The primary adducts from benzene readily eliminate HBr to give N-phenylsuccinimide. ..... in which only bromination of benzene by a radical pathway can occur ...
An investigation of the photodecomposition of N-bromosuccinimide; the generation and reactivity of succinimidyl radical1 Fu-LUNGL u , YOUSRYM. A . NAGUIB,MASAYUKI KITADANI, A N D YUANL. CHOW Deprrrctn~,nt ofC'het~rislry, Sitnot1 Froser Utiicosicy, Blrrrznby, B . C . , Crrr~orlrrV5A I S 6
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Received July 19. 1978' Fu-LUNGLu, YOUSRYM. A. NAGUIB, MASAYUKI KITADANI, and YUANL. CHOW.Can. J. Chern. 57, 1967 (1979). The photolysis of acetonitrile solutions of N-bromosuccinimide (NBS) in the presence of ethylene oxide and an excess of olefins or benzene in the -30 20°C range was shown to generate the succinimidyl radical in competition with the bromine atom reactions. The succinimidyl radical preferentially attacked a n bond to give 1-succinimidyl-2-bromoalkanes, rather than abstracting alkyl hydrogens. Allylic bromination also occurred and competed with the 1,Zaddition. Formation of 3-bromocyclohexene in the presence of cyclohexene at 10-20°C could be effectively reduced at lower temperatures or in the presence of a high concentration of ethylene oxide; the 1,2-addition process was favored at low temperature but surprisingly not facilitated significantly in the presence of ethylene oxide. The primary adducts from benzene readily eliminate HBr to give N-phenylsuccinimide. The attack of a bromine aton1 on a carbonxarbon double bond may become an important step in the 0 -30°C range. The ease of the addition reaction suggests that this succinimidyl radical may have a Z electronic configuration.
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KITADANI et YUANL. CHOW.Can. J. Fu-LUNGLu, YOUSRYM. A. NAGUIB,MASAYUKI Chern. 57. 1967 (1979). On a montre que la photolyse de solutions de N-bromosuccinimide (NBS) dans I'acttonitrile en presence d'oxyde d'kthylkne et d'olefines en excts ou de benzene a des temperatures allant de -30 a +20° donne lieu a la formation de radicaux succinimidyles en competition avec les reactions des atomes de brome. Le radical succinimidyle attaque de preference une liaison rc pour conduire a des succinimidyl-1 bromo-7 alcanes plutBt que d'arracher les hydrogenes des groupes alkyles. La bromation allylique se produit aussi et elle est en competition avec I'addition-1,2. On peut reduire d'une f a ~ o nefficace la formation du bromo-3 cyclohextne en presence de cyclohexene a 10-20°C en abaissant la temperature ou en presence de concentrations &leveesd'oxyde d'ethylene; la reaction d'addition-1,2 est favorisee a basse temperature mais elle n'est toutefois pas rendue beaucoup plus facile en presence d'oxyde d'ethylene. Les adduits primaires avec le benzkne eliminent facilement du HBr et donnent lieu a la N-phknylsuccinimide. L'attaque d'un atonle de bronie sur la liaison double carbonexarbone peut devenir une Ctape importante a des ten~peraturesde 0 a -30°C. La facilitt avec laquelle le radical succinimidyle s'additionne suggere qu'il peut avoir une configuration electronique Z. [Traduit par le journal]
Introduction Ziegler et a1 (1) have demonstrated that the decomposition of N-bromosuccinimide (NBS) in the presence of olefins selectively brominates allylic positions under the appropriate conditions, i.e., thermolysis and (or) photolysis of NBS in boiling carbon tetrachloride. This allylic bromination has been shown to occur by a radical chain mechanism (2-7) and has been extensively utilized in synthesis primarily because of its simplicity and good yield (2-5). The original Ziegler reaction conditions have been used for allylic bromination without much 'Some of these results have been presented at the 52nd annual meeting of the Chemical Institute of Canada at Winnipeg; June 4-7, 1978. A communication of this work submitted t o the Journal of American Chemical Society in September 28, 1977 was not accepted. 'Revision received January 25, 1979.
modification until recently, when it was recognized that conditions affect the NBS decomposition pathways significantly (11-14). Interest in this allylic bromination was heightened when the bromine radical chain mechanism proposed earlier by Goldfinger and co-workers (9) was confirmed in 1963 (8); a straightforward succinimidyl radical chain mechanism proposed by Bloomfield (10) was shown to be not operating at least under Ziegler's conditions (1 1). The Gol&nger Mechanism
121 CH2=CHCH2131 (CH2C0)2NBr
+ Br' + CH'=CH~H- + HBr + HBr (CH2C0)'NH + BrZ -+
0008-4042/79/151967-IO$OI .00/0 @ 1979 National Research Council of CanadalConseil national de recherches du Canada
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CAN. J. CHEM. VOL. 57. 1979
Tlle BloomfieM Mechanist??
