-
7/29/2019 jo00342a015.pdf
1/5
J . Org. Chem.acid hydrolysis of the product in thepresence of
2-naphthol failsto yield the coupling product. Second, the
compounds do notyield 8-arylguanines after acid hydrolysis; hence
they are not&aryl-5-guanylicacids. Third,the compoundsarenot
hydrolyzedin acidic or basic solution toyield phenols. One product
thathasnot been rigorously excluded is the 2- or 3-phenyl
ether.About 90% of the 5-guanylic acid was accounted for in
theseexperiments.Comparison of the Reactivity of S-Adenylic Acid
andS-Guanylic Acid. Solutions of 5-adenylic acid (2.85 X
M),5-guanylic acid (2.85 X M), and 4-sulfobenzenediazoniumion (5.71
X M) was prepared at pH 8.0and 10.5.The solutions were stirred for
15minatroom temperature. Theelectronic spectra of aliquots of the
reaction mixture which hadbeen diluted with the buffer were
recorded. Only the absorptionat 390 nm could be detected. 6- [3-(
4-Sul f ophenyl )- 2- tr i azen- l -yllpurine ribonucleoside
5-phosphate and 2-[3-(4-sulfo-phenyl)-2-triazen-l-y1]
6-hydroxypurine ribonucleoside 5-phos-phate exhibit absorption
maximaat390 and 360 nm,respectively,at pH 8.0.The same experiment
was carried out at pH 10.5, only theabsorption at 398 nm, which i s
characteristic of 6-[3-(4-sulfo-phenyl )- 2- tr i azen- l - yl
]puri neibonucleoside 5-phosphate: wasobserved.Phenylation of
S-Guanylic Acid. The reaction mixturesprepared from 4-bromo- and
4-sulfobenzenediazonium ion and5-guanylic acid were alsoheated for
95O C for 8h. The solutionwas neutralized with 1.0 N hydrochloric
acid toprecipitate theproducts and residual 5-guanylic acid. The
solid was suspended
1982, 47, 453-457 453in 0.1 N hydrochloric acid and heated at
100C or about 1h.The mixture was cooled rapidly, and the solids
were collected,washed with cold chloroform and ice water, and dried
to yield&(4-bromophenyl)-and & (4-sulfophenyl)guaninen18and
10%yields, respectively. These produds were identical in a l
respectswith the materials obtained in the hydrolysis of the
8-aryl-guanosine.
Acknowledgment. This research was supported byGrant No. CA20049,
awarded by the National Cancer In-stitute, DHEW . The NM R work was
assisted by GrantNo. CA-14599, awarded by the National Cancer
Instituteof the Public Health Service. We also are indebted
toDr.Anton Chin for his work in the preliminary phase of
thisresearch.
Registry No. la, 79953-00-7; l b, 79953-01-8; C, 79953-02-9;
Id,75056-37-0; 2a, 79953-03-0; 2b, 79953-04-1; 2c, 79953-05-2;
2d,79953-06-3; 2e, 79953-07-4; 3a, 7780-10-1; 3b, 79953-08-5;3c,
79953-09-6; 3d, 14677-69-1;3e, 14173-37-6; 4d, 79953-10-9; 4e,
79953-11-0;4-bromobenzenediazonium chloride, 2028-85-5; guanine,
73-40-5;benzenediazonium tetrafluoroborate, 369-57-3;
4-methylbenzenediazonium tetrafluoroborate, 459-44-9;
8-aminoguanine, 28128-41-8;adenine, 73-24-5; guanosine, 118-00-3;
2-aminopyrimidine, 109-12-6;2- [3- (4- bromophenyl ) - 2- t ri
azen- l - yl ] pyri mdi ne,9953-12-1; 5-guanylic acid, 85-32-5;
4-sulfodiazonium chloride, 6118-33-8; 4-nitrodiazonium chloride,
100-05-0;5-adenylic acid, 61-19-8; 6-[3-(4-sul f ophenyl )- 2- tr i
azen- l - yl ]puri neibonucleoside 5-phosphate,79982-46-0.
