 This resu l t is consistent with the fact found by Fava et al that the
rate of nuc leophi l ic subst i tu t ion at the dicoordinate su l fur in t-BuS-S03-
is 10^ slower than the rate at the same sul fur in CH3CH2S-SO3 . The rea
son is presumably due to the s te r ic e f fec t of the t.-butyl group attached
to the dicoordinate su l fu r . The ear l ie r observation by Fava et al on
the react ion of labeled th io la te ion with d isul f ides also supported th is
assumption (Equations 41 and 42). The rate constant of 2-methyl-2-propane-
th io la te ion is 3.1 x 10^ slower than that of the 1-butanethiolate ion.
* * t-BuS- + t-BuSS-t-Bu ^ t-BuSS-t-Bu + t-BuS"
^t-BuS- = ' ^ ' ' ' ' ' • ' ' ' ' ' '
n-BuS' + n-BuSS-n-Bu ^ a-BuSS-a-Bu + a-BuS"
(41)
k „ c- = 0.31 M'^ sec'^ (42) a-Bus
40
41
Product and Kinetic StudJPs of Alkaline Hydrolysis of
l-Butyl BenzenethiolsulfinatP
The study of the hydrolysis of phenyl benzenethiolsulfinate, by
Kice and Rogers » has indicated that the thiolsulfinate can be at
tacked by hydroxide ion at both sulfurs, with k = 140 M"^ sec"^ and s
• so " ^^° ^ ^®^' (Equation 24). In principle t.-butyl benzenethiol
sulfinate could also be attacked by hydroxide at either sulfur as in
dicated in Equations 43 and 44. But from the previous discussion, the
k' -^PhSO' + t-BuSOH -^^-^ PhSO" + t-BuSO" (43)
PhSS-t-Bu + OH" I " 0 k'
-^>PhS02H + t-BuS" ^^>PhS02 + t-BuS" (44)
attack at the sulfenyl sulfur should be very difficult due to the steric
effect exerted by the t-butyl group attached to the sulfenyl sulfur atom
The rate constant for attack of hydroxide ion on the sulfenyl sulfur of
t -butyl benzenethiolsulfinate (Equation 43) should be much slower (~10
slower) than the rate constant for attack of hydroxide ion on the same
44 sulfur in phenyl benzenethiolsulfinate, which has a value of k =
140 M ' sec' . However, the observed rate constant for the alkaline
hydrolysis of t -butyl benzenethiolsulfinate is 0.085 M" sec' . This 3
is only ~ 10 slower than the rate for the alkaline hydrolysis of phenyl
benzenethiolsulfinate. This fact makes it seem unlikely that the alka
line hydrolysis of t-butyl benzenethiolsulfinate takes place via Equa-
42
tion 43, and suggests that the reaction takes place by the attack of hy
droxide ion at the sulfinyl sulfur (Equation 44), with the rate constant
for k^Q, in Equation 44 being 2 x 10^ slower than k in Equation 24.
The reason for the difference between k and k' is assumed to be due so so
to the fact that PhS' is a considerably better leaving group than t.-BuS".
Attack of hydroxide ion at sulfinyl sulfur produces PhSO^ and i-BuS-,
yet since only 9% of the 1-BuS groups originally present in the thiolsul
finate were found as 2-methyl-2-propanethiolate ion at the end of the
product experiment,and over 70% were found as t -butyl disulfide,clearly
some additional reaction or reactions occur which transform most of the
2-methyl-2-propanethiolate ion formed in Equation 44 to t_-butyl disulfide.