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A or IIV [ 5 ] (CH2CO),NBr - d ( C H , C O ) N '
-t- Br'
There have been numerous contributions to our knowledge of the mechanism of the NBS decomposition; these have been summarized in a review by Thaler (6). The major credit should go to Goldfinger who has suggested that NBS reacts rapidly with HBr to generate a low concentration of bromine, i.e., a concentration high enough to maintain the propagation step [4] but low enough to avoid ionic addition to olefins as an integral part of the chain processes; in other words, NBS serves as a reservoir of bromine. This proposal has been nicely substantiated recently (8a, 11). In recent years, decompositions of NBS have been studied in bromination reactions of less active substrates (e.g., haloalkanes and alkanes) under a variety of conditions (e.g., uses of methylene chloride or acetonitrile as solvent to effect a homogenous reaction) much different from Ziegler's heterogeneous conditions ( I 1-1 5). These studies that related to controversies over the anchimeric assistance of a neighboring bromine atom (12-15) led to the discovery that product patterns obtained in the comparative bromination of these alkanes with bromine and with NBS under homogeneous conditions are different (12, 13). This has led to the recognition of the succinimidyl radical as the possible chain carrier in the NBS decomposition (i2, 13). Although esr studies have not succeeded in identifying the succinimidyl radical directly, its presence in NBS decompositions has been indicated in the formation of ~ ~ b r o m o p r o p i o n yisocyanate l (16) and in the esr determination of the stable nitroxide resulting from 2-nitroso-2-methyl-propane trapping (17). Oxidation of 1-phenylethanol in the deco~nposition of N-iodosuccinimide has been claimed to involve the succinirnidyl radical as the chain carrier (18). It is noteworthy that the decomposition of NBS in the presence of benzyl ethers has been reported t o give a-succinimidyl benzyl ethers (19); these results offer a new facet to the mechanism of NBS decomposition. Theoretical calculations by simple molecular orbital theory carried out by Hedaya et al. (20) suggested that a succiniinidyl radical nlay have C
electronic configuration in the ground state, as in A, because of the electronegativity of the acyl groups attached to the nitrogen radical center; they have also predicted that a C radical should be more reactive and less selective. This prediction seems to be borne out by the recent works of Traynham and Lee (13) and Skell and co-workers (12) who have concluded that the succinimidyl. radical is as reactive and unselective as the chlorine atom. Recent I N D O calculations have shown that the ground state of the succinimidyl radical may have a rI (or C,) configuration, as in C, and that the C, configuration A (excited state) correlates with the electronic ground state of the acyl isocyanate radical in the p-scission (21). Our interests in radical species at amino centers have led us to recognize the peculiar reactivities of these radicals. In particular, the aminyl (22, 23) and N-alkylamidyl ( R C O ~ R (24,25) ) radicals exclusively abstract allylic hydrogen atoms from olefins, while aminium radicals exclusively add t o carbon-carbon double bonds (23, 26). In general, it may be expected that amine centered radicals with electron deficiency preferentially add to olefins (25). This expectation has been supported by Lessard and co-workers (27, 28) who have demonstrated that halogen substituted amidyl radicals ( X C H ~ C O ~ ~add H ) more efficiently as both the electronegativity and number of halogen atoms X increase. As the nitrogen center of the succinimidyl radical carries two electron-withdrawing acyl groups which could enter into delocalization with the lone pair electrons if it has C configuration as in B, one might expect the succinimidyl radical to have electrophilic character.
There is scattered evidence that the decomposition of NBS in the presence of olefins does give rise to addition products, although in poor yields (29-34); in some cases the adducts have been implied to arise by ionic addition pathways because of the observed orientations (32-34). However, others may have involved a direct attack of succinimidyl radical on the double bonds, particularly if substrates carry unreactive l~ydrogetu (1, 29-32), e.g., CH2=CH2-
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LU ET A L .
CH,CN. These considerations have led us to reinvestigate the decomposition of NBS hoping to shed some light on the generation and reactivity of the succinimidyl radical. Recent communications by Skell and co-workers (35) on the addition of NBS to olefins and aromatic compounds when decomposed in the presence of these substrates prompts us to publish our results. The complexity of the reaction pattern of NBS in the presence of an olefin has been discussed in detail by Thaler (6). Some of the complications arise from a reversible bromine atom addition to olefins as in reaction [8], the reversibility of reaction [2], and the instability of some allylic bromides. A general strategy of generating the succinimidyl radical would be to suppress the bromine atom chain mechanism (reactions [2]-[4]) so that NBS decomposition and propagation involving NBS as in reactions [5] and [7] can be encouraged. In practice, the HBr-Br, cycle has to be efficiently interrupted, and direct or indirect succinimidyl radical generation as in reactions [5] and [7] has to be accelerated. Our plan was to use ethylene oxide to scavenge HBr, and a high concentration of an olefin to scavenge molecular bromine (1 1); the latter was also the substrate in the present scheme. Epoxides have been used to scavenge HBr in bromination of ketones and aralkanes (36, 37). Acetonitrile was the choice as the solvent owing to its small tendency to donate hydrogen to electrophilic radicals and its reasonably good solubilizing power for NBS (15).
Results In acetonitrile NBS exhibits an absorption maximum (shoulder) at 245 nm (E 317) which tails above the 300 nm region (E,,, 13). This allowed us to carry out the decomposition of NBS by photolysis in a Pyrex apparatus with temperature controls. However, since there were no reliable kinetic data on reactions [I]-181, an a priori prediction of temperature effects on the reaction pattern was not possible. Acetonitrile solution containing NBS (0.047 M) and ethylene oxide (0.35 M) reacted instantaneously at - 10°C with HBr to show a pale yellow color and a weak absorption of bromine at 405 nm. A good amount of ethylene bromohydrin was also obtained from this reaction, indicating that HBr reacted rapidly with both substrates. Qualitatively, it was shown that HBr reacted with ethylene oxide with the order of decreasing efficiency in acetonitrile > methylene chloride > benzene z carbon tetrachloride. Photolysis of NBS in acetonitrile in the presence of 3,3-dimethyl-1-butene (neohexene, l a ) and ethylene oxide under nitrogen was carried out in a Pyrex
1969
apparatus immersed in a cooling bath. Generally the concentration of NBS was 0.016-0.02 M and the molar ratio of NBS, olefins, and the oxide was 1: 10:8. The reaction mixture was vigorously stirred with a magnetic stirrer and its progress was monitored with potassium iodide - starch paper test and (or) iodimetry. For each experiment, a dark reaction was run simultaneously to confirm that thermal reactions occurred to only a small extent ( ~ 5 % or ) not at all. The photolysate was evaporated to give a crude product which, in this case, exhibited a clean nmr spectrum essentially similar to that of l-succinimidyl-2-bromo-3,3-dimethylbutane (2a, 85% yield by vpc). The gc-ms analysis of the crude product showed that it contained trace amounts of four minor products; the minor components had no succinimido moiety (no peak at m / e 100) and were not likely to be BrCH,C(CH,),CH=CH,. Recrys-
tallization of the crude product afforded 2a in a 65% yield which showed physical constants and analysis in agreement with an addition product. Reduction of 2a with zinc powder and sodium iodide gave 30. The nmr spectrum of the latter exhibited, in addition to two singlets, symmetrical multiplets of type A,B, at z 8.54 and 6.48; the latter chemical shift is commonly found for a methylene attached to a succinimido group. Under similar photolysis conditions, the photodecomposition of NBS in the presence of 1-hexene (lb) gave a 60% yield of 1-succinimidyl-2-bromohexane (2b) and a small amount of 1,2-dibromohexane (4b). Reduction of 26 with zinc powder and sodium iodide gave N-hexylsuccinimide (3b) synthesised from hexylamine and succinic anhydride (38). Both photoadditions indicated that the succinimido group attached itself to the less substituted carbon, and the bromine to the more substituted carbon of the double bond. The photoaddition of NBS to cyclohexene under similar conditions gave con~plexproduct patterns (as shown by vpc analysis) which varied noticeably with a slight change of the conditions. The crude product showed an ir absorption at 2260 cm-' typical of an isocyanate group and gave reproducible vpc analyses.