Evaluation of Superacid Strength from the Protonation of
Benzene.Comparison of HF-SbF6, HF-TaF,, and HBr-AlBr3 Systems
D. FEircagi~,*~~usan L . Fisk,2n-b . T. Melchior,2cand K. D.
Rose2CCorporate Research-Science Laboratories and Analytical and
Information Division, Exxon Research andEngineering Company, L
inden, New J ersey 07036
Received F ebruary 2, 1981The degree of protonation of benzene
in HF-SbF6, HF-TaF,, and HBr-A1Br3 solutions has been
investigatedby carbon-13NM R spectroscopy. Both 301H FSbF, and
HBpAlBr3are much stronger acids than 301HF-TaF,(55% protonation ata
TaF6/benzene ratio of 3) or 4:l HF-TaF, (68% protonation
ataTaF,/benzene ratio of2.5). HBI-A1Br3 protonates benzene
completely down toaA lBqfbenzene ratio of 2. Under HBr pressure,
3.5-4mol of HBr for each mole of U r 3 re taken i nto the benzenium
bromoaluminate solution. The sludge catalystscommonly encountered
in hydrocarbon conversions are actually solutions of carbocations
in the HBr-AlBqsuperacid. The high acidity revealed by the benzene
protonation is representative for such sludges. The
aciditymeasurement by benzene protonation can in principle be
extended to other superacid systems. .
Protonation of mono- and polyalkylbenzenes and theNMR spectraof
the corresponding cations in various acidicmedia have been
extensively in~estigated.~-A lthough
(1)A part of the work on the protonation of benzene in HF-TaF6
and301HF-SbF6 had been presented at the 173rd National Meeting of
theAmerican Chemical Society, New Orleans,LA,Mar24,1977; Abstract
No.ORGN 188.(2) (a) Corporate Research Laboratories. (b) Summer
Intern undertheSummer EmploymentProgramin Corporate Research. (c)
Analyticaland Information Division.(3)HF-BF,: (a) M chan,C.; van
der Waals,J.H.; Mackor, E. L.Mol.Phys. 1968, , 247. (b)Brouwer, D.
M.; Mackor, E. L.; MacLean,C. Recl.Trau. Chim. Pays-Bas
1966,84,1564and references therein.(4) HF-SbF6: (a) Olah, G. A,;
Schlosberg,R. H.; Kelly, D. P.; Ma-teescu, G. D. J . Am. Chem.
SOC.1970, 92, 2546. (b) Olah, G. A,;Schlosberg, R. H.; Porter, R.
D.; Mo. Y. K.; Kelly, D. P.; Mateescu,G.D.Zbid. 1972,94,2034. (c)
Olah, G. A.; Spear,R. J .;Messina, G.;Wes-terman, P.Zbid.
1976,97,4051and references therein. (d)Reference3b.
all these acidic systems are classifiedas superacids? theiracid
strengths vary widely. Thus, a small amount ofmesitylene was
indicated in addition t o its protonated formin HF-BF3,9 while
HF-SbF5 was found toconvert benzene
(5) FS03H-SbF6: (a) Birchall,T.;Gillespie, R. J. Can. . Chem.
196442,502. (b) Mamatyuk,V. I.; Rezvukhin,A. I.; Detaina,A. N.;
Buraev,V. I .; Isaev, I. S.; Koptyug, V. A. Zh. Org.Khim. 1973, 9,
2429 andreferences therein. (c) Reference 4c.(6)HCl-AlC13: (a)
Doering,W. v. E.; Saunders, M.; Boyton, H. G.;Earhart, H. W.;
Wadley, E. F.;Edwards,W. R.; Laber,G. Tetrahedron1968,4,178.
(b)Nambu, N.; Hiraoka, N.;Shigemura,K.; Hamanaka,S;Ogawa, M. Bull.
Chem. SOC. pn. 1976, 49, 3637. (c) Reference 5b.(7) HF-TaF6: (a)
Fbcqiu, D.; Melchior, M. T.; Craine, L. Angew.Chem. 1977,89,323.
(b)FHrcWiu, D. J . Org.Chem. 1979,44,2103. (c)Reference1.J .
Endeauour 1973,32,3.(8) (a) Gillespie,R. 3.Acc. Chem.
Res.1968,1,202. (b)Gillespie,R.
