8
Halogenation with N-haloamines in strong acids. I. The nature of the chain propagating radical1 J. SPANSWICK~ AND K. U. INGOLD Diuision of Clzemisf~y, Nafional Research Council of Canarla, Otta,t,a, Canada Received September 8, 1969 The N-haloamine halogenation of 1-chlorobutane in 4 M sulfuric acid in acetic acid as solvent is a radical chain process in a-hich alninium radicals arc thc principal hydrogen atom abstracting species. With AT-chloroamines, a concurrent chlorine atoll1 chain is promoted by impurities such as lnolecular chlorine, hydrogen chloride, and chloride ion. Canadian Journal of Chemistry, 48, 546 (1970) Introduction The halogenatioil of organic substrates with N-chloroaniines and N-bronioamines in strong acids3 is a free radical chain process. The intra- molecular reaction, which is the well known Hoffman-Loffler reaction (1) gives predomi- nantly 6-halogenated arnii~es.~ The halogenation can be initiated both photochemically and by the addition of one electron reducing agent, such as ferrous ion. There is abundant evidence (1-4) that the reaction involves i~ltramolecular hydro- gen atom abstraction by an aminium radical, i.e. \A by a protonated amino radical H-N'. The / reaction can be represented by Scheme 1. The intermolecular halogenation with AT- haloamines in strong acids has usually been assumed to involve a similar series of reactions, i.e. \T X \+ RH \+ H-N-x H-N' ---+ H-N~H + K / / / \+ \+ R' + H-N-X ---t H-N' f RX / / These reactions have been extensively investi- gated during the past 4 or 5 years by Minisci and co-workers in Italy (5) and by Neale and co- workers in the United States (6). In addition to the analogy that can be drawn with the Hoffman- Loffler reaction, the evidence favoring the participation of aminium radicals in the inter- molecular halogenations include the following observations. (i) Halogenation is highly selective. In particu- 'Issued as NRCC No. 11058. *NRCC Fellow 1968-1969. lar, chloroamiile chlorinations are ~nuch more 3Alllong the acids rnhich are generallq employed in Selective than free radical chlorinations which these reactions are sulfi~ric acid, \arious mixtures of sul- illvolve chlorine atoms as the hydrogen atom furic acid and acetic acid, and trifluoroaceiic acid. 4Tl,ese are usually subsequently cyclired to pyrrolidines abstracting 'pecies (5d-5i). by treainlent with base. (ii) Chloriilations with an N-chloroamine and Can. J. Chem. Downloaded from www.nrcresearchpress.com by UNIVERSITY OF TASMANIA on 11/13/14 For personal use only.

Halogenation with N -haloamines in strong acids. I. The nature of the chain propagating radical

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Page 1: Halogenation with               N               -haloamines in strong acids. I. The nature of the chain propagating radical

Halogenation with N-haloamines in strong acids. I. The nature of the chain propagating radical1

J. SPANSWICK~ AND K. U . INGOLD Diuision of Clzemisf~y, Nafional Research Council of Canarla, Otta,t,a, Canada

Received September 8 , 1969

The N-haloamine halogenation of 1-chlorobutane in 4 M sulfuric acid in acetic acid as solvent is a radical chain process in a-hich alninium radicals arc thc principal hydrogen atom abstracting species. With AT-chloroamines, a concurrent chlorine atoll1 chain is promoted by impurities such as lnolecular chlorine, hydrogen chloride, and chloride ion.

Canadian Journal of Chemistry, 48, 546 (1970)

Introduction

The halogenatioil of organic substrates with N-chloroaniines and N-bronioamines in strong acids3 is a free radical chain process. The intra- molecular reaction, which is the well known Hoffman-Loffler reaction (1) gives predomi- nantly 6-halogenated arni i~es .~ The halogenation can be initiated both photochemically and by the addition of one electron reducing agent, such as ferrous ion. There is abundant evidence (1-4) that the reaction involves i~ltramolecular hydro- gen atom abstraction by an aminium radical, i.e.