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1970
CAN. J . CHEM. VOL. 57. 1979
The photodeconiposition of NBS at - 10°C gave the trans-1-bromo-2-succinimidylcyclohexane (5, ca. 30%) as the major product in addition to trans-1,2dibromocyclohexane (6), trans- l -bromo-2(2'-bromoethoxy)cyclohexane (7), and 3-bromocyclohexene (8). By vpc analysis, several components in lesser amounts were detected, two of which were shown by gc-ms analysis and comparison with authentic mass spectra t o be 2-cyclohexenol and 2-cyclohexenone. Varying amounts of succinimide were also isolated. By careful chromatography, a small amount of what appeared to be a mixture of 1- and 3succinimidylcyclohexene was also isolated in the tail fractions; the postulate was based on the presence of succiniinidyl and cyclohexene moieties in the fraction as judged from nmr, ir, and mass spectra. both 10 and 11 were formed from the primary adducts, such as 9 or its isomer, by bromine addition and HBr elimination, respectively. The photolysis of NBS in acetonitrile containing benzene and ethylene oxide was carried out with a 450 W Hanovia lamp through a Corex filter in the presence of neohexene (neohexene-benzene, ca. 1/15) to scavenge bromine. In this case the reaction Both 6 and 7 were prepared by the addition of was very rapid and a good yield of 11 (54%) was bromine to cyclohexene in the presence of ethylene obtained. In cases where the ratio of neohexene t o oxide. Adduct 5 showed ir absorptions at 1770, 1700, benzene was higher, small amounts of adduct 2a and 1170 cm-' and a strong tn/e peak at 100 (for were also formed but the ir absorption at 2260 cm-' C,H,NO,+) common to compounds containing a was not present in the crude products. When a lamp succinimido moiety. Zinc powder reduction of 5 gave with less power or a Pyrex filter was used, the time the known N-cyclohexylsuccinimide independently for completion of the NBS decomposition was longer synthesised by another route (38). The trans con- and the yields of 11 were much lower. Vapor phase figuration in 5, 6, and 7 was decided by typical chromatographic analysis indicated that a considerdoublets of a triplet arising from the diaxial orienta- able amount of succinimide was formed and the tion of the 1,2-protons. A similar photolysis at yields of dibromide 4a were at least as much as those -20°C gave a lighter colored crude product and a of 11 and that no other peak, such as that of bromobetter yield of adduct 5 (38x). benzene, was present in the spectra. In view of the extensive formation of bromine, as Photolysis of NBS in benzene in the presence of ethylene oxide was run in a similar manner a t 10°C witnessed by the isolation of 10, in spite of the t o give an isomeric mixture of 10 as the major presence of ethylene oxide, photolysis of bromine in product and a small amount of N-phenylsuccinimide. the presence of ethylene oxide and NBS in benzene The isomeric mixture (40x yield) could not be was examined using a filter to cut off the light source separated by chromatography; these chromato- below 334 nm. NBS decreased slowly owing partially graphic fractions showed similar mass spectral to the interference from the precipitated succinimide, patterns but drastically different ninr patterns, and but no reaction of NBS and bromine was observed gave good analyses for CloH,oBr,N02 arising from in the dark. From the photolysate, in addition t o a the addition of NBS and Br, t o benzene. These nearly quantitative yield of succinimide, a large fractions contained structural and stereochemical amount of a mixture of polybrominated benzene isomers of 10. By extensive chromakgraphy and derivatives was obtained without a trace of benzene recrystallization, small amounts of .pure isomers derivatives containing the succiniinido moiety. having mp 204-207°C and mp 153-157°C were Photobromination of benzene has been reported to isolated; the complexity of their nmr spectra did not yield hexabromocyclohexane slowly through the permit determination of structures. The structure of bromine radical addition (39, 40). More detailed studies on the NBS decomposition 11 was readily decided by direct comparison with an authentic sample prepared by the reaction of in the presence of cyclohexene were carried out in succinic anhydride and-aniline. 1t was assumed that order to explore the effects of changes in the con-
1971
LU ET AL.
TABLE1. Product patterns of NBS decompositions in the presence of cyclohexene and ethylene oxide in acetonitrile" Percentage of productsh
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Exp. No.
Time (h)
Temp. ("c)
5
8
6
7 (CH2CO)2NH
Remark
OUnless specified otherwise, solutions of 0.01 13 mol of NBS in 240 m L o f acetonitrile were photolysed with a 100 W Hanovia lamp in a Pyrex apparatus and the mole ratio of NBS:cyclohexene:elhylene oxide was 1: 10:8. bThe Y, yields were based o n NBS. 8 is 3-bromocyclohexene, CNo ethylene oxide added. *Air leaked in inadvertently. =The mole ratio o f NBS:cyclohexene:ethylene oxide was 1: 10:20. JPhotolysed with a 450 W Hanovia lamp in Corex filter.
ditions. For this purpose a series of photolyses in acetonitrile was carried out at various temperatures under a set of standardized conditions, e.g., apparatus, reagents, and procedures of operation and vpc analyses, as described in the Experimental. The vpc peaks of 5-8 did not change significantly (e.g., _+ 1%) within experimental error, but the peak of succinimide was partially superimposed on an unknown peak which increased with time; the percentage yields of succinimide were therefore less reliable (Table 1). As the temperature decreased from 20 to -30°C, the yields of adduct 5 increased and those of 3-bromocyclohexene (8) decreased but the yields of ionic products 6 and 7 remained substantial regardless of the conditions. Ethylene oxide had no effects on the yield of 5 but at higher concentrations it almost suppressed the formation of 8. The presence of trace amounts of air caused erratic changes in the yields as well as photolysis times. The material balance based on the bromine was better than 90% except for a few cases which were at the 70% range. The material balance based on the succinimidyl moiety (30-80%) was better at higher concentrations of ethylene oxide and increased as the temperature was lowered. The time required for complete photolysis increased as the temperature was lowered, but particularly sharply in the range - 10 to -30°C. Photodecomposition of NBS in the presence of ethylene oxide alone in acetonitrile was shown to induce the formation of bromine, the light yellow color of which persisted even when all the NBS was decomposed. Under these conditions, ethylene bromohydrin, succinimide, and P-bromopropionamide were identified as the major products in
addition to a small amount of bromoacetaldehyde and many other minor components.