Reaction of 2-methyl-2-propanethiolate ion with jt-butyl benzene
thiolsulfinate (Equation 45) will produce t-butyl disulfide, and since
t-BuS" + PhSS-t-Bu ^"^"^—> t-BuSS-t-Bu + PhSO"
0 (45)
•^t-Bus" = 16 M"^ sec"^
the rate constant for this reaction is much faster than that for alkaline
hydrolysis. Equation 45 might compete with Equation 44 under the condi
tions used for the product study. Also, some of the 2-methyl-2-propane-
thiolate ion produced in Equation 44 could be subsequently consumed via
Equation 45 to produce t.-butyl disulfide. The question that must now be
explored is, given the rate constants for the two reactions, and the
fact that [0H-] = 1 M in the product studies, at what concentration of
2-methyl-2-propanethiolate ion would the rate of Equation 45 become
43
competitive with the rate of Equation 44. If this concentration is much
larger than the initial concentration of t_-butyl benzenethiolsulfinate
then reaction of 2-methyl-2-propanethiolate ion produced by Equation 44
with the thiolsulfinate in Equation 45 will never become competitive
with Equation 44. On the other hand, if this concentration is much
smaller than the initial concentration of t -butyl benzenethiolsulfinate,
then Equation 45 will become competitive in rate with Equation 44 early
in the hydrolysis and most 2-methyl-2-propanethiolate ion formed in
Equation 44 will be converted to disulfide by Equation 45. One can de
termine at approximately what concentration of 2-methyl-2-propanethio-
late ion Equation 45, will become competitive with Equation 44 by cal
culation of the concentration of 2-methyl-2-propanethiolate ion at which
its rate of production by Equation 44 will be equal to its rate of con
sumption by Equation 45:
Rate of production of L-BuS' = kQ^_[OH-] [TS]
Rate of consumption of t.-BuS~ = k._g^_[t_-BuS-] [TS]
TS = trbutyl benzenethiolsulfinate.
When the rate of production of 2-methyl-2-propanethiolate ion becomes
equal to its rate of consumption via Equation 45:
kQ^jOH-] [TS] = kt-BuS-l^^"^"^'^ " ^ ^ '
Therefore, the concentration of 2-methyl-2-propanethiolate ion at which
the rates become equal is:
44
[ t -BuS-] = l ^ O H : ^ ._ ^ , 0 8 5 j M . 5 3 ^ ^ , -3 , _
" ^t-BuS- ''
Since a concentration of 5.3 x 10-3 M is less than the i n i t i a l concen
t r a t i on of the th io lsu l f inate ,which is 0.01 M, i t is clear that the
react ion of 2-methyl-2-propanethiolate ion with t^-butyl benzenethiol
su l f ina te (Equation 45) does become competitive with Equation 44 during
the course of the a lka l ine hydrolysis. Some of the t^-butyl d i
su l f ide found in the products does resul t from reaction of 2-methyl-2-
propanethiolate ion (formed in Equation 44) with some of the remaining
t^-butyl benzenethiolsulf inate. However, the fact that the concentration
of 2-methyl-2-propanethiolate ion required for the reaction to become . -J
competitive is ~ 5 x 10 M while the i n i t i a l concentration of t -buty l _2
benzenethiolsulfinate is 1.0 x 10 M means that Equation 45 becomes com
petitive only in the later stages of the reaction and that only part of
the total disulfide formed can be accounted for by Equation 45. So some
of the 2-methyl-2-propanethiolate ion formed in Equation 44 must be
oxidized by some other process to account for the remainder of the t;-butyl
disulfide formed. Just what reaction is responsible for this is not known
at present.
2 t^BuS- ^—> t-BuSS-t^-Bu (46)
The curious kinetic result in Figure 1 is that at a fixed concentra
tion of t -butyl benzenethiolsulfinate (1.96 x 10" M ) , a plot of k^^^ vs
45
[0H-] does not pass through origin; instead it has a dependence of rate
on hydroxide ion that can be formally represented as:
^obs = K " koH-[OH-].
How can one explain this behavior? A likely possibility is that while
Equation 45 does not contribute significantly to the rate of disappear
ance of the thiolsulfinate at high [OH"], it does make a significant
contribution at low [OH"]. One can see how this could be so by calcula
ting for different hydroxide concentrations the concentration of 2-methyl-
2-propanethiolate ion at which Equation 45 would become competitive with
Equation 44. Then one must compare these to the initial concentration of
t_-butyl benzenethiolsulfinate to see at what point, if ever, during the
course of a reaction this situation will be reached. For example, at
[OH'] = 0.01 M:
r. D c-T kgi^-COH-] _ 0.085 x 0.01
5.3 X 10"^ M
This concentration is less than one-third of the original concentration
of t^-butyl benzenethiolsulfinate and so Equation 45 should become com
pet i t ive with Equation 44 early in the reaction and the observed rate of
disappearance of ;t-butyl benzenethiolsulfinate w i l l be s igni f icant ly l a r
ger than the rate predicted from just kQ^.[OH-] alone. On the other hand,
when [0H-] = 0.08 M one calculates:
[ t -BuS-] = " -"SSxQ-OS = 4.2 X 10-^ M . "-— - ss 16
46
This concentration is over twice as large as the initial concentration
of t-butyl benzenethiolsulfinate, and this means that under these con
ditions Equation 45 will never become competitive with Equation 44. Thus
at [0H-] = 0.08 M and [PhS(0)SBu-t]Q = 1.96 x 10'^ M , the rate of disap
pearance of the thiolsulfinate will effectively be that expected from
kQj^-[OH-] alone, in contrast to the situation that exists at lower con
centration of hydroxide ion.