\ A

by a protonated amino radical H-N'. The /

reaction can be represented by Scheme 1. The intermolecular halogenation with AT-

haloamines in strong acids has usually been

assumed to involve a similar series of reactions, i.e.

\T X \+ RH \+ H-N-x H-N' ---+ H - N ~ H + K / / /

\+ \+ R' + H-N-X ---t H-N' f RX

/ /

These reactions have been extensively investi- gated during the past 4 or 5 years by Minisci and co-workers in Italy (5) and by Neale and co- workers in the United States (6). In addition to the analogy that can be drawn with the Hoffman- Loffler reaction, the evidence favoring the participation of aminium radicals in the inter- molecular halogenations include the following observations.

(i) Halogenation is highly selective. In particu- 'Issued as NRCC No. 11058. *NRCC Fellow 1968-1969. lar, chloroamiile chlorinations are ~nuch more 3Alllong the acids rnhich are generallq employed in Selective than free radical chlorinations which

these reactions are sulfi~ric acid, \arious mixtures of sul- illvolve chlorine atoms as the hydrogen atom furic acid and acetic acid, and trifluoroaceiic acid.

4Tl,ese are usually subsequently cyclired to pyrrolidines abstracting 'pecies (5d-5i). by treainlent with base. (ii) Chloriilations with an N-chloroamine and

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Page 2: Halogenation with               N               -haloamines in strong acids. I. The nature of the chain propagating radical

SPANSWICK AND IXGOLD: HALOG rENATION IN STROh-G ACIDS. I 547

brominations with the same N-bromoamine show the same selectivity. For example, the isomer distributions for the chlorination and bromination of methyl hexanoate are practically identical (5e, 5i).

(iii) The relative reactivities of ring substituted toluenes towards chlorination by the piperidi-

+ nium radical, (CH,),N'H. can be correlated by means of the Hammett equation using o' con- stants (6). The p value of - 1.36 is comparable to that for bromination with Br' (- 1.46) but is much greater than for chlorination with C1' (- 0.66).

(it.) Further evidence that aminium radicals are probably present in these systems is provided by the facile free radical chain addition of chloro- amines to olefins (5a-c. 7).

Our interest in the kinetics and absolute rate constants for free radical reactions in solution was stirnulatcd by the information outlincd above. Intermolecular haloge~lations with N- haloamines in strong acids appeared suitable for studying the absolute reactivities of nitrogen cation radicals, about which there is little infor- mation presently available. After the completion of a kinetic study on some of these reactions, which is described in the following paper, a report by Tanner and Mosher (8) appeared which suggested that these intermolecular halogena- tions actually involved the halogen atom as the chain propagating radical. These authors found

no significant experimental differences in the isomer distribution from the cuprous chloride, ferrous sulfate, and ferrous chloride initiated chlorination of 1-chloropropane with four different N-chloroamines. Furthermore, an iden- tical isomer distribution was obtained for the photochemically initiated chlorination of 1- chloropropalle using molecular chlorine as the chlorinating agent - a reaction that must involve

chlorine atoms as the chain propagating species. Similar results were obtained for the relative rates of reaction of a number of different hydro- carbons ton ards N-chloroamines and molecular chlorine. In addition, the bronlination of 1- chloropropane with N-bromodiethylamire (Cu' initiated) and with molecular bromine (photo- initiated) gave the same isomer distribution of brominated products but a distribution that was distinctly different from that for the chlorinations.

Since our kinetic studies could not distinguish unequivocally between an aminium radical chain and a chlorine atom chain, we undertook a rein- vestigation into the nature of the radical involved in these reactions. The isomer distribution for the halogenation of 1-chlorobutane was chosen as the most suitable diagnostic tool for distin- guishing between the two radicals. The results of this ~7ork are reported in the present paper. These results show unequivocally that for pure chloro- anlines the aminium radical is the pri~lcipal species propagating the chain in intermolecular halogenation^,^ the change to a chlorine atom chain being promoted by in~purities such as molecular chlorine, hydrogen chloride, and, under certain circumstances, chloride ion.