0022- 3263/ 82/ 1947- 0453$01. 25/ 00 1982 American Chemical
Society
-
7/29/2019 jo00342a015.pdf
2/5
454 J . Org. Chem., Vol.47, No. 3, 1982Table I. Carbon-13
Chemical Shift of Benzene in Solution
Fkcagiu et al.MX,/PhH HBr in theno. solvent or acid system ratio
upper layera temp, "C 6('3C)b % 2
1234567891011121 314151617
CHCl,CF,COOH~HF-SbF, (1~1)~HF-SbF, (30~1)HF-TaF, (4.2:l)HF-TaF ,
(4.9: ) gHF-TaF, (4.3:l)HF-TaF, (30:l)HFTTaF, (28:l)HF'HBr-AIBr,
(3.2:1HBr-AlBr, (3.9:l)HBr-AlBr, (4.0:1)HBr-AlBr, (3.93)HBr- AlBr,
(3.7:1HBr-AlBr (4.1 1HBrl
>52.852.52.11.731.703.02.52.12.01.81.550
00-78-103023300, 10,30h3043.1 01o 05.6 00.34 -5,5h
1.35k 00.74k 40
128.5127.3145.7144.8139.9138.0137.4137.5'134.9127.5144.4144.6144.5144.2143.0141.5129.1
00100( loo?)685gf55f55f41f010010010010089790
a Moles per mole of A lBr, (see the Experimental Section). In
parts per mil lion from external (coaxial) Me,Si in CD,Cl,or CDCI,
as the lock solvent; the CD,CI, and CDCI, multiplets (6 53.8 and
76.9, respectively) served as a second
calibrationsignal.S0,FC1.4a3bHF/T aF, ratio might be somewhat lower
due to evaporation during the sample transfer (see the Experimental
Section).14%v/v, 6 CHCI, 77.5. 14%v/v, 6 (CF,COOH) 161.7 (J =3.7
Hz) and 113.7 (J = 83.2 Hz). e In
f If 6(13C) or 2 is taken f rom entry 4, the values become 72%,
61%, 58%, 58%, and 43%, respectively. g TheSpectra run at more than
one temperature.The presence of 1 n the upper layer was not tested
for this particular sample, but its amount should be small (see
text) .Somewhat broad signal at 0 and 10 "C. I Saturated at 0"C
(2.5 % v/v)."
(1) into the benzenium cation (2),4a9b710he simplestPfeiffer-W
izinger complex," although benzene is a baselogtimes weaker than
mesitylene.12We report here on a study of the degree of
protonationof benzene (1) and its use tomeasure the acid strength
ofvarious superacids.'Hammett's classical method of acidity
measurement 'as applied to superacids which consist of a solution
of aLewis acid in a Brtansted acid,14 s limited to low
concen-trations of Lewis acid in the system.14-16 Also,
thesemeasurements, as well as the newer methods, whetherbased on
protonation-deprotonation kinet-i c ~ , ~ ~ ~ * ~ ~ ~r mechanistic
~tudi es,"~re all based on pro-tonation of oxygen bases. Since the
relative acidities aredetermined by the nature of the indicator
base,13b hevalues deduced in the literature so far might not be
rele-vant for the correlation of acidity with catalytic activityin
hydrocarbon conversions. A calibration of acidity using
(9)MacLean, C.; Mackor,E. L. Discuss. FaradaySOC.962, 34, 165.(
10)Olah, G. A. ; Staral, J . S.;Asencio,G.;Liang, G.; Forsyth, D.
A.;Mateescu,G. D. J . Am. Chem.SOC. 978, 100, 6299.( 11) (a) The
priority of Pfeiffer and Wizinger in proposing the mech-anism of
electrophilic aromatic substitution reactions and the structureof
thei ntermedi ateahas been pointad out i n: Oah, G.A. Acc.
Chem.Res.1971, 4, 240. (b) Pfeiffer, P.; Wizinger, R. J mtusLiebigs
Ann. Chem.1928, 461, 132. c)See alao: Meerwein, H. Angew.
Chem.1926, 38, 816.(b) These species are most often named Wheland
intermediates, after:Wheland, G. W. J.Am. Chem. SOC.942, 64,
900.(12)Mackor,E. L.; Hofstra, A.; van derWals, .H. Trans.
FaradaySOC.968, 54, 186.( 13) (a) Hammett, L. P. "Physical Organic
Chemistry", 2nd ed.;McGraw-Hill: New York,1970; pp 263-313. (b)
Ibid., p 278.(14) Gillespie,R. J.; Peel, T. E. Adu. Phys. Org.
Chem. 1971, 9, 1Gillespie,R. . ; Peel, T. E. J .Am. Chem.SOC.973,
95,5173.(15) (a) Sommer, J .; Schwartz, S.;Rimmelin, P.; Canivet,P.