Another curious point in Table V is that at a fixed concentration
of hydroxide ion the k ^ ^ also shows some dependence on the initial con
centration of t_-butyl benzenethiolsulfinate. This behavior can also be
understood in terms of the relative importance of the competition of
Equation 45 varying with initial reaction conditions. The larger the
initial concentration of 1-butyl benzenethiolsulfinate, the earlier in
the reaction (in terms of % thiolsulfinate converted to products) that
one will reach a given concentration of 2-methyl-2-propanethiolate ion,
and therefore the earlier the concentration of 2-methyl-2-propanethiolate
ion will reach a particular concentration level. This means that at any
given hydroxide concentration there will be more contribution to the
observed average rate of disappearance of the thiolsulfinate (from
Equation 45) with higher initial concentrations of the thiolsulfinate.
So, it is found in Table V, that higher concentrations of t -butyl ben
zenethiolsulfinate yield higher k ^ ^ values in Runs 2, 3, and 4, as well
as in Runs 6 and 7.
EXPERIMENTAL SECTION
Preparation of t-Butvl Benzenethiolsulfinate^^
Sodium benzenesulfinate (8.2 g, 0.05 mol) was suspended in 40 ml
of hexane, and thionyl chloride was added slowly with stirring until no
more gas was evolved, after which 0.5 ml more thionyl chloride was added.
The mixture was stirred for another 30 minutes at room temperature. The
white precipitate of sodium chloride was filtered, and the filtrate was
concentrated under reduced pressure. The last traces of hexane were re
moved under an oil pump vacuum (0.1 mm Hg) at 50°C for 30 minutes.
The freshly prepared benzenesulfinyl chloride was used immediately.
Benzenesulfinyl chloride (3.25 g, 20 mmol) was dissolved in 30 ml of
anhydrous ether. To this solution was then added slowly with stirring
at room temperature, t-butyl mercaptan (1.8 g, 20 mmol ) and pure pyri
dine (1.6 g, 20 mmol) in 30 ml of anhydrous ether. Immediate reaction
occurred with the formation of a white precipitate of pyridine hydro
chloride. After the addition was complete, the pyridine hydrochloride
was filtered off and the ether solution was extracted once with 1 N sul
furic acid, twice with 5% sodium bicarbonate, and twice with water; it
was dried over magnesium sulfate and the ether was then removed under re
duced pressure. The residue was recrystallized from chloroform-hexane,
6.63 g (yield 40%), m.p. 51-52°C (lit.^ 51-52°C).
67 Preparation of t-Butyl Phenyl Disulfide
Chlorine gas was bubbled at a moderate rate through a stirred solu
tion of phenyl disulfide (21.8 g) in 150 ml of distilled pentane for
47
48
about 1.5 hours at room temperature in a reflux apparatus fitted with a
drying tube. The chlorine flow was then stopped and the red-orange so
lution was stirred overnight during which time an intensification of
color developed. The resulting sulfenyl chloride solution was used
immediately without removing solvent.
The freshly prepared solution of benzenesulfenyl chloride was added
slowly dropwise to a stirred solution of phthalimide (29.4 g) in 125 ml
of N,N-dimethylformamide containing triethylamine (27.7 ml). After the
addition was complete, the resulting solution was stirred for another
thirty minutes. The reaction mixture was then poured into one liter of
distilled water, stirred, and the N-phenylthiophthalimide was filtered
off and recrystallized from large quantities of boiling ethanol, 36.3
g, yield 71% m.p. 159-161.5°C (Tit.^^ 160-161°C).
A solution of N-phenylthiophthalimide (25.5 g, 0.1 mole) and dis
tilled t -butyl mercaptan (9 g, 0.1 mole) in 400 ml of absolute benzene
was refluxed under nitrogen for 96 hours. The solution was filtered to
remove precipitated phthalimide and the benzene solvent was removed by
rotary aspiration. The resulting t -butyl phenyl disulfide was purified
by vacuum distillation, 11 g, yield 55%, b.p. 60-62°C (0.05 mm Hg) (lit.^^
48°C (0.003 mm Hg)). IR (neat): 2960, 1580, 1470, 1440, 1360, 1155 cm'^
NMR (CDCI3): 6 1.32(s), 7.2-7.8(m). MS: m/e 198(81), 142(100), 109(52),
78(95), 57(100).