Experimental Preparation o f N-Haloatni~zes

hr-Cl~lor.orlinief/~yIa~~~irze It \?as prepared by the chlorination of 0.5 M aqueous

dimethylamine uith 0.5 IM sodiurn hypochlorite solution (91.. The chloroamine was extracted with ether, washed, and carefully dried over sodium sulfate. N-Chlorodi- methylamine hydrosulfate was precipitated as a crystalline salt by the addition of 90% of the calculated volume of concentrated sulfuric acid froin a syringe to the stirred solution, which wa? cooled to 0.. In order to obtain a product pure enough for kinetic ~ o r k , it is important that the salt should be precipitated in a crystalline form. The white prismatic crystals \\ere quickly washed with cold anhydrous ether, and the ether was removed by vacuum pumping for 15 ~ n i n in the dark. The still crys- talline product was dissolved in the solvent of choice and stored at - 78' in the dark.

N- Chloropiperidine It was prepared by the same method but the dry

ethereal solution was concentrated at room temperature and the residue distilled. The chloroanline was immedi- ately redistilled and stored at liquid nitrogen temperature in the dark.

Our kinetic studies showed that when the chloroamines were prepared and stored in this way, their rate of reac- tion with the substrate being chlorinated (decanoic acid)

'Further support for this view is contained in the pre- ceding paper of this issue by F. Minisci et al. (5j).

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Page 3: Halogenation with               N               -haloamines in strong acids. I. The nature of the chain propagating radical

548 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970

in the dark at 30" was extremely small. Furthermore, in the absence of added photo-initiators the reaction was initiated only very slowly by light of 3650 A wavelength. However, when the fullest precautions were not taken, both the background thermally initiated reaction and the photo-initiated reaction could become quite large.

N-Bvomodimethylainine I t was prepared in the same way by the bromination

of 0.5 M aqueous dimethylamine with 0.5 M sodium hypobromite solution. The colorless crystalline hydro- sulfate was dissolved in the solvent of choice and stored at liquid nitrogen temperature in the dark. N-Bromo- dimethylamine can be stored in this way for at least 2 weeks but at room temperature the formation of bromine is discernible after several hours.

Chain Initiation a,a'-Azo-bis-cyclohexylnitrile (ACHN) was employed

as photo-initiator and cc,a'-azo-bis-isobutyronitrile (AIBN) as the thermal i n i t i a t ~ r . ~ The reactions were carried out in sealed an~poules which had been degassed by the freeze-thaw method. The photo-reactions were initiated by light from a 200-250 V, 250 W ME/D B.T.H. ultraviolet lamp. The light bean1 was passed through a Corning C.S. No. 7-51 filter (maximum transmittance 3650 A). The thermal reactions were kept in the dark in a thermostatted water bath for the required period of time. At the end of the reaction period the ampoules were opened, the clear reaction mixture poured into ice/water and extracted with carbon tetrachloride, and the extract analyzed by gas-liquid chromatography (g.1.c.).

Other reactions \?'ere initiated by the addition of elec- tron reducing agents (Cu', Fez+, Co2+, Ce3+). The finely powdered metal salts were added in one lot to the stirred solution of the reagents and stirring was con- tinued through the reaction period. The slurry was poured into iceiwater and extracted with carbon tetra- chloride for g.1.c. analysis.

Analysis The N-haloamines were analyzed for positive halogen

by reaction with potassium iodide and titration of the liberated iodine (3). It was necessary to buffer the satu- rated potassium iodide solution (30% acetic acid, 70% isopropanol) with sodium acetate in order to obtain reproducible titrations.

The chlorinated and brominated isomers of l-chloro- butane were analyzed by g.1.c. using a 12 ft by 1!8 in. column (20% methyl silicone gum rubber on an 80-100 mesh silanized diatolnaceous earth support) and a flame- ionization gauge detector. We are indebted to Dr. Tanner for authentic samples of the various isomers. All analyses are averages of not less than three separate detern~ina- tions.