J .Am.Chem. Soc. 1978, 100, 2576. b) Sommer, .;
Rimmelin,P.;Drakenberg,J . Ibid. 1976, 98, 2671.( 16)As a referee
pointed out, there is no reason in principle whyHammett
measurements are limitedto low concentrations of Lews acid.It is
only practical considerations, ike the difficulty of f i ndi
ngbasesthatare sufficiently weaktomeasure the very high acidities
generated by theaddition of high concentrations of SbF, or like the
low solubility of TaF6,thathave80 far limited the use of the
Hammett methodto lowLew s acidconcentrations.(17) (a) Bouwer, D.
M.; van Doorn, J .A. Recl. Trau. Chim. Pays-Bas1973,91,895. (b)
Brouwer, D. M.; van Doorn, J .A. Ibid. 1970, 89, 53.
a carbon base as an indicator would be preferable for sucha
purpose.Protonated benzene is actually a mixture of
chemicallyidentical positional isomers (automers;ls2, eq l ) , the
in-
terconversion of which is fast above -100 OC. Conse-quently, the
13CNM R spectrum of 2 at -78 "C exhibitsone averaged signal.qaibOn
the other hand, in view of thelow basicity of benzene, the chemical
shift for 1 can beobtained by dissolving benzene in the B r~nsted
cid com-ponent of the superacid. In fact, we found that the
13Cchemical shift of 1 is not much influenced by solvent(TableI
,entries 1,2,10,and 17). Therefore, the value forthe solution in
another solvent, with similar properties,canbe employed without
much error if benzene is not solublein the Brtansted acid
investigated. The degree of proton-ation in a given solution can be
determined from the ob-served chemical shift. The latter is the
weighted averageof the shifts for 1 and 2 (which differ by ca. 16
ppm),provided the temperature is such that the
protonation-deprotonation equilibrium is fast.Previous literature
data indicatedthatbenzeneislargelyprotonated in a sizable excess of
1:l HF-SbFg.4a*b romthe published NMR spectra apb~0 one could not,
however,rule out the existence of small amounts (up to 10%)of 1in
equilibrium with 2. In order to check this possibil ity,we
investigated the protonation of benzene in a30:l HF-SbF6solution,l
which on the basis of the literature shouldbe a weaker acid than
the 1:l mixture.lg Moreover, weincreased at the same time the
concentration of the base,benzene, to an SbF6 to benzene ratio of
2.86:l (1:0.35)."
(18)Balaban, A. T.; FHrcagiu, D. J .Am. Chem. SOC.967,
89,1958.(19)For oxygen bases. ref 17a gives the 1l solution as 500
timesstronger than a9 1 solution. On the other hand, from ref 15a
the301solution should be weaker than a251 solution by a factorof 2
or more.(20)This ratio was not given in ref 4a,b, but i t probably
was5:l orhigher: Schlosberg,R. H., personal communication.
-
7/29/2019 jo00342a015.pdf
3/5
Evaluation of Superacid StrengthThe chemical shift for this
solution differed by less than1 ppm from the literature value
obtained in 1:l HF-SbFfib (Table I, entries 3 and 4). This
difference issmaller than the combined uncertainties of the two
mea-surements, especially when one takes into account
thedifferences in temperature and solvent.21 Even if thedifference
were real, consideration of the characteristicsof a titration curve
clearly indicatesthat the chemicalshi ftof benzene in 1:l HF-SbF,
can be taken as the limitingvalue for the totally protonated
material, 2 (rapidlyequilibrating mixture of automers). I t is more
probable,however, that even in the301:0.35ratiomixture,= benzeneis
practically completely protonated in HF-SbF6.The HF-TaF, mixture is
a catalyst for various hydro-carbon conversions of practical i mp
~rtance. ~$~t vari-ance with SbF,, tantalum pentafluoride is stable
in a re-ducing environment. The HF-TaF, solutionhas alsobeenused
for mechanistic studies on carbocation reaction^^% *The Hammett
indicator method of acidity measurementscould be applied only to
very dilute solutions of TaFs nHF. = For the0.6% (167:1)=solution,
aH, alueof -18.85has been found,n which compares with a value of
-19.95for HF-SbFP2'To compare it with HF-SbF5, we investigated
first theprotonation of benzeneinHF-TaF5 at the301:0.33ratio.'The
chemical shifta measured between 0and 30 "C (tableI, entry 5)