Discussion Both the failure of the dark reaction and the orientation of the NBS addition to neohexene and 1-hexene as found in 2 are straightforward indications that the succinimidyl radical has been generated in the photolysis and initiates the attack on the n-bond followed by attack of bromine atom donors as in reactions [9] and [lo]. In the photolysis, radical halogenation processes probably involving allylic hydrogen abstraction also occur in competition with the 1,2-radical addition. Indeed the yields of 1,2addition, 2 and 5, at 0" to - 10°C increase as the number of allylic hydrogens decrease as in cyclohexene < I-hexene < neohexene; substantial amounts of 3-bromocyclohexene 8 under the comparable conditions might indicate relatively facile allylic hydrogen abstraction from cyclohexene. As shown in Table 1, the 1,2-addition product 5 increases and 8 decreases as the temperature is lowered. Such temperature effects have been observed by Touchard and Lessard (27) in the photochemical reaction of N-bromoacetamides with cyclohexene. The succinimidyl radical also attacks benzene efficiently under the photolysis conditions to give the 1,4-adduct 9 (or 1,2-adduct) as the primary product. [9] CHz=CHCHZ-
+ (CH2CO)zN' -+ ( C H ~ C O ) ~ N C H ~ C H C H ~ -
[lo] ( C H ~ C O ) ~ N C H ~ ~ H C H ~ -
+ (CH2CO),NBr(or Bi) -+ (CH2C0)2NCH2CHBrCH2-
+ (CHZCO),Nm
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CAN. J . CHEM. VOL. 57. 1979
Judging from the excellent yield of adduct 2a, the abstraction of hydrogens by the succinimidyl radical from the tertiary butyl group does not compete to any significant extent with its addition to the x-bond of neohexene in spite of the conclusion reached by Traynham and Lee (1 3) and Skell and co-workers (12) that the succinimidyl radical is a voracious (and indiscriminate) hydrogen abstractor. In view of the complex product patterns observed in the detailed studies in the presence of cyclohexene, 3-bromocyclohexene 8 might be formed by both bromine and succinimidyl radical processes. While the strongly reactive succinimidyl radical (12, 13) requires only low activation energies in hydrogen abstraction or addition, the bromine atom, being a moderate (and selective) hydrogen abstractor needs a relatively higher energy (13). This implies that hydrogen abstraction by a bromine atom (reaction [2]) is retarded more than the succinimidyl radical reactions [6] and [9]) as the temperature is lowered. Therefore at -30°C succinimidyl radical reactions should be the major pathways in which addition reaction [6] occurs overwhelmingly over abstraction reaction [9]. Since the mode of the succinimidyl radical reactions is not expected to change significantly over a small temperature range, the increased yields of 8 in the - 10-20°C range are most likely derived from the operation of bromine radical reaction [2]. An extrapolation of this trend (experiments 2-7) predicts that this process would probably operate more extensively at temperatures higher than 20°C; this may explain the lack of the formation of 5 in a similar experiment (in the absence of ethylene oxide) communicated by Skell and co-workers (12). However, it must be noted that at 20°C, even in the absence of ethylene oxide, a reasonable amount of adduct 5 can be formed (experiment 1) indicating that the presence of ethylene oxide is advantageous but not very effective in promoting the succinimidyl radical addition. Epoxides have been known to scavenge HBr efficiently in the photobromination of ketones in carbon tetrachloride (36). In the present cases, ethylene oxide in polar solvents such as acetonitrile appears to scavenge HBr efficiently as shown by the formation of ethylene bromohydrin. Unfortunately, as shown in Table 1, it surprisingly exerts no effect on the yield of the 1,Zaddition process but appears to retard the halogenation process only if the ethylene oxide/NBS molar ratio is of the order of 20: 1. Its effects on the NBS decomposition are probably much more complex than expected as shown by the unpredictable variations of the yields of 6 and 7 as the concentrations of ethylene oxide increase (experiments 1, 2, and 10).
The uneven variation of the yields of dibromide 6 in contrast to the monotonic change of those of 7 might suggest that 6 is formed by dual pathways of ionic addition as well as bromine radical addition, while 7 is formed by the single ionic pathway. The reversibility of reaction [8] has been suggested independently by generation of P-bromocarbon radicals which preferentially undergo elimination with a small activation energy (-5 kcal/mol) (41). At - 30°C, reaction [8] may not revert efficiently and constitutes the major pathway for formation of 6. At temperatures higher than 20°C, reaction [8] reverts efficiently as demonstrated (41) and regenerates the bromine atoms for allylic bromination by Goldfinger's mechanism (9). The persistent formation of the bromination products 6 and 7 under various conditions in spite of fairly efficient scavenging of HBr by ethylene oxide leads us to suspect there might be another pathway for bromine formation since the bromine generated by reaction [3] is expected to be a relatively minor amount under the conditions. Skell and co-workers (35) have recently suggested that reaction [I 11 occurs [ I l l (CH2C0)2NBr
+ Br' + (CH2CO)ZN'+ BrZ
in the NBS photodecomposition in neopentane and methylene chloride, both of which are unreactive substrates. Further, they have proposed that the succinimidyl radical generated in reaction [ l l ] is a II-radical, different from that generated in reactions [5], [7], and [lo] (a C radical). The formation of bromine from NBS by a radical pathway is hinted from two experiments: (i) photolysis of NBS in acetonitrile with ethylene oxide in the absence of an olefin to scavenge radicals efficiently and (ii) photolysis of bromine in the presence of NBS in benzene in which only bromination of benzene by a radical pathway can occur slowly (39, 40). Although the complexity of the reaction pattern does not allow us to pin-point the reaction for the bromine formation, reaction [ l l ] is a straightforward possibility provided the failure of the succinimidyl radical reaction with benzene can be satisfactorily explained. However, possible mechanisms other than reaction [I I] and, indeed, other pathways to account for the decomposition of NBS, cannot be excluded at this stage. The overwhelming addition reactivity of the succinimidyl radical generated from reactions [5], [7], and [lo] to olefinic and benzene x-bonds suggests that this radical is highly electrophilic and, in turn, hints that it has the C rather than the rI electronic configuration. It is known that a carbon radical having the unpaired electron in an sp2 orbital is generally more electrophilic than that in
1973
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LU ET A L .