All reactions were run to completion, that is, they were continued until the N-haloamine was completely con- sumed. For the h~chloroamines this means that both chloroamine and any free chlorine were consumed but for N-bromodimethylamine, a certain amount of molec- -

6A number of other potential thermal initiators uhich were tested proved unsatisfactory in the sulfuric-acetic acid solvent. Di-t-butylperoxyoxalate and t-butylhy- ponitrite decomposed too rapidly and 2,2,3,3-tetraphenyl- butane was insoluble. Azo-isopropane was an ineffective photo-initiator.

ular bromine remained in the reaction mixture at the point where it was worked-up for analysis. Because all the halochlorobutane isomers are stable under the reac- tion conditions, no serious attempt was made to deter- mine the overall efficiency with which the halogen is transferred from the N-haloarnine to the 1-chlorobutane, since this can have no bearing whatever on the nature of the radical halogenating agent which yields the observed mixture of isomers. A few, rather rough, quantitative measurements with equimolar N-chloroamine and 1- chlorobutane indicated a 50 % or greater yield of dichlo- robutanes with relatively little trichlorobutanes. Whether the true yield of chlorinated 1-chlorobutanes was 100% based on chloroamine or whether some chlorine was lost as HCI, or by chlorinating the anline itself or the acetic acid solvent was not determined. For N-bromodimethyl- amine the yield of bromochlorobutanes also appeared to be - 50% or greater.

All the work described in this paper was carried out at 30". The solvent was 4 M sulfuric acid in acetic acid except for a few experiments with the molecular halogens in CCI4 and CFCI,.

Results and Discussion

The isomer distributions listed in Table 1 for the chlorination of 1-chlorobutane with molecu- lar chlorine show no significant solvent effect on change from the non-polar solvents, CCl, and CFCI,, to the sulfuric acidlacetic acid medium. The same isomer distribution is produced by photo and by thermal initiation. The individual isomers were shown to be stable in the acid medium for at least 24 h, as has been previously reported by Tanner and Mosher. Similar results, but with the different isomer ratios that charac- terize a bromine atom chain, are given in Table 2 for brominations with molecular bromine.

Chlorination with N-chlorodimethylamine (Table 3) and iV-chloropiperidine (Table 4) gave isolner distributions which are quite different from that obtained froni a chlori~le atom chain. Both of the AIBN initiated reactions and also the ACHN photo-initiated N-chloropiperidine reaction gave very similar isomer distributions. This isomer distribution7 (4.7, 10.3, 77.9, 7.1) is the most different from that for chlorine atoms (5.1, 23.5, 45.6, 25.8), and probably most nearly revresents the isomer distribution of a Dure un- hindered aminium radical reaction in this particu- lar solvent.' The isomer distributions for the

7That is, the mean of runs 16-19 and 27-30. 'The higher selectivity observed in more acidic solvents

(see e.g, experiment 26 and various papers by Minisci et 01.) probably represents a real etTect of acidity on selectivity. The possibility that there is some chlorine atom chain in 4 IM H,SOd:'AcOH even with AIBN initia- tion which does not occur in 16 M H2S04(85 %)/AcOH cannot, however, be entirely ruled out.

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Page 4: Halogenation with               N               -haloamines in strong acids. I. The nature of the chain propagating radical

SPANSWICK AND INGOLD: HALOGENATION IN STROKG ACIDS. I

TABLE 1

The isoluer distribution for the chlorination of 1-chlorobutane with lllolecular chlorine (initial 1-chlorobutane N 0.25 1M)