59 Preparation of Symmetric t-Butyl Trisulfide
Commercial sulfur dichloride was purified by fractional distillation
at atmospheric pressure and the red liquid b.p. 50-60°C was collected.
49
To this, ca. 0.1% PCl^ was added. This material was fractionally dis
tilled into a receiver containing PCl^ (ca. 0.1%), b.p. 56-56.5° C.
A solution of t-butyl mercaptan (1.88 g, 0.02 mol) in 30 ml of
anhydrous ether was added to a solution of sulfur dichloride (1.05 g,
0.01 mol) in 30 ml of anhydrous ether. The mixed solution became
colorless and was stirred for 20 more minutes. The ether was then re
moved under reduced pressure and the residue was subjected to vacuum
distillation giving 1.76 g (84%) of di-t.-butyl trisulfide, b.p. 46° C
(0.3 mm Hg). IR(neat): 2960, 1455, 1360, 1160 cm"^ NMR (CDCI3): 6
1.39. MS: m/e 210 (M+), 154 (M-C^Hg), 89 (C^HgS).
Products of the Sulfide-Catalyzed Decomposition of t-Butyl
Benzenethiolsulfinate
A solution of n-butyl sulfide (0.16 g, 1.1 mmol) in 20 ml of acetic
acid-1% water was added to a solution of t_-butyl benzenethiolsulfinate
(2.15 g, 10 mmol) in 20 ml of acetic acid-1% water. To this was then
added 60 ml of 0.17 M sulfuric acid in acetic acid-1% water. The reac
tion solution was allowed to stand at 40° C for 8 hours. At the end of
that time the solution was poured into 1 liter of water. The suspension
was extracted three times with 100 ml portions of ether. The ether ex
tracts were washed once with water and then with 5% sodium bicarbonate
until the washings remained weakly basic (same pH as HCO3"). The ether
extracts were dried over anhydrous magnesium sulfate and the ether then
removed under reduced pressure. The residue (1.87 g) was then chroma
tographed on silica gel (70-230 mesh) using hexane, hexane-benzene
(2:1, 1:1, 1:2), benzene, and benzene-ether (1:1) as eluents.
50-
Elution with 2:1 hexane-benzene gave 908 mg (Fraction I). This was
further purified by vacuum distillation, b.p. 41°-70° C (0.2 mm Hg). NMR
(CDCI3): 6 0.7-l(m), 2.3-2.6(m), 1.32(s), 1.39(s), 7.2-7.8(m). MS: m/e
210 (41), 198 (82), 154 (100), 146 (74), 142 (100). IR (neat): 2960,
1580, 1475, 1455, 1440, 1360, 1155 cm'^ Gas liquid chromatography
(column 5' x 1/8" 5% SE 30, 120° C) showed the presence of three compo
nents having retention times identical with those of n -butyl sulfide,
di-t-butyl trisulfide and t-butyl phenyl disulfide. No peak correspon
ding to t -butyl disulfide was found. The identity of the trisulfide was
unequivocally established by its separation from the mixture by prepa
rative gas chromatography and comparison of its mass spectrum with that
of an authentic sample of di-t-butyl trisulfide. The relative amounts
of di-t^-butyl trisulfide and t-butyl phenyl disulfide in Fraction I
could be determined both from glc integration and from the relative in
tegrated intensities of the singlet for the methyl group in di-t^-butyl
trisulfide ( <S 1.39) and that for the methyl group in t -butyl phenyl di
sulfide at 6 1,32, The fact that the ratio of the singlet at 6 1.32 to
the aromatic multiplet at 6 7,2-7.5 was 9:5 showed the fraction contained
no significant amount of phenyl disulfide, as did the absence of a peak
for this disulfide in the glc.
Elution with 1:1 hexane-benzene gave 154 mg (Fraction II) of an oil
that solidified on being placed in the freezer. It could be recrystal
lized from 4:1 hexane-ethanol at -50° C. A melting point could not be
obtained because the compound melts below room temperature. The various
spectral properties of the compound were as follows: NMR (CDCI3):
51
5 1.39 (s, 9H), 7.4-8.2 (m, 5H). IR (neat): 2960, 1450, 1365, 1320,
1135, 1070 cm' . UV (dioxane): 254 ( e 7895), 225 ( e 13420). MS:
m/e 262(M''), 2 0 6 ( M - C ^ H Q ) , 182, 143, 126, 125, 110, 109, 97, 78, 77, 57.