a p or sp3 orbital (42). In analogy to this general correlation and also owing to a polar resonance contribution as in B, the C electronic configuration of the succinimidyl radical is expected to be more electrophilic than its IT electronic configuration. In view of some theoretical calculations (21) assigning the C configuration to a higher energy (excited) state species, it would be a challenge to identify C as well as IT radicals and to correlate their electronic configurations with their reactivities by quantitative means. The addition of NBS to benzene has been observed previously (43) in a decornposition using benzene as the solvent. The comparable yields of 11 obtained under present conditions (54%) and by Skell and co-workers (45%) in the absence of ethylene oxide (35) seem to indicate, in agreement with the above conclusion, that ethylene oxide has very limited influence on the generation of the succinimidyl radical. The surprisingly good yield of tribromo adducts 10 (40%) in benzene in the presence of ethylene oxide might also be taken as an indication that bron~ineis formed by alternative pathways such as reaction [Ill. The coupling products of the cyclohexadienyl radical, such as those obtained in homolytic additions of phenyl radicals to arenes (42), have not been isolated, probably owing to a rapid reaction of this radical with bromine donors to give 9 (or the 1,2-isomer). It is expected that not only the trans-adduct 5 but also the corresponding cis-adduct is forrned in the NBS photoaddition to cyclohexene as is generally observed in radical addition to cyclohexene (26). The probable presence of a mixture of 1- and 3-succinimidylcyclohexei~esuggests that these con~pounds inight be formed by dehydrobroinination of the cisadduct during work-up since in this compound the C-Br bond can readily take the axial orientation favorable to ar7ti-elimination. In conclusion, we can say that the photodecomposition of NBS in the presence of olefins and benzene containing no allylic hydrogen occurs by the succinirnidyl radical addition to give straightforward addition products, but that in the presence of olefins containing readily abstractable allylic hydrogen it is cornplicated by various side reactions involving the bromine atorn and bron~ine.
Experimental Unless otherwise specified, the following conditions are used. Nuclear magnetic resonance spectra were recorded with a Varian A56160 or XL-100 equipped with a Nicolet 1080 conlputer in CDCI3 using TMS as the internal standard. Infrared spectra were recorded as Nujol mulls or neat liquids with a Perkin Elmer model 457 grating spectrophotometer and mass spectra with a Hitachi-Perkin-Elmer RMU-6E instru-
ment at 80 eV. Melting points were recorded with a Fisher Johns hot stage and are uncorrected. The vpc analyses were carried out with a Varian Aerograph series 1200 with a flame ionization detector. Elemental analyses were performed by Mr. M. K. Yang, Department of Biosciences, Simon Fraser University, using a Perkin Elmer 240 microanalyser. Nitrogen was scrubbed through a Fieser solution, followed by concentrated sulfuric acid and potassium hydroxide pellets. N-Bromosuccinimide was recrystallized from water t o give white crystals, mp 181-183"C, and was kept in a dark desiccator. Preparatiotls of Autl~et~tie Cotnpout7ds N-phenylsuccinimide ( l l ) , N-cyclohexylsuccinimide, and N-hexylsuccinimide 3b were prepared by the method described by Cava et a/. (38) using succinic anhydride and the respective anlines as the reactants. 11 was recrystallized from cyclohexane to give white needles; mp 154-155°C (lit. mp (43) 155.2-155.7"C); ir: 1780(w), 1710(s), 1595(w), 1500(w), 1445(m), 1395(m), 1190(s), 700(m), and 6OO(m) c m - I ; nmr T : 2.65 (m, 5H) and 7.15 (s, 4H); 13C nmr 175.9(s), 131.6(s), 128.7(d), 128.l(d), 126.l(d) and 27.9(t); ms mle (%): 175(100), 120(34), 119(45), and 93(28). N-Cyclohexylsuccinin~idewas sublimed t o give crystals; mp 44.5-46°C (lit. (44) mp 41-42°C); ir: 1765(rn), 1690(s, b), 1399(s), 1378(s), 1190(s) cm-I; nmr s : 6.03 (tt, J = 12 and 4 Hz, lH), 7.35 (s, 4H), 7.7-8.8 (m, IOH); 13C nmr: 176.6(s), 51.2(d), 28.3(t), 27.7(t), 25.5(t), 24.6(t); ms tnle (%I: 181 (2.5, M + ) , 138(10), 101(20), 100(100), and 55(28). 3b was distilled as an oil; ir: 1770(m), 1700(s), 1400(m), 1172(m), 1130 and 825(m) cm- ; nmr T : 6.50 (t, J = 7 Hz, 2H), 7.32 (s, 4H), 8.7(m) and 9.1 1 (t, J = 5.5 Hz, 3H); 13C nmr 177.0(s), 38.6(t), 31.0(t), 27.9(t), 27.4(t), 26.2(t), 22.2(t), and 13.7(q) ppm; ms tnle (%): 183 (54, M + ) , 168(9), 140(14), 113(80), 100(100), and 84(35). Addition of bromine t o neohexene gave a colorless 1,2dibromo-3,3-dimethylbutane (4b) which darkens on storage. The distilled oil showed ir : 1380(s), 1320(m), 1260(m), 1230(m), 900(n1), and 875(m) cm-'; nmr T : 5.90 (m, 2H), 6.45 (dd, J = 11 and 9.5 Hz, IH), 8.85 (s, 9H); nIs tnle (%): 246(0.2), 244(0.6), 242(0.3), 231(1.8), 229(3.4), 227(1.8), 165(29), 163(31), 147(9.3), 149(9.7), and 83(32). 3-Bromocyclohexene was prepared according t o Ziegler et al.'s method (1) and was distilled at 64"C/14 Torr; ir: 3040(m), 1640(w), 1188(s), 862(s), and 732(s) c m - I ; nmr s : 3.96 (m, 2H), 5.16 (m, WL,2 = 9, IH), 7.1-8.1 (m, 8H); ms tnle: 162, 160, and 8 1 (100%).