Duration of Exoeriment no. Solvent Initiation experiment Cl-CH2--CH2--CH,--CH,

1 CCl, hv 5 min 5.8 24.5 45.1 24.6 2 CFC1, hv 5 min 6.2 23.4 44.7 25.7 - Z CFCI: hv 5 min 6 . 0 24.3 43.7 26.0 4* 4 M H,SO,~ACOH - 6 h 4.4 22.4 45.9 27.3 5 * 4 IM H2S04/AcOH - 24 h 4 .9 22.9 46.9 25.3 6 4 IM H,SO,/AcOH hv 5 min 4.0 24.1 46.2 25.7 7f- 4 M H2S04/AcOH - 24 h 4 .7 23.4 46.3 25.6 X 4 M H,SO~lAcOH AIBN 12 h 4 .7 22.9 46.3 26.1

-$ k ~ l , hv - 10.5 27.6 45.3 13.7 - 85 % H2SO4/15 % AcOH 11 v - 5.4 20.9 52.0 21.6

*Products of experlment 3 stored In 4 M H2S04/AcOH. 'yProducts of experlment 6 after 24 h. $Results of Tanner and Mosher (8).

TABLE 2

The isomer distribution for the bromination of 1-chlorobutane with molecular bromine (initial 1-chlorobutane - 0.25 M)

Duration of Experiment no. Solvent Initiation experiment Cl-CH,-CH,-CH, -CH3

hv 10 min 25.2 26.0 48.8 Trace hv 10 lnin 24.4 27.5 48.1 Trace hv 10 min 21.4 26.2 48.6 3.8 hv l h 23.6 23.6 52.8 Trace

23.7 26.7 49.6 Trace 23.0 28.0 49.0 Trace 23.8 26.8 49.4 Trace

--

*Flushed continuouslv with N,.

photo-initiated N-chlorodimethylamine reaction diverge slightly from that of an aminium radical towards that of a chlorine atom, which implies that chlorine atoms may play a sinall role in this reaction. Presumably some chlorine atoms are generated either by the direct photolysis of the chloroamine or, more probably, by photolysis of the corresponding N,N-di~hloroamine.~

Initiation of the Me2NCl reaction with ferrous sulfate gives ail isomer distribution that is some- what closer to that of chlorine atoms than the photolytic reaction. This implies that chlorine atoms can play a more important role in the FeSO, initiated reaction. It is worth noting that molecular chlorine was formed in these FeSO,

gNeale and Walsh (4a) have shoun that N-chlorodi- iz-butylamine undergoes a non-radical disproportionation to yield N,N-dichloro-n-butylamine. In the absence of an externally added initiator, the dichloroamine is believed to be the important photo-initiator in the Hoffman- Loffler rearrangement of the chloroamine. Such a disproportionation is less likely to occur with N-chloro- piperidine than with Me2NCl.

initiated reactions in sufficient concentration to escape from the solvent so that its characteris- tic odor became unpleasantly easily detected. In contrast to the Me2NC1 reaction, the initiation of the N-chloropiperidine reaction with ferrous sulfate, ferrous chloride, cuprous chloride, and cerous sulfate gave isomer distributions which are essentially identical to that which we believe corresponds to an unhindered aminium radical in this solvent. It would appear that molecular chlorine is obtained considerably less readily from N-chloropiperidine than from N-chloro- dimethylamine. However, isomer ratios charac- teristic of a chlorine atom chain were obtained with N-chloropiperidine when the reaction was initiated with cobaltous chloride, and cerous chloride, and with cerous sulfate in the presence of chloride ion. The odor of chlorine in the pres- ence of these three catalysts was unmistakable.

The addition to N-chloropiperidine of anhy- drous hydrogen chloride (Table 5) , or the addi- tion of chloride ion (experiment 47), incline the

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Page 5: Halogenation with               N               -haloamines in strong acids. I. The nature of the chain propagating radical

550 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970

TABLE 3

The isomer distribution for the chlorination of 1-chlorobutane nith AT-chlorodin~ethylamine in 4 M H2S0,1AcOH (initial 1-chlorobutane - 0.25 M ; N-chloroamine - 0.25 M)

Duration of Exueriment no. Initiation [InitiatorI~M experinlent (h) C1-CH2----CHZ--CH2--CH3

AIBN AIBN AIBN AIBN

ACHNIhv ACHNIhv

Izv FeSO,

4 .5 17.6 63.7 14.2 1 .2 6.1 87.3 5.4

Trace 6.6 87.8 5.4 5.1 23.5 45.6 25.8

TABLE 4

The isomer distribution for the chlorination of 1-chlorobutane with N-chloropiperidine in 4 >%f H2S04/AcOH (initial 1-cl~lorobutane - 0.25 M ; N-chloroamine - 0.25 M)

Duration of Experiment no. Initiation [In~tiator]~\l experi~llent (h) CI-CH2--CHz--CH2--CH,

-- - -- -- - - .

