PHOTOBROMINATION WITH N-BROMOSUCCINIMIDE
Oct. 20, 1963 TABLE VI1
RELATIVEREACTIVITIES OF ARALKYL HYDROCARBONS TOWARD THE TRICHLOROMETHYL RADICAL' From retardation of telomerization, 91.5' (ref 10)
CCls, 40' (this work)
Hydrocarbon
Toluene Ethylbenzene Cumene Diphenylmethane Triphenylmethane 3 Per a-hydrogen atom.
1 ,001 50 260
1 .OOh 3 1 4 2 8.0 16.7
50
*
160 Assumed.
better except for bromotrichloromethane (Eastman practical grade) which was about 96 mole per cent pure. All reagents except for bromotrichloromethane and elemental bromine were dried by percolation through activated silica gel before use. Procedure for Reactions with Elemental Bromine.--..lpproxiniately 25 mmoles of each of two hydrocarbons and 15 mmoles of bromobenzene were accurately weighed out and diluted t o 50 ml. with carbon tetrachloride. This was the reaction mixture. Since the brornobenzene did not react under the bromination conditions i t was used as the internal standard for the g.1.c. analysis. Experiments in which t h e internal standard was added after bromination gave the same results as those in which the bromobenzene was present during reaction. The reaction mixture vias placed in a 100-ml. three-necked round-bottomed Pyrex flask equipped with a nitrogen inlet tube t o remove hydrogen bromide from the solution, a cold finger condenser containing Dry Ice and carbon tetrachloride, a dropping funnel for 'bromine, and a magnetic stirrer. The flask was immersed in a 40 i 0.5" water bath and illuminated with a 100-watt unfrosted tungsten
[CONTRIBUTIOS FROM
THE
3139
lamp. Enough bromine was added dropwise to react with approximately half of t h e hydrocarbons present. The rates of addition of bromine and nitrogen were varied in several reactions. After the reaction was completed, the reaction mixture was hydrolyzed t o remove lachrymatory products and analyzed by g.1.c. In competitive brominations involving toluene no products of ring substitution could be detected by g.1.c. indicating t h a t less than 17; of the toluene reacting in these reactions was converted to products of nuclear substitution. Procedure with Bromotrich1oromethane.-.%pproximately 25 mnioles of each of two hydrocarbons and 15 mmoles of bromobenzene were accurately weighed and diluted with carbon tetrachloride. -4bout 5 ml. of the rtaction mixture and an equal volume of bromotrichloromethane were placed in a Pyrex ampoule, degassed b y freeze-thawings, and sealed. The tube was immersed in a 10 i 0.5' water bath and illuminated with a 2 7 , s watt sun lamp. Under these conditions about half of the hydrocarbons was consumed in 2 1 hr. The tubes \\-ere opened and the contents analyzed by g.1.c. soon after t h e reaction was quenched. The choice of column and gas Chromatograph was dictated by the hydrocarbon mixture being analyzed. For mixtures which had widely separated boiling points, a n F. and M . model 500 gas chromatograph was used and the temperature was hand programmed during the analysis. hnother procedure for mixtures with widely separated boiling points was t o add two internal standards and make two separate analyses a t uniform teniperatures. For analyses wherein no temperature change was required during the analysis a Perkin-Elmer Model 154D vapor fractomcter with thermistor detector was used. The columns used, either separately or in series, were a %-in.diisodecyl phthalate, a 1-m. methyl silicone grease, a 1-in. Apiezon N, a 2-m. n-propyl tetrachlorophthalate, a 1-m. b,P'-oxydipropionitrile, and a 1-m. column of silicon grease ( G . E. XF1105) colunin in which 5yc of the methyl groups have been replaced by cyanomethyl groups.
DEPARTMENT O F CHEMISTRY,
IOX'JA STATE
Directive Effects in Aliphatic Substitutions. XIX. N-Bromosuccinimide
UNIVERSITY, AMES,
IOWA]
Photobromination with
BY GLENA. RUSSELL~ A N D KATHLEEN M. DESMOND RECEIVED MARCH27, 1963 Competitive photobromination of aralkyl hydrocarbon in methylene chloride solution a t 40" by N-bromosuccinimide yields a relative reactivity series so similar t o t h a t observed in photobromination by molecular bromine t h a t it appears t h a t under the reaction conditions both reactions involve the bromine atom as the hydrogen abstracting species.