-
Pl~oto~leeott~positiot~ of NBS 111 tile Presetlee of3,3-Ditnet/1yl-l-b~1e A solution of NBS (2 g), the olefins (10 g), and ethylene oxide (2 mL) in acetonitrile (70 mL) was purged with nitrogen for a few minutes and was irradiated with a Hanovia lamp (100 W) at - 10°C for 3.5 h. At intervals, small samples were withdrawn for iodimetric tests or titrations t o follow the progress of the reaction. The zero hour sample kept at - 10°C in the dark showed no decomposition after 4 h as shown by titration. The photolysate was evaporated t o afford a white solid (2.72 g). The oil showed a nmr spectrum which was essentially that of adduct 20 in addition t o small signals a t T 6.77, 7.74, and 9.00, and a vpc trace which contained one major and four minor peaks (2 3%). By quantitative peak matching the major peak was shown to correspond to 20 (85% yield); gc-ms analysis showed that the first two minor components contained no bromine atom and showed strong nz/e at 57 and 56. The fourth peak showed a very strong peak at tnle 126 which was probably derived from (CH3),CCH(OCH2 CH2Br)CH2Br. Recrystallization of the solid (1.56g) from
-
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1974
CAN. J . CHEM. VOL. 57, 1979
the Presence of 1-Hexetie A solution of NBS (2 g), I-hexene (10 mL), and ethylene oxide (2 mL) in acetonitrile (70 mL) was irradiated under nitrogen for 2.5 h at 0°C until iodimetric tests were negative. A similar mixture kept in the dark at 0°C was iodimetrically titrated after 3 h to give the same amount of the titrant as the zero hour sample. The photolysate was evaporated to give a residue (2.53 g) which showed an ir absorption at 2260 cm-'. This residue taken up in methylene chloride was extracted with water; the methylene chloride was dried and evaporated to give an oil (2.18 g); nrnr T : 8.5 (m, 9H), 7.25 (s, 4H), 6.2 (m, 2H), and 5.7 (m, 1H); ir: 1705,1770, 1400,1160, 820, and 735 cm-'. Chromatography of the oil taken up in methylene chloride on silicic acid gave a forerun (128 mg) containing several minor and one major component. This oil was separated by preparative tlc to give 1,2-dibromohexane (4b); ir: 2960, 2930, 2860, 1460, 1430, 1220, 1150, 965; nrnr T : 5.7-6.7 (m, 3H), 8.0 (m, 2H), 8.5 (m, 4H), 9.1 (m, 3H); ms mle: 185, 187, 189, 163, and 165. The next nine fractions (1.83 g) contained I-succinimidyl-2-bromohexane 2b with a trace of impurities. A portion was further chromatographed to give pure2b as anoil; ir: 1775,1702,1400,1160, and 819 cm-'; nrnr T : 5.49 (m, lH), 5.8-6.5 (m, 2H), 7.24 (s, 4H), 7.9-8.8 (m, 6H), and 903 (t, 3H); 13C nrnr ppm: 177.1(s), 51.0(d), 44.9(t), 35.4(t), 29.l(t), 27.8(t), 21.8(t), 13.4(q); ms nile (%): 263 (5, M+), 261(5), 182(74), 164(3), 162(3), 138(13), 112(47), 100(100), 99(20), 86(41), 84(68), 83(46), and 82(31). Adduct 2b (156 mg), sodium iodide (150 mg), and zinc powder (900 mg) in 80% acetic acid (3 mL) were stirred for 24 h. The filtrate was evaporated and the residue was diluted with water. The aqueous phase was extracted with carbon tetrachloride. The latter phase was washed with dilute sodium thiosulfate solution and was worked up to give an oil (80 mg) of N-hexylsuccinimide; the ir, nmr, and mass spectra were superimposable with those of the authentic samole.
in petroleum ether as eluent to give trans-1,2-dibromocyclohexane 6 (56 mg) and ether 7 (28 mg). These products showed spectra identical with those of authentic samples. Fractions 4-7 (917 mg) showed one spot on tlc with a trace of impurity and decomposed on distillation at 120°C. A portion of this oil was subjected to preparative tlc to give an oil of tratis-1-succinimidyl-2-bromocyclohexane (5); ir: 1770(m), 1700(s), 1450(m), 1176(s), 966, 820 cm-'; nrnr T : 5.14(dt,J= l l a n d 4 H z , l H ) , 5 . 8 8 ( d t , J = 11 a n d 4 H z , lH), 7.30 (s, 4H), 8.30(m); 13C nrnr ppm: 176(s), 57.l(d), 50.3(d), 37.l(t), 29.l(t), 27.5(t), 26.2(t), 24.5(t); ms m/e (%): 262 (47, M ), 260(47), 180(100), 162(89), 160(89), 138(100), 110(55), 100(100), 96(60), 84(63), 82(100), and 81(100). Anal. calcd. for ClOHl4NO2Br: C 46.17, H 5.42, N 5.40; found: C 46.35, H 5.46, N 5.31. Fractions 8-10 (85 mg) were rechromatographed to give the adduct 5 (60 mg) and an oily fraction (15 mg). The total yield of the adduct 5 (977 mg) was 32%. Fraction 11 (22 mg) showed one major spot and four minor spots on tlc and the following physical constants; ir: 3025, 1760(m), 1700(s), 1380(s), 1170(s) cm-' ; nrnr T: 4.15(m), 6.2(m), 6.8(m), 7.35(s); ms m/e (%): 179(100), 151(75), 150(70), 122(120), 121(loo), 100(280), 81(175), and 80(290). Adduct 5 (120 mg) was stirred with zinc powder and sodium iodide in 80% aqueous acetic acid to afford a crude product (62mg). The oil was chromatographed to afford N-cyclohexylsuccinimide; the ir, nmr, and mass spectra and the vpc retention time were identical with the sample prepared by other routes (38). (2) In a separate experiment, a mixture of the same composition was photolysed at -20°C for 5 h. The photolysate was worked up as before to afford a light brown oil (2.72 g) and succinimide (444 mg). The ir spectrum of the oil exhibited a weak absorption at 2260 cm-'. The vpc spectrum of the oil showed the peaks matching with succinimide, 3-bromocyclohexene (S), trans-l,2-dibromocyclohexane (6), ether 7, and adduct 5 in addition to four trace amount peaks. The peak of 5 was the major and others were about 116 to 117 in area of the peak of 5. The oil was chromatographed on a silica gel column to afford adduct 5 (1.14 g, 38%). By gc-ms analysis, the first two trace amount peaks following the solvent peak were shown to possess the mass spectral patterns of 2-cyclohexenenone and 2-cyclohexenol (46). (3) A solution of NBS (2 g), cyclohexene (12mL), and ethylene oxide (4 mL) in acetonitrile (240 mL) at 0°C was purged with nitrogen and was irradiated with a Hanovia lamp (100 W). Samples were withdrawn at every hour and iodimetrically titrated. The solution was kept under a slight positive pressure of nitrogen at all times. The zero hour sample kept at 0°C in the dark for 15 h gave a titration volume the same as that at the zero hour. The same experiment was repeated at - 10°C.