27 AIBN 0.01 24 5.3 10.3 78.0 6.4 28 AIBN 0.01 24 4.9 10.1 78.6 6.4 29 ACHNIhv 0.015 3 5.1 10.4 78.3 6 .2 30 ACHN/lzv 0.015 3 5.3 10.3 78.1 6.3 31 hv - 20 6.6 14.2 69.8 9 .4 32 FeSO, 1 . O 2 6.5 10.3 76.9 6.3 3 3 FeS04 1 .O 2 5 .4 10.0 78.9 5.7 34 FeCI, 1 .O 2 5 .3 9 . 6 78.7 6 4

F~CI; CuCl CuCl CuCl CuCl CuCl

*Molecular chlorine was formed in these reactions. iChlorine atom chain.

isomer distribution towards that for a chlorine atom chain. Hydrogen chloride must therefore be able to react with the chloroamine to generate molecular chlorine (10) as vlas previously claimed by Tanner and Mosher (8). However, in contrast to Tanner, we must agree with Minisci et ak. ( 5 4 that hydrogen chloride is not formed in suficient quantity under the condiiions em- ployed in these chlorinations for its reaction

with chloroamine to significantly affect the isomer distribution.

The complete conversion of an aminium radical chain to a chlorine atom chain is not acconlplished by the addition of small amounts of molecular chlorine (Table 5) . For example, the addition to the N-chlorodimethylamine reac- tion of 10 mole "/, of chlorine (based on M-chlo- rodimethylamine) gives an isomer distribution

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Page 6: Halogenation with               N               -haloamines in strong acids. I. The nature of the chain propagating radical

SPANSWICK AND INGOLD: HALOGENATION IN STRONG ACIDS. I

TABLE 5

The influence of chlorine and hydrogen chloride on the isomer distribution for the chlorination of 1-chlorobutane with chloroanlines in 4 11.1 H2S0,1AcOH

(Initial 1-chlorobutane N 0.25 M; N-chloroamine - 0.25 M ; reaction ~nitiated with 0.01 M AIBN and run for 12 h)

% C12 or Experiment no. Chloroamine HCl* C1-CH2---CH2----CH,-CH3

1-87 - 100 2 CI; 5.1 23.5 45.6 25.8 27-30? ((332) sNC1 0 5 .1 10.3 78.3 6 . 3

50 (CHZ),NCI 2% HCl 4.9 8 .3 80.7 6 .1 51 (CHZ)5NCl 8 % HCl 4.0 12.9 72.4 10.7

*Approximate mole % based on added chloroamine concentration. ?Average isomer distribution for these experiments.

TABLE 6

The isomer distribution for the bromination of I-chlorobutane with N-bromodimethylamine in 4 .%I H2S0,/AcOH (initial 1-cl~lorobutane - 0.25 M ; IV-bromoamine - 0.15 M)

Duration of Experiment no. Initiation [InitiatorIM experiment (h) GI-CH2---CH,--CH,--CH,

-- - -- --

52 AIBN 0.01 30 4 . 6 8 . 8 77.3 9 .3 53" ACHNihv 0.015 1 4.2 8 .5 80.3 7 .0

*Considerable amounts of Br2 formed during the reaction. iDimethylaminium radical chain. $Bromine atom chain.

which roughly corresponds to a one-third chlorine atom chain and a two-thirds aminium radical chain. This must mean that chlorine atoms and aminiun~ radicals are of roughly equal importance as hydrogen atom abstracting agents in this particular system. Provided the HCl- chloroamine reaction is fast, the alkyl radicals derived from 1-chlorobutane must attack molec- ular chlorine and the protonated chloroamine at very roughly comparable rates.