There is considerable evidence t h a t photobromination by N-bromosuccinimide (NBS) proceeds through a free alkenyl or aralkyl radical. For example, the occurrence of allylic rearrangementYand the essentially complete racemization observed in the bromination of ( --)-ethylbenzene-cr-d4 are consistent with such a radical mechanism. However, the question as to whether the free radical is formed by process X or B or by both A and B has never been completely resolved. HBr
+ KBS --+ Br2 + succinimide (SIT) Br2 + R.--+ RBr + Br. Br. + RH -+ HBr + R. S . + R H --+ SH + R. R . + KBS -+ R B r + S .
(A)
(B)
Competitive brominations of substituted toluenes has been interpreted as evidence in support of process B5 and process A.6 cis-trans Isomerization of unreacted olefins has also been interpreted in terms of mechanism (1) This research was supported by a grant from the Petroleum Research Grateful acFund administered by the American Chemical Society knowledgment is hereby made t o the donors of this Fund ( 2 ) .4lfred P. Sloan Foundation Fellow 1959-1963, (3) C F. Bloomfield. J Chcm Soc., 114 (1944) (4) H . J Dauben, Jr . and I. I, LlcCoy, J A m C h e m S o c , 81, 5404 (1959). (5) E. C Kooyman, R van Helden, and A F Bickel, K o n i n k l . X e d A k a d W e l e n s c h a p , Pvoc., 66B,75 (1953) ( 6 ) F . I.. J Sixma and R . H Riem, ibid , 61B. 183 (1058).
B4 as well as process h.' The formation of dibromides in NBS reactions with certain olefinss would seem to support process X or a competing ionic process whereas the formation of 1: 1 adducts between NBS and other olefinsg can be interpreted in terms of process B . Another complication rests in the fact t h a t as usually performed the Wohl-Ziegler reaction utilizes refluxing carbon tetrachloride as a solvent. Both KBS and succinimide are rather insoluble in carbon tetrachloride and a third possible reaction mechanism involving succinimidyl radical propagation occurring on the surface of NBS crystals has been suggested.'" T o avoid this problem we have studied the competitive bromination of a series of aralkyl hydrocarbons in methylene chloride solution a t -IOo, a solvent in which NBS is soluble to the extent of ca. 0.25 X.
Results T h e competitive photochemical bromination of toluene and ethylbenzene by N B S in refluxing methylene chloride solution was carefully studied as a function of hydrocarbon and NBS concentrations. Sufficient NBS was employed so that upon completion of ( 7 ) B P. McGrath and J .If Tedder, Pvoc Chem. Soc , 80 (1961) (8) P L. Southwick, L A P u r s d o v e , and P. h-umerof, J A m C h r m Soc 72, 1600 (19.70); E J . Corey. rbid , 75, 2251 (1953) (9) P. Couvreur and A . Bruylants, B d l . soc ckirn. R r l g e s , 61, 2 5 3 (1952); W . J . Bailey and J Bello, J . O v g C h a m . , 20, ,525 (19,55) (10) L Horner and E . H . Winkelmann, A n g r u C h e m , 71, 349 (1959)
3140
TABLE I RELATIVEREACTIVITY O F TOLUENE A N D ETHYLBENZENE IN SBS BROMINATION IN METHYLESECHLORIDE AT 40' --Toluenea--Initial
0.169 ,166 ,147 ,210 ,247 ,390 ,396 ,365 ,482 ,124 ,798 1.27 10.lj.'