the Presence of Cyclohexene (1) A solution of NBS (2 g), cyclohexene (12 mL), and ethylene oxide (2 mL) in acetonitrile (70 mL) was kept under nitrogen at -10°C and was photolysed for 3 h until the KI-starch test showed negative. The photolysate was evaporated and the residue taken up in methylene chloride (3 4 mL) was filtered. The solid (544 mg) was identified as succinimide by ir and nrnr spectroscopy and by mixture mp 121-122°C. The methylene chloride solution (containing 2.5 g of the residue) was chromatographed on a silica gel column to afford 12 fractions. Fraction 2 (378 mg) decomposed to tar on attempted microdistillation at 10 Torr. A portion of this oil (150 mg) was subjected to preparative tlc o n silicic acid using 10% CHzClz
Photolysis of NBS in the Presence of Benzene (1) In the Absence of Neohexene A benzene (350 mL) solution containing NBS (1.5 g) and ethylene oxide (5 mL) was irradiated with a Hanovia lamp (200 W) in a Pyrex vessel under nitrogen at 5-10°C for 12 h until the photolysate gave a negative KI-starch test. The residue (2.2 g) obtained after evaporation of the solvent was chromatographed on a silicic acid column using methylene chloride containing 0-5% methanol as eluent. The first four fractions (455 n?g) gave very similar ms and ir but different nrnr patterns. The semi-solids showed major mass spectral peaks at mle 338, 336, 334, 257, 256, 254, 238, 236, 234, 176, 175, 158, 156, 120, 119, 100, and 93, and major ir absorptions at about 1710(s, b), 1150, 1180, 770 cm-'. Recrystallizations
petroleum ether gave white solid of I-succinimido-2-bromo3,3-dimethylbutane (20, 65%); mp 85-87°C; ir: 1700 and 1150 cm-'; nmr r: 8.85 (s, 9H), 7.25 (s, 4H), and 5.66.5 (m, an ABC system, 3H); 13C nmr in ppm: 176.2(s), 63.9(t), 41.5(t), 34.6(s), 27.6(t), 26.9(q); ms tn/e (%): 263(25), 261(25), 248(10), 246(10), 207(92), 205(93), 183(17), 182(100), 166(38), 149(6), 127(1I), 126(90), 112(65), 100(92), 84(49), 83(78), 82(18). Anal. calcd. for CloH16N02Br: C 45.81, H 6.15, N 5.34; found: C46.04, H 6.16, N 5.26. Adduct 2a (156 mg) was stirred in 80% acetic acid (3 mL) with sodium iodide and zinc powder (900 mg) for 48 h (45). A usual work-up gave a solid (59 mg) which was recrystallized from cyclohexane to afford white crystals; mp 9699°C; ir: 1760(w), 1695(s), 1405(s), 1355(s), 1305(w), 1235(s), and 1150(s) cm-'; nrnr T : 9.08 (s, 9H), 7.32 (s, 4H), 8.54 and 6.48 (A2BZ,multiplets, 2H each); 13C nmr: 176.4(s), 40.2(t), 34.7(t), 29.1(s), 28.4(q), and 27.5(t). It1
Itz
-
+
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L U ET AL.
of these fractions from methanol or methylene chloride gave crystalline compounds with wide melting ranges. The fifth and sixth fractions were rechromatographed to afford a similar semi-solid (81 mg) as above and a crystalline compound (120 mg) identified as N-phenylsuccinimide (11) by comparisons of ir, nrnr, and mass spectra with those of the authentic sample. Elution with 5% MeOH-CH2CI2 gave succinimide. The semi-solid fractions were combined and chromatographed on an alumina column to afford many fractions, one of which (60 mg) was recrystallized from MeOH-CH2C12 to afford white crystals; mp 204-207°C; ir: 1700, 1185, 1165, and 780 cm-'; nrnr r : 7.14 (s, 4H), 5.2-4.83 (m, 3H), 4.66-4.38 (m: lH), and 4.00 (m, 2H); ms n ~ / e(%): 419(0.2), 417(0.5), 415(0.5), 413(0.2), and lOO(100). Anal. calcd. for C l o H , o N B r 3 0 2 : C 28.85, H 2.40, N 3.37; found: C 29.20, H 2.45, N 3.15. (2) In the Presence of Neohexene A solution of NBS (599 mg, 3.3 mmol), benzene (2.6 mL, 29 mmol), ethylene oxide (1.5 mL, 23 mmol), and neohexene (0.3 mL, 2 mmol) in acetonitrile (100 mL) was irradiated with a 450 W Hanovia lamp through a Corex filter for 20 min at 0°C under nitrogen. The solution was evaporated to afford a brown oil (769 mg), The oil (384 mg) was chromatographed on silica gel using MeOH-CH2C12 as eluent. The first fraction (53 mg) was polybrominated compounds and was not investigated further. The fractions (144 mg) collected with 0.1-0.2% MeOH-CH2C12 were mixtures containing some 11. The fractions (191 mg) eluted with 0.5 1% MeOH were crystallized from cyclohexane to give 11; the ir, nmr, and mass spectra were superimposable with those of the authentic compound. In a separate experiment, a solution of benzene (10 mL), NBS (893 mg), neohexene (0.3 mL), and ethylene oxide (2 mL) in acetonitrile was irradiated in a Pyrex vessel with a 100 W Hanovia lamp at O°C for 25 h. Half of the solution was treated with triethylamine (1 mL) for 8 h at room temperature. The solution was worked up in the usual manner to afford an oil. Analysis by the vpc peak matching method gave three peaks corresponding to dibromide 4a (3%), succinimide (31%), and 11 (13.6%). The other half of the photolysate was evaporated and the residue was analysed by vpc as above to give dibromide 4a (4.6%), succinimide (40%), and 11 (8.6%). In both cases neither adduct 2b nor bromobenzene were detected. Photolysis of Bromine in the Presence of NBS A solution containing NBS (3.55 g), bromine (880 mg), and ethylene oxide (6 mL) in benzene (130 mL) was photolysed through a G.W.C. filter3 (cut off at 334 nm) at about 10°C. Aliquots withdrawn at intervals were treated with a drop of cyclohexene and tested with KI-starch paper. The zero hour sample kept for 40 hours in the dark was similarly treated with cyclohexene and titrated iodimetrically to show no decomposition. It took 35 h for NBS to disappear during which white solids precipitated. The solid (1.13 g) was shown to be succinimide by an ir spectral comparison. The filtrate was evaporated to give an oil. The oil was chromatographed on silica gel to give a mixture of polybromobenzenes (1.93 g) and a small amount of succinimide. Rechromatography of the former gave five fractions that were mixtures, in various proportions, of brominated benzenes. By gc-ms analysis of each fraction, bromobenzene, dibromobenzene, tribromobenzenes, tetrabromobiphenyl, and tetrabromocyclohexadiene were identified in addition to minor amounts of other
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3We are grateful to Dr. G . Pfundt, Max-Plank-Institut fiir Kohlenforschung, Miilheim a.d. Ruhr, who provided the filter.