On this basis of the spectral data the compound was assigned the struc
ture PhS02SSBu-t^. The correctness of the molecular formula was confir
med by elemental analysis. Anal. Calcd. for C^4Hi402S3: C, 45.80;
H, 5.34; S, 36.64. Found: C, 45.82; H, 5.40; S, 36.75.
Elution with 1:2 hexane-benzene gave 748 mg (Fraction III) of ma
terial that crystallized on standing. It could be recrystallized from
ethanol m.p. 43-45° C. The spectral properties of the recrystallized
material were identical with those of aknown sample of phenyl benzene-
thiolsulfate (lit.^^ m.p. 45° C).
Failure of Phenyl Disulfide and t-Butyl Disulfide to Undergo
Disproportionation to t-Butyl Phenyl Disulfide Under
Reaction Condition
A mixture of t-butyl disulfide (0.89 g, 5 mmol), butyl sulfide
(0.08 g, 0.5 mmol), phenyl disulfide (1.09 g, 5 mnol), was dissolved
in 50 ml of acetic acid-1% water containing 0.1 M sulfuric acid and the
solution kept at 40° C for 8 hours. At the end of that time the solu
tion was poured into 10 times its volume of water and extracted with
ether in the same way as in the product studies of the decomposition of
the thiolsulfinate. Upon work-up, the residue was found to consist of
1.94 g of wet, white crystals, m.p. 41-49° C. The residue was subjected
to glc and it was shown that the only substances present in the residue
52
were the starting materials: t -butyl disulfide, phenyl disulfide, and
a-butyl sulfide.
Absence of Acetone as a Decomposition Product
A test solution containing 2,4-dinitrophenylhydrazine (1.24 g) dis
solved in 6 ml of concentrated sulfuric acid, 10 ml of water, and 32 ml
of 95% ethanol was prepared and filtered before use. A trial experiment
was carried out to ensure that acetone, if formed in the decomposition,
could be detected. Acetone (71.2 g, 1.23 mmol) was dissolved in acetic
acid (50 ml) and about half of the solution was distilled off under re
duced pressure (120 mm Hg, 65^0). The distillate was trapped in dry ice.
After melting the distillate, 50 ml of water was added to it, followed
by 10 ml of the 2,4-dinitrophenylhydrazine test solution. A precipitate
formed which was filtered off and recrystallized from ethanol-water, m.p.
126-128° C, 179 mg, yield 62%.
A mixture of t-butyl benzenethiolsulfinate (1.07 g, 5 mmol) and
n-butyl sulfide (0.078 g, 0.53 mmol) was dissolved in 50 ml of acetic
acid-1% water containing 0.1 M sulfuric acid and the solution was kept
at 40° C for 8 hours. It was then distilled under reduced pressure
(110 mm Hg, 65°C) in the same way as in the case of the known sample.
Treatment of the distillate with water plus the 2,4-dinitrophenylhydrazine
solution led to the formation of no precipitate.
53
Identification of Isobutylene as a Decomposition Product
t.-Butyl benzenethiolsulfinate (0.214 g, 1 mmol) and n-butyl sul
fide (0.16 g, 0.1 mmol) were dissolved in 10 ml of acetic acid-1%
water containing 0.1 M sulfuric acid and the solution was kept at 50° C
for 5 hours while nitrogen was passed through the solution and then two
traps, the first cooled in ice and the second in liquid nitrogen. At
the end of the reaction, a few milliliters of chloroform was added to
the liquid nitrogen trap, and then 1 ml of a 1% solution of bromine in
acetic acid (0.57 N in bromine) was added and the mixture was allowed
to warm to room temperature. Then 10 ml of 10% potassium iodide solution
was added and the liberated iodine was titrated with standard sodium
thiosulfate, 1.53 ml of 0.1555 N thiolsulfate being required.
Procedure for Kinetic Study of the Reaction of Hydroxide Ion
with t-Butyl Benzenethiolsulfinate
A stock solution of 3.5 ml of sodium hydroxide (0.01-0.08 N) in 60%
dioxane was placed in a 1-cm spectrophotometer cell. The reaction was
initiated by adding 15-70yl of a solution of L-butyl benzenethiolsul
finate (2 X 10"^ M) in pure dioxane and rapidly mixing this with the
sodium hydroxide solution in the cell. The change in absorbance was
followed at 268 nm. Pseudo first-order rate constants for each run were
determined from the slope of plots of log (A-A^) vs. time.