The most conclusive proof that the aminium radical is involved in these systems is the similarity of the isomer distribution for the bromination of I-chlorobutane by N-bromodimethylamine (Table 6) with the isomer distribution already assigned to the aminiuin radical. Perhaps the most interesting result in these particular experi- ments was the fact that although considerable amouilts of n~olecular bromine were forined in the photo and thermal initiated reactions, the isomer distributions did not change froin that due to an aminium radical. The bromine atom isomer distribution is of course distinctly different from

that for the aminium radical. The lack of influence of the bromine is most readily accounted for by the fact that hydrogen atom abstraction from 1-chlorobutane by a bromine atom will be an endothermic process and will therefore occur rather slowly. The molecular bromine may play a role in brominating the alkyl radicals

R' + Br, + RBr + Br'

but the resulting bromine atom does not continue the chain by attacking the I-chlorobutane.

In the absence of ACHN, the photo-initiated reactions with N-cl~lorodiinetl~ylainii~e (experi- ment 22) and N-chloropiperidine (experiment 31) give isomer distributions ahich are inter- mediate between those for an aminium radical and for a chlorine atom. The experimental con- ditions normally employed in the Hoffman- Loffler reactioil involve the direct photolysis of the chloroamine. The present results suggest that direct photolysis leads to both chlorine atom and ami~lium radical chain reactions. Since a chlorine atomisless selective in hydrogen atom abstraction

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Page 7: Halogenation with               N               -haloamines in strong acids. I. The nature of the chain propagating radical

CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970

TABLE 7

Summary of isomer distribution for the chlorination and bromination of 1-chlorobutane in 4 12/1 H,SO,/AcOH

Experiment no. Reactant Initiation Radical Cl-CH2--CH2---CH2--CH3

16-19 Me,NCl AIBN

27-30 (CH2),NCI AIBN, ACHNIhv

52-56 Me2NBr AIBN, ACHNIhv FeS04

1-8 c12 hv, AIBN

Br' 23.6 26.4 49.5 0 .5

than an aminium radical and since, in the Hoffman-Loffler reaction, chlorine abstracts intermolecularly while the aminium radical abstracts intramolecularly, we should expect that the yield of the desired cyclized product will be increased by all procedures that reduce the importance of the chlorine atom chain. It is there- fore not surprising to find that the procedures that Neale and Walsh (4a) have reported increase the yield of cyclized product from the photo- initiated Hoffman-Loffler reaction of N-chloro- di-n-butylamine are all consistent with a decrease in the importance of the chlorine atom chain. These procedures include (i) increasing the chloroamine purity, (ii) rapid flushing of the system with nitrogen (this will remove HC1 and Cl,), (iii) increasing the light intensity and decreas- ing its wavelength. (This will increase the rate of radical reactions relative to non-radical reac- tions. Presumably some HC1 or C1, is formed by non-radical pathways.)

Neale and Walsh have also reported (4a) that for chloroamines for which intramolecular abstraction (Hoffman-Loffler reaction) is pos- sible, this process occurs more slowly than the addition of the aminium radical to 1,3-dienes but more rapidly than their addition to simple olefins. Minisci et al. have reported (5h) that intermolecular abstraction from methyl hex- anoate is competitive with intramolecular ab- straction for both N-chloro-rz-butylamine and N-chlorodi-rz-butylamine. Similarly, 12 % of benzyl chloride is formed from toluene and N- chlorodi-n-butylamine (5b). However, in the

reaction of N-chlorodi-n-butylamine with 1- chlorobutane in 4 M H,SO,/AcOH (initiation, ACHNIhv, and FeSO,) and in 85% H,SO,/ AcOH (initiation FeSO,) we could detect no trace of any dichlorobutanes. Presumably 1- chlorobutane is considerably less reactive towards aminium radicals than methyl hexanoate or toluene. From the relative concentrations of chloroamine and 1-chlorobutane, we estimate that for these particular reagents kinte,ikintra <

M - 1 .