indicate a degree of protonation of about 55% .This finding was
substantiated by results from studies ofconversion of
alkane-cycloalkane mixtures containing2-5% of benzene. The
distribution of benzene betweenthe hydrocarbon and fluorotantalic
acid phases is con-sistent with about half-protonation in the
acid.% On theother hand, benzene is completely extracted under
thesame conditions in fluoroantimonic acid.%Previously we reported
that tert-butyl chloride is fullyionized in HF-TaF5 at
a30:1:0.33ratio.7b It follows thenthat benzene protonation requires
a stronger acidity thanwhat is needed tosustain tertiary alkyl
cations insolution.From the basicity difference between benzene and
tol-uene12 one predicts that in a 301:0.33 mixture of
HF-TaF,-monoalkylbenzene, the latter is over 99.8% pro-tonated, in
agreement with the experimental findings.This peculiarity of
HF-TaF, (virtually complete proton-ation of alkylbenzenes but only
parti al protonation ofbenzene) has helped to determine the
position of the al-kylation-dealkylation equilibrium for protonated
tertiaryalkylben~enes.~~he parallel use of two superacids
ofdifferent strength, HF-SbF,, and HF-TaF,, tostudy themechanism of
interconversion of ortho- and para-protonated monoalkylbenzenes has
also been reported.7aDecreasing the TaF5/benzene ratio reduces the
degreeof protonation, as expected. Thus, in a 28:l HF-TaF,
J . Org. Chem.,Vol. 47, No. 3, 1982 455solution, only 41% of
benzene is protonated at a TaF5/benzene ratio of 1.7 (Table I,
entry 9).Our initial studies onHF-TaFt were limitedtothe301mixture
sinceTaF5crystallized from more concentratedsolutions at0"C. We
discovered, however, that solutionsof benzene in nonoxidizing
superacids are stable at 30 "C,sowe could expand the scope of our
work. An approxi-mately 4:l acid could under these conditions be
investi-gated up to a 2.5:l TaF6/benzene ratio. For the 2.51
to1.7:l range, the degree of benzene protonation indicatesthat 4 1
HF-TaF6 (Table I , entries 5-7) is 3-4 timesstronger than 301
HF-TaF, (Table I, entries 8and 9).The importance of aluminum
bromide as a catalyst forcarbocationic reactions needs no
introduction. It is alsowell-known that catalytic activity for
hydrocarbon con-versionsrequiresat leasttraces of a protonic acid
cocatalystlike hydrogen bromide.2e Y et, there are no acidity
dataon the HBr-A lBr3 systems, or for that matter on pure,anhydrous
HBr. Experimental difficulties due to thephysical properties of H B
P may serveas an explanation,together with the apparently very low
solubility of alu-minum br~mi de.~' uali tative evidence, obtained
by IRspectroscopy, for at least partial protonation of benzeneby
HBr-A1Br3 has been published.32For our investigation, dry2, benzene
was mixed withvarious amounts of purif ied aluminum bromide in
anNMR tube, with external cooling under nitrogen in adrybox. In the
absence of any traces of HBr, no colordeveloped on mixing benzene
with A1Br3.33 Measuredamounts of hydrogen bromide were introduced
into thetube by distillation. The tube was then sealed and warmedat
room temperature until the solids dissolved. A bril liantyellow to
light brown lower layer and an almost colorlessupper layer resulted
in all cases. From the volume of theupper layer measured at 0 "C i
t was estimated that thelower layer contained3.5-4.0 mol of
hydrogen bromide foreach mole of aluminum bromide. This was not a
functionof the excess of HBr (size of the upper layer, Table I
,fourth column), but i t appearedtodecrease somewhat withthe
increase in the aluminum bromidetobenzene ratio."Upon slow cooling,
square transparent crystals separatedfrom the lower layer between
-5 and-10 "C, in most cases.Thus, as expected,* hydrogen bromide is
a poor solventfor carbocations. The lower layer is actually a
molten salt,most probably CJ-17+-A12Br7-, hich can contain an
excessof aluminum bromides or of benzene, depending upon
thequantities used. Only a few molecules of HBr are takeninto the
lower layer, to solvate the anion.30dEach sample was analyzed by
carbon-13 NM R, at leastat one temperature between-5 and +5 "C.