V O l . 85
GLENA. RUSSELLAND KATHLEEN M. DESMOND
Final
-EthylbenzeneOInitial Final
ktoluem
XBS'
ketilylbenzene
0.136 ,147 ,134 ,191 ,213
0.133 0.00635 0.16' 13.4 ,0230 ,142 .14' 14.9 ,140 ,0112 .ISh 14.2 ,0678 ,290 .24h 15.5 ,290 ,0424 .22'mC 13.0 ,290 ,0678 .38 15.0 ,191 .44 ,308 ,365 ,0114 13.7 ,413 ,313 ,0514 .40 13.4 ,390 ,544 ,0398 .34d 15.5 ,108 .80 12.7 ,620 ,798 ,772 .09c'4 .80 14.7 ,652 1.12" 1.246 ,188 ,60 14.6 9 . 2 0 ' 1 3 . 1 j e 3.23" ll".',' 13.9 Av. 14.2 f 0.9# Moles per liter. * Initially homogeneous. 20 mole 76 (based o n S B S ) of molecular bromine added. 45 mole 70 (based on S B S ) of molecular bromine added. e Millimole in an initial volume of 85 ml J S B S added over a 2-hr. period as a solution in methylene chloride. g Standard deviation.
sluggishly if a t all with NBS and attempts to include these compounds in a reactivity scale failed owing to their inhibition of the Wohl-Ziegler reaction. Pertinent results are summarized in Table 11. Assuming that the consumption of the hydrocarbons involves only attack of an atom or radical on hydrogen atoms a to a double bond or the aromatic ring, the relative reactivity series of Table 111can be constructed. No attempt has been made to estimate experimental uncertainties except for the first four hydrocarbons in the table.
Discussion The similarity of the reactivity sequence observed for the bromination of aralkyl hydrocarbons with molecular bromine and with NBS appears to be strong evidence for the occurrence of mechanism A in NBS brominations, a t least for aralkyl hydrocarbons under our reaction conditions. The uniqueness of this reactivity series has been discussed previously. l1 The greater reactivity observed for allylic hydrogen atoms than for similar types of benzylic hydrogen atoms is qualitatively in agreement with reactivity data toward the t-butoxy12 and phenyl13 radicals.
TABLE I1 Hydrocarbon h
Ethylbenzene Cumene Cumene Cumene Cumene Cumene Cumene Ethylbenzene Ethylbenzene Ethylbenzene Ethylbenzene Indan Indan Indan Indan Cyclohexene Cyclohexene Cycloliexetie Cyclohexene Cyclohexene Cyclohexene Cyclopentene Cyclopentene Cyclooctene Cyclooctene Millirnolcs '&
COMPETITIVE BROMINATIOSS WITH NBS IS METHYLESE CHLORIDE AT 40' [BID" Nnsa Hydrocarbon B [a lo" [Alfa Average value, Table I Toiuene 9.20 17 22 8 8 40 9.90 Toluene 10.4 13 13 5 3 20 11 3 Toluene 10.2 9.80 8.3' Toluene 15 3 8 60 5.02 5.9 3.02 Ethylbenzene 6 38 3 95 11.8 9.5 10 7 7.30 Ethylbenzene 6 60 20.0 Ethylbenzene 25 0 12.7 26 39 6 6.97 4.55 7.0 Diphenyltnethane 6 95 2 62 6.97 4.22 7.2 Diphenylmethane 7 57 3 00 5.58 .%llylbenzene 8 38 3.10 6.5 5 52 4.54 2.62 6.5 e all^ Ihenzene 6 82 4 40 18.0 11 21 9 12 4 21 .0 Ethylbenzene 13 4 18.8 15,7 16 27 0 Ethylbenzene 11.7 20 22.6 Tetralin 11 9 5 90 12.3 17 11 2 22.85 Tetraiin 6 40 14 2 13.0 11 11 6 4 40 Ethylbenzene 14.3 12 9 47 2 51 15.9 Ethylbenzene 17 1 24.3 7.65 20 Cyclopentene 19 95 23.0 104 72 6 91.0 Cyclopentene 95 0 64 5 49 7 72.6 13.6 67 Cycloheptene 58.1 13.5 49 46 2 37 4 Cyclolieptene 10.5 10 9 5.70 8.0 Cycloheptene 6 90 11 2 10.3 5.70 8.0 Cycloheptene 7 60 8 83 i 25 9.90 6.45 9.5 Cycloheptene 5.54 9.4 9.88 Cycloheptene 8 42 6 55 Containing 45°C (based on S B S ) of elemental bromine
the reaction about 50yoof the total toluene and ethylbenzene initially present was consumed. T h e concentrations of toluene and ethylbenzene after bromination were measured by gas-liquid chromatography (g.1.c ) and the relative reactivities of the hydrocarbons calculated by the equation
where the subscripts refer to final and initial concentrations. Pertinent data are summarized in Table I. Having established the reproducibility of the method, a variety of other aralkyl hydrocarbons and cycloalkenes were competitively brominated. The reactions involving aralkyl hydrocarbons were complete in 10 to 40 niin. while those involving cyclodkenes ar allylbenzenr required 7-21 hr. for complete consumption of the NBS. Conjugated and 1,3-dienes react very
Vol., cc.