1975
brominated benzenes. These fractions showed no ir absorption or nrnr signals typical of the succinimido moiety. Reaction of HBr with NBS and Ethylene Oxide T o a solution of NBS (1 67 mg) and ethylene oxide (0.35 mL, 0.007 mol) in acetonitrile (20 mL) was added an acetonitrile solution of HBr (0.01 M, 1.5 mL) at - 10°C. The solution became pH 6-8 immediately. The uv spectrum of this pale yellow solution showed a weak absorption at 404 nm for bromine. This solution was evaporated and the residue was worked up in the usual manner to give ethylene bromohydrin as an oil; nrnr r : 7.22(s, D 2 0 exchangeable, lH), 6.09(t, J = 5.2 Hz), 6.48(t, J = 5.2 Hz). Reaction of Bromine Cyclol~exet~e and Ethylene Oxide T o a solution of cyclohexene (14 mL) and ethylene oxide (2.3 mL) in methylene chloride (100 mL), bromine (3.2 mL) was added dropwise at 0°C. The solvents were evaporated to give an oil (14.6 g). The oil was distilled to afford a fraction at bp 112-113"C/14 Torr which showed nrnr and ir spectra identical to those of 6 (46). The vpc analysis of the forerun showed no peak corresponding to 3-bromocyclohexene. The residue (1.5 g) was chromatographed on silica gel using petroleum ether as eluent to afford an oil (916 mg) which showed one peak on vpc analysis. Preparative tlc of a portion of this oil gave 1-bromo-2-(2'-bromoethoxy)cyclohexane (7); ir: 2930, 2860, 1445, 1190, and 1100 (s, b); nrnr r : 6.10 (t, J = 5.5 Hz, 2H), 6.60 (t, J = 5.5 Hz, 2H), 6.00 (dt, J = 8 and 3 Hz, lH), 6.54 (dt, J = 6 and 1.5 Hz, 1H); ms m/e (%): 288(6), 286(1 I), 284(6), 165(28), 163(31), 109(41), 107(44), 97(60), and 81(100). Anal. calcd. for C8H14Br20: C 33.59, H 4.93; found: C 34.07, H 4.96. Pl~otodecompositionof NBS in the Presence of Ethylene Oxide A solution of NBS (1.17 g) and ethylene oxide (20 mL) in acetonitrile (140 mL) was irradiated with a 450 W Hanovia lamp at 0-5°C under nitrogen. During irradiation, a yellow color developed. A sample treated with a drop of cyclohexene gave a colorless solution; potassium iodide starch test with this solution was used to follow the progress of the reaction. After 5 h, the solution remained yellowish but NBS was consumed. The zero hour sample kept in the dark showed n o decomposition by iodimetry. The crude product was analysed by gc-ms to give many peaks. Most of them showed mass spectral patterns indicative of one or two bromines. Among them, the major peaks corresponding to ethylene bromohydrin, succinimide, and a-bromopropionamide and a minor peak of bromoacetaldehyde were identified by their mass spectral patterns and (or) peak matchings. The mass spectrum of a-bromopropionamide showed prominent peaks at m/e 153, 151, 109, 107, 72 (loo%), and 44. Pl~otoreactiotzof NBS in the Presence of Cyclohexene In a conventional Pyrex photocell, acetonitrile (240 mL), NBS (2.017 g, 0.01 1 mol), cyclohexene (12 mL, 9.72 g), and ethylene oxide (4 mL, 3.56 g) were placed at O°C. The mole ratio of NBS, cyclohexene, and ethylene oxide was 1: 10:8. The solution was purged with nitrogen for 5 min and brought to the desired temperature. While the solution was kept under a small positive pressure of nitrogen, it was irradiated with a 100 W medium pressure Hanovia lamp. Irradiation was continued until the photolysate responded negative to KI-starch tests. On completion of irradiation, the photolysate (50 mL) was evaporated in a rotary evaporator. A portion of the crude oil and biphenyl were weighed into a 5 mL volumetric flask and diluted with acetonitrile to the mark. This solution was analysed with a 10% SE 30 on Chromosorb Q 100/120, 118 in.
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1976
C A N . J . CHEM.
x 6 ft stainless steel column with oven temperature 100-200°C (4"C/niin programming, the flow rate 33 n~L/minat 26 psi). Under these conditions the retention times (in brackets, in minutes) are 3-bromocyclohexene (8, 2.75), succinimide (3.9, t~nt~s-dibromocyclohexane(6, 5.0), ether 7 (10.5), adduct 5 (16.3), 2-cyclohexenone (2.0), 2-cyclohexenol (2.3), unknown 1 (6.3), and biphenyl (7.8). Experiments at different temperatures were repeated to ascertain the reproducibility. For known compounds, correction factors were first established by injecting the known concentrations of biphenyl and a pure sample to be analysed. The measured areas and concentration gave the correction factors. For each sample injected, the peak areas were measured and the absolute contents of each component calculated from these correction factors. The percentage yields of each compound are summarised in Table I . An acetonitrile solution of 3-broniocyclohexene always gave two peaks at 0.5 and 2.5 niin, even when an analytically pure compound was used. The correction factor was calculated on the second peak. The peak area corresponding to succinimide increased gradually as the time elapsed on storage of the crude product; typically, area ratios of succinimide to biphenyl changed from 0.0776 at 5 h to 0.32 at 50 h after isolation.
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