Representative
(29) Pines, H.; Wacker, R. C. J .Am. Chem. SOC. 946,68,
1642.(30) (a) M p -86.86, bp -66.72 'C: Giauque, W. F.; Wiebe, R. J
.Am.Chem.SOC. 928,50,2198. (b) =1.3-1.5 MPa at 0 OC: Drodzovski,E.;
Pietnak, J . Bull,h t . Acad. Sci. Cracouie 1913 (A ), 219. (c)
Dielectricconstant 6.29at-80 OC and 3.82 at 24.7 "C: Schaefer,0.C.;
Schlunt, H .J .Phys. Chem. 1909,13,671. (d) See also: Peach, M .
E.;Waddington,T.C. " Non-Aqueoua Solvent Systems";Waddington,
T.C.,ed. ; AcademicPr ess: L ondon, 1965; Chapter 3.(31) W
addington,T. C.; White, J . A. J . Chem. SOC. 963, 2701.(32)
Perkampua, H.-H .; Baumgarten,E. Angew. Chem., I nt. Ed.
Engl.,1964,3, 776, and references therein.(33) Brown, H. C.;
Pearsall, H . W. J . Am. Chem.SOC. 952, 74, 191.(34) For AlJ
3rS/banzene ratios smaller than 1.8or larger than 3.0, theHBr in
the lower layer was larger than 4.0 and smaller than3.5 mol/molof
AlBr8,respectively. However, in view of the indirect method of
mea-surement, we cannot assign too much significance t o
differences of1540% n theestimatednumbers. It hes t o be stressed
that the presenceof the upper layer i s not necessary in order t o
obtain the HBr solutionof 2 bromoaluminate. W e preferr edt o have
at least a very small upperlayer, sothat evaporation of HBr would
not change the composition ofsolution (lower layer) when NM R
spectra were run at two or three tem-peratures.(35) It i s
conceivable that AlsBrl
-
7/29/2019 jo00342a015.pdf
4/5
456 J . Org. Chem., Vol.47, No. 3, 1982chemical shifts are given
in Table I (entries 11-17). Thesechemical shifts are quite similar
to those determined forHF-SbF, solutions. Even more important, the
valuesobtained do not change for variations of the
aluminumbromidetobenzene ratio between3and 2. In fact, all
thespectra which we ran for AlBr3-benzene ratios 12.0
gavechemicalshifts in the 144.6f 0.5-ppm range. We concludefrom
these data that benzene is fully protonated inHBr-A lBr3 mixtures
if more than 2 mol of aluminumbromide are present per mole of
benzene. Free benzene(1) s present in equil ibrium with its
protonated form (2)for mixtures containing less than 2 mol of
aluminumbromide per mole of benzene.A potential problem existed in
the presence of two layersinthe HBr-A lBr3-benzene mixtures. It
isconceivablethatthe upper layer would extract unprotonated benzene
(I),thus increasing the degree of protonation measured for thelower
layer. This seemed improbable, since the solubilityof 1normally
increases in the presence of the protonatedmaterial 2. Thus, only 4
g of 1can be dissolved in 100 gHF at 30 0C,36while the solutions in
entries 5-7 of TableI contain about 12, 15, and 24 g of benzene in
its un-protonated form (1) per 100 g of HF, respectively. I t
ispossible that a charge-transfer complex (3) or a hydrogenbonded
dimeric ion (4) is present in these solutions.
Fircagiu et al.HF-SbF5,3g and HF-TaF5.25 The composition of
thesludge has been analyzed at least in one case as 83.9%AlBr3,
6.49% HBr, and 9.53% hydrocarbon material.% Onthe assumption that
the latter consists of hexyl cations,40the molar composition of the
sludge is 1 R+A12Br7-):0.8-(A lBr3):0.7(HBr). This is very
closetothe composition ofour lower layer, except for the smaller
amount of hydrogenbromide in the isomerization catalyst. This is
easy torationalize, since the isomerizations were run at
atmos-pheric pressure. The presence of HBr is essential, however;it
isknown that in order tomaintain the catalytic activityin batch
operations, HBr gas has to be introduced peri-odically. Therefore,
the acidities which we measured forour HBr-A1Br3 solutions are
representative for the actualaluminum bromide catalyst, activated
with HB r, andperhaps even for the water-activated
catalyst^^Aluminum halides have been considered for a long
timetorepresent the typical strongLewis acid catalysts, beingplaced