50 25 25
--i r 7
80 50 25 25
25 50 60
50 70 70 50 50 50 100 100
50 25 50 50 25
kA/kr!
1 4 . 2 rt 0 . 9 14 16 13 1.05 1.0 1 .o 2.3 2.0
0.71 0.78 3.6 3.7 1.06 1.05 11.1 11.7 0.21 .20 .15 .14 .75 .67 .46 .44
However, the magnitude of the difference is somewhat ~ u r p r i s i n g 'as ~ is the fact t h a t the a-hydrogen atoms of the C+& cycloalkenes are considerably more reactive than the a-hydrogen atoms of allylbenzene (see Table 111). Admittedly the reactivities of the cycloalkenes, as determined by the disappexance of the hydrocarbons, involves a side reaction le iding to the dibromocycloalkenes. l'ields of 1,2-dibromocyclohexane in the range of 6-10y4 have been reported in this reaction.' (11) G . A . Russell, C. DeBoer, and K M Desmond. J . A M . Che7n. Soc., 81, Xj (1983). (12) C Walling and U'. Thdler, ibid., 83, 3877 (1961). (13) Unpublished work with D r . Robert F Bridger. (14) Toward the 1-butoxy radical a t 4 0 a an a-hydrogen atom of cycluhexene is about 11. 5 times asreactive as an a-hydrogen atom o f ethylbenzene (ref. 12) while toward the phenyl radical a t 60' the relative reactivities are P . 6 i : l (ref. 1 3 ) . (13) K Ziegler. A . Sgath, E . Schaaf, JV.Schumann, and E . U'inkelmann, A n n , 611, 80 (194'2.): W. J. Bailey and J . Bello, J . O m . C h e m . . 20, 689
Oct. 20, 1963
PHOTOBROMINATION WITH
RELATIVEREACTIVITIES I N BROMINATiON REACTIONS AT 40"
NBS (CHZC12)
Hydrocarbon
a
Ref. 16
1.0b 14 f 1 15 i 1 6.5 f 1 49 47 19 780 160 1100 590
Molecular bromine" (CC14)
NBS (per a-hydrogen atom)
1.0b 11 f 1 12 f 1 6.5 i 1
1.Ob 21 f 2 45 i 3 10 & 1 . 5
36 36 28 600 160 825 440
Assumed
However, correction for this reaction would not appreciably effect the reactivity series of Table 111. The effect of all the alkenes studied, including allylbenzene, upon the rate of the brominations is surprising in view of the high reactivity of the alkenes observed in competition with aralkyl hydrocarbons. The competitive reactions involving only aralkyl hydrocarbons proceeded about 20-50 times as fast as those involving the olefins. Moreover, the competitive reaction between ethylbenzene and cyclohexene required about 20 times as long for complete consumption of the NBS as the competitive reaction between ethylbenzene and toluene. Possibly olefins inhibit reaction A and the slow reaction observed proceeds via mechanism B . Sixma and Riem have, however, argued that, since a t low concentrations of molecular bromine cyclohexene yields 80% of the allylic bromide, NBS bromination of alkenes involves process A and the high yields of allylic bromides formed in NBS reaction result from the low bromine concentrations demanded by process A. If the Wohl-Ziegler bromination is a process involving long kinetic chains, consumption of an occasional molecule of bromine by addition to the olefin would result in chain termination if no alternate chain propagation reaction is available for the alkenyl radical. Consumption of bromine in this way could force the alkenyl radical to react with NBS rather than molecular bromine and thus inhibit process A and force the reaction to proceed (slowly) by process B. The occurrence of process C would also force the reaction to proceed via a succinimidyl radical instead of a bromine atom. Br Br. Br
>C-C
C=C
C!-C
C-C