Conclusion

The isomer distributions obtained in the present work are summarized in Table 7. There can be little doubt that N-haloamines yield aminium radicals and that under carefully con- trolled conditions, anlinium radicals are by far the most important hydrogen atom abstracting agents in these systems. Furthermore, although a chlorine atom chain is promoted by the hydro- gen chloride -- chloroamine reaction, small amounts of chlorine or hydrogen chloride do not have a very important effect on the dichloro- butane isomer distribution. The negligible effect of bromine on the isomer distribution in the presence of bromoamine is probably due to the low rate of hydrogen atom abstraction by bro- mine atoms.

1. M. E. WOLFF. Chem. Rev. 63, 55 (1963), and refer- ences cited therein.

2. S. WAWZONEK and P. J. THELE~ . J. Amer. Chem. Soc. 72, 2118 (1950). S. WAWZ~NEK and T. P. CUL- BERTSON. J. Amer. Chem. Soc. 81, 3367 (1959).

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SPANSWICK AND INGOLD: HALOGENATION IN STRONG ACIDS. I 553

3. E. J. COREY and W. J. HERTLER. J. Amer. Chem. SOC. 82, 1657 (1960).

4. (a) R. S. NEALE and M. R. WALSH. J. Amer. Chem. Soc. 87, 1255 (1965). (b) R. S. XEALE, M. R. WALSH, and N. L. MARCUS. J. Org. Chem. 30, 3683 (1965).

5. (a) F. MISISCI, R. GALLI, and M. CECERE. Tetra- hedron Lett. 4663 (1965). (0) F. MIXISCI, R. GALLI, and R. BERNARDI. Tetrahedron Lett. 699 (1966). (c) F. MINISCI, R. GALLI, and M. CECERE. Tetra- hedron Lett. 3163 (1966). (d) F. MINISCI, R. GALLI, A. GALLI, and R. BERNARDI. Tetrahedron Lett. 2207 (1967). (e) F. MINISCI, R. GALLI, and R. BER- NARDI. Chem. Comn~un. 903 (1967). (f) F. MINISCI, R. GALLI, and R. BERNARDI. Chim. Ind. (Milan), 49, 594 (1967). (g) F. MINISCI, R. GALLI, and R. BERNARDI. J. Chenl. Soc. 324 (1968). (11) F. MINISCI, R. GALLI, R. BERXARDI, and M. PERCHINU~IXO. Chin?. Ind. (Milan), 50, 453 (1968). (i) F. MINISCI. Private communication. (j) F. MINISCI, G. P. GAR- DIN, and F. BERTINI. Can. J. Chem. This issue.

6. R. S. NEALE and E. GROSS. J. Amcr. Chcm. Soc. 89, 6579 (1967).

7. (a) R. S. NEALE and R. L. HIXMAN. J. Amer. Chem. Soc. 85, 2666 (1963). ( 6 ) R. S. NEALE. J. Amer. Chem. Soc. 86, 5340 (1964). (c) R. S. NEALE. Tetra- hedron Lett. 483 (1966). (d) R. S. NEALE. J. Org. Chem. 32, 3263 (1967). (e) R. S. NEALE and N. L. MARCUS. J. Org. Chem. 32, 3273 (1967). (f) R. S. NEALE and N. L. MARCUS. J. Org. Chem. 33, 3457 (1968).

8. D. D: T A ~ E R and M. W. MOSHER. Can. J. Chem. 47, 715 (1969).

9. G. H. COLEMAN. J. Amer. Chem. Soc. 55, 3001 (1933). H. BOCK and K. L. KOVIPA. Chem. Ber. 99, 1347 (19661.

10. S. K O ~ ~ O R I ; M. OKAHARA, and Y. HORADA. Kogyo Kagaku Zasshi, 66, 1850 (1963).

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