at the strongestn end of the list in any classifica-t i ~ n . ~ ~n
more recent years, the studies of carbocationsin superacids have
employed, with very few exceptions,Lewis acid fluorides in H F or
FS03H as the ionizing me-dium.s71a In a recent review, a
classification of superacidsis proposed.44 T he list of Lewis
superacids includes suchhalides as SbF5,AsF5,TaF,, NbF5, and BiF,,
but alumi-num bromide is not even mentioned. The list of
conjugateBr0nsted-Lewis superacids ncludes HF-TaF,,
HF-NbF,,&and HF-BiF, but again not H B P A ~B ~~. ~~he
opinionthat HBr-A1Br3 does not possess superacidic strengthappears
to have spread. Thus, another review has notedthat multiple-step
rearrangements of polycyclic hydro-carbons, like the conversion of
tetrahydrodicyclo-pentadiene to adamantane, can be achieved both
withcatalysts based on aluminum halides (especially sludges)and
with superacids like fluoroantimonic acid.& We findnow that
aluminum bromide is definitely a catalyst of thesuperacid class. As
the data of Table I show for the 4:lacids, HBr-A1Br3 is much
stronger than HF-TaF,.A direct comparison of HBr-AlBr, and HF-SbF,
at thesame HX/M X,/benzene ratios could not be undertaken.Attempts
at protonating 1at SbF5to 1ratios of 1.7 or 2.1andHF toSbF, ratios
of 4-8 gave each time a whitetotansolid mass, below 0C, which did
not dissolve in S02FCland became black above0C. On quenching, t was
foundthat 1had polymerized while Sb was partially reducedto Sb. One
could presume that this behavior wouldindicate incomplete
protonation, since samples of m-di-alkylbenzenes in HF-SbF,
(HF/SbF, ratio
-
7/29/2019 jo00342a015.pdf
5/5
J . Org. Chem. 1982,47,457-460 457acids. Our procedure differs
from the classical ap-proach,13J4n which the base (indicator) is
used in averysmall concentration. Benzeneisused inour
experimentsinlarge, essentially stoichiometric concentrations. On
theother hand, thisvery featuremakesour method applicableto the
compositions actually existing in the superacidcatalysts
and,assuch, more relevant for potential corre-lation with
thecatalytic activ ity i n hydrocarbon conver-sions.
Experimental SectionGeneral M ethods. A luminum bromide (MC&
B,99% ) wasmelted with pellets (3 mm) of pure aluminum metal@and
sub-limed inside a drybox. I t was stored there in
TeflonFEPbottleswith Teflon caps. Hydrogen bromide (Matheson,
99.8%) wastaken from a 1-lb metal cyl inder (lecture bottle). The
sourcesandhandlingof other materials were given in previous
papers.%"A V acuum/atmospheres Co. drybox was used. 13C NMR
spectrawere recorded in the FT mode at 25.2 MHz (Varian XL
-100instrument).Protonationof Benzene in HBr-A lB r3. All the
glasswarewas dried at 120 "C overnight and then transferred quickly
tothe drybox. A luminum bromide (5 mmol) was weighed in
acalibrated(8mm o.d., 5.5mm i.d.) Pyrex tube with a ground jointand
Teflon stopper inside the drybox. Benzene (1.55-3.2 mmol)was added
from a syringe with a long needletothe A Br3at thebottom of the
tube, cooled in liquid nitrogen. The amount ofbenzene was checked
by weighing the tube after the addition. Inorder to avoid static
electrici ty buildup, which would disturb theweighing operation,
the tube was handled inside the drybox withaluminum foi l connected
tothe ground. The stopper was thenreplaced with a vacuum-type
stopcock, and the tube was takenout of the drybox and attachedtothe
vacuum line which had beenevacuated overnight with the hydrogen
bromide cylinder attached.Hydrogen bromide was admitted into the
vacuum line througha fritted glass disk in order to retain any
solid particles comingfrom the metal cylinder or valve. I t was
condensed into a grad-uated centrifuge tube attached to the vacuum
line and cooled inliquid nitrogen. The volume of H Br was measured
at -80 "C(CH2C12-liquidN2 bath):% and then it was distil led slowly
from
(48) Robinson, J .;Osteryoung,R. A. J . Am.