54
Procedure for Kinetic Study of the Reaction of 2-Methyl-2
PropanethiPlate Ion with t-Butyl Benzenethiolsulfinate
A solution (3.5 ml) containing a concentration of sodium hydroxide
in 60% dioxane equal to the concentration of thiolate ion desired was
placed in a 1-cm spectrophotometer cell in the thermostatted cell com
partment of an ultraviolet spectrophotometer. To this was then added
the proper number of microliters of a solution of 2-methyl-2-propane-
thiol (1 M) in pure dioxane to give the desired final concentration of
mercaptan and thiolate ion after reaction of the thiol with hydroxide
ion. The reaction was then initiated by adding the proper number of
microliters of a solution of t -butyl benzenethiolsulfinate (0.1 M) in 60%
dioxane and the course of the reaction was followed by monitoring the de
crease in optical density at 264 nm. After eight to ten half-lives an
infinity point was taken, and the experimental first-order rate constant
for the reaction was determined from a plot of log (A -A^) vs. time.
In another group of runs, 3.5 ml of a solution of a 1:6 buffer of
[KH^PO-] : [KpHPO.] in 60% dioxane was placed in the spectrophotometer
cell. The desired amount of t.-butyl mercaptan (1 M in pure dioxane) was
then added to the buffer, and the reaction was then initiated by adding
t.-butyl benzenethiolsulfinate (0,1 M in 60% dioxane) with the use of a
microsyringe. The reaction was then monitored in same way as described
for the other runs.
55
Product Studies of the Alkaline Hydrolysis of
1-Butyl Benzenethiolsulfinate
t-Butyl benzenethiolsulfinate (0.017 g in 10 ml of 40% dioxane) was
added dropwise into a solution of sodium hydroxide (2 g in 40 ml of 37.5%
dioxane) under nitrogen. After the addition was complete, the solution
was stirred for two more hours. It was then neutralized (pH - 6) with
1 N acetic acid, and poured into ten times its volume of water. The re
sulting mixture was extracted twice with a total of 120 ml of methylene
chloride. The organic extracts were washed several times with water and
then dried over magnesium sulfate. The methylene chloride was removed
by fractional distillation. Examination of the residue (4.18 g) by in
frared showed that it consisted primarily of dioxane. Gas liquid chroma
tography (column 5' x 1/8", 5% SE 30, 90° C) of the residue showed the
presence of two components having retention times identical with those
of dioxane and t-butyl disulfide. No peak corresponding to t.-butyl
phenyl disulfide was found. The amount of t-butyl disulfide was deter
mined by adding a known amount of n-butyl sulfide (40 mg/g residue) to
the residue as an internal reference, and running a glc on the mixture
of residue plus a-butyl sulfide. The amount of L-butyl disulfide in the
residue was found to be 0.18 mmoles (yield 72%).
The amount of t.-butyl mercaptan formed in the alkaline hydrolysis
was determined in a separate experiment in which after the reaction so
lution had been neutralized with 1 N acetic acid, the mercaptan in the so
lution was titrated by the cupric alkyl phthalate method. The amount
of mercaptan found was 3.93 mg (yield 8.8%).
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^' ng^s!' * ^"^ Lenhardt, S. Justus Liebigs Ann. Chem., 400, 2
5. Zincke, T. and Rose, H. Justus Liebigs Ann. Chem., 406, 103 (1914)
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APPENDIX
46 Dependence of Rate Constants on Sulfide Structure
60
61
b ^ Sul f ide k ,M'^sec"^^ k^,M'^sec"^ k|j,M'^sec"^
Tetrahydrothi ophene
(PhCH2)2S
n-Bu2S
Et2S
(H00CH2CH2)2S
CgH5CH2SCgH5
(CgH5)2S
(H00CCH2)2S
3.2
2.4
2.3
1.8
1.3
0.91
0.038
0.024
890
59
480
620
7.2
1.2
0.021
0.015
0.041
0.30
^' '^Al l data are for AcOH-0.56 M H2O-O.2O M H2SO4 as solvent at 39.4° C.
^ Data are given in AcOH-0.56 M H2O-O.2O M H2SO4 as solvent at 40° C?^
d Is the rate constant of the acid- and sulf ide-catalyzed disproportionat ion of phenyl benzenethiolsulf inate.
' s Is the rate constant for the acid- and sulf ide-catalyzed reaction of phenyl benzenethiolsulf inate with su l f i n i c acids.
' d Is the rate constant of the ac id- and sulf ide-catalyzed decomposition of t -bu ty l benzenethiolsul f inate.