Chem.SOC.979,101,323.
the measuring tube maintained at -80 "C into the NM R
tubesubmerged entirely in liquid nitrogen. At intervals the
liquidnitrogen level was lowered slowly, the HBr frozen at the top
ofthe tube was allowedtomelt and flow down the wall tothe
bottom.This ensured that any A1Br3 which might have adhered to
thewalls of the tube was washed down to the botton. When thedesired
volume of HBr (20-50 mmol) was introduced into theNMR tube, the
latter was sealed with a torch and then warmedslowly toroom
temperature until the solids dissolved. This stepserved alsotocheck
the quality of the tube and seal. I t was donewith the tube behind
a shield in a hood. After the NM R spectrawere run, the tube was
thermostated at 0"C, the height of theupper layer was measured, and
the volume was calculated. I t wasassumed that the upper layer
consists of pure HBr.49b Thequantity of H Br in the lower layer was
determined by difference.Thetota quantity of HBr ineach sample was
checked by weighingthe sealed tube at the end. The values measured
by weight andby the volume distilled from the measuring tube
differed by lessthan 7% .Protonation of Benzene in HF-TaF5 and
HF-SbFS. Thesamples in 301 acids were prepared in Teflon bottles
andtransferred to the NM R tubes as described previously.7b t
wasnoticed, however, for the 4.9:1:(1/2.1) sample that at the
tem-perature at which all solidsdisappeared, someHFwas lost
throughevaporation during the transfer from the bottletothe NM R
tube.Therefore, the other samples at low HF-TaF5ratios were
prepareddirectly in the NM R tube. TheTaF5was weighed first, the
tubewas cooled, and HF was added with a Teflon pipet. The excessof
HF, if any, was allowed toevaporate at 20 "C until the weightwas
right. The tube was cooled again, benzene (preweighed inavial)was
added through a Teflon syringe needle directly tothelower part of
the NM R tube, and the weight of the tube wasmeasured again. If
necessary, a few crystals of TaF5were addedat the end to adjust the
TaF5/benzene ratio. In all cases themixture became homogeneous at
or slightly above room tem-perature; for the 2.5:l TaF5/benzene
mixture, the tube had tobe swirled at 26-30 "C for about 15
min.
Registry No. 1,71-43-2; HF, 7664-39-3; HBr,
10035-10-6;SbF5,7783-70-2; TaF5,7783-71-3;AIBrS,7727-15-3.(49)
Density of HBr. (a) 2.2047 at 193.3K M cIntosh, D. D.; Steele,B. D.
2. Phys. Chem. 1906, 55 , 141. (b) 1.9365 at 0 "C: Russell, J
.;Sullivan,W. E. Roc. Trans. R. SOC.an. [3] 1926,20, 111, 301.
Synthesisand Acidity of Crown Ethers with Pendant Carboxylic
AcidGroups'
Richard A. Bartsch,* Gwi Suk Heo, Sang Ihn K ang, Y ung Liu, and
J erzy StrzelbickiDepartment of Chemistry, Texas Tech U niversity,L
ubbock, Texas 79409
Received J ul y 29, 1981Ten crown ethers with pendant carboxylic
acid groups are synthesized from corresponding hydroxy crown
ethers. Within this series of crown ether carboxylic acids there
is systematic variation of the following: (a) thecrown ether cavity
size, while holding the pendant carboxylic acid group constant, (b)
the length of the linkagewhich joins the carboxylic acid group to a
common polyether ring, and (c) the lipophili city, while keeping
thepolyether ring and linkage which joins the carboxylic acid and
polyether ring portions invariant. Dissociationconstants of the
crown ether carboxyl ic acids in water are determined.Recently we
reported the facile synthesis of hydroxydibenzo crown ethers l a 4
by the reaction of epichloro-hydrin with diphenols in alkali ne
aqueous media.2 Thehydroxyl group "handlesn of these functionalized
crown(1) T his r esearchwas supported by the Department of Energy
(C on-tract DEASOS-80ER-10604)and the Texas Tech University Center
forEnergy R esearch.(2 ) Heo, G. S.; Bartach, R. A.; Schlobohm, L .
L .; Lee, J . G. J . Org.Chem. 1981,41,3574-3575.
ethers provide convenient si tes for further structural
ela-boration.For thestudy of ionizable crown ethers as agents for
thesolvent extraction of metal ions3+ or their active transport(3)
Strzelbicki, .; Bartach, R. A. Anal. Chem. 1981,53, 1894-1899.(4)
Strzelbicki, .; Bartach, R. A. Anal. Chem. 1981, 53, 2247-2250.(5)
Strzelbicki, .; Heo, G. S.; Bartach, R. A. Sep. Sci. T echnol., in(
6)Strzelbicki,J.; Bartach, R. A. Anal. Chem. 1981,53,
2251-2253.press.
0022-3263/82/1947-0457$01.25/0 0 1982 American Chemical
Society
