59s
STANLEY J. CRISTOL, JOHN Q. WEBERAND MARGARET C. BRINDELL
Vol. 78
[COSTFZBUTION FROM TKE DEPARTMENT O F CHEMISTRY, UNLVERslTY OF COLORADO]
Mechanisms of Elimination Reactions. XVI. Reaction af sodium Iodide with cisand trans-2-Bromo-1-cyclohexylXitrates and Arenesulfonates' BY STANLEY J. CRISTOL,JOHN Q. WEBEEAND MARGARET C. BRINDELL RECEIVED APRIL 18, 1955 Rates of reaction M ith sodium iodide in n-propyl alcohol of czs- and trans-2-bromocyclohexyl nitratffi,p-toluenesulfoiiates, and ~-bnomobenzenesulfonare~ and of trans-1,2-dibromocyclohexanewere measured at various temperatures. Rate5 of elimination were 30 t o 70 times faster with the trans-bromohydrin arenesulfondtes than with the corresponding czs kmners, whlle both isomers of the bromohpdrin nitrates reacted a t substantially identical rates. The relatively Iow (or negligible) differences in reactivity are rationalized in terms of a rate-determining displacement of arenesulEonate or of bromide by iodide precediiig elimination in appropriate cases
IV. G. Young and his co-workers2 have studied the compounds ran are listed in Table I. The stoithe rates and products of elimination of bromine chiometric equation for the over-all reactions is from acyclic dibromides with potassium iodide in I 1 methanol and with zinc-copper couple in ethanol, Br-C-C-Y + 31- + >C=C< + Br- + Y - f 13and they have shown that elimination proceeds 1 1 largely in a trans fashion, although some of the re(1) actions were not entirely stereospecific.2e Sehubert and his co-workers3 have studied debromination and the extent of reaction was followed by titration with zinc (in water), magnesium (in tetrahydrofu- with standard sodium thiosulfate solution. Calcuran) and sodium in ammonia with acyclic dibro- lations of rate constants were conducted as demides with respect to the stereochemistry of the scribed by Young,2 but no corrections for salt efelimination process. Elimination of other groups FectsZcappeared necessary. The rates were generby these reagents apparently has not been studied ally followed to 20-607, reaction, depending upon systematically, and elimination from cis and trans the reaction temperature, as beyond these v d w s the isomers of cyclic dibromides could not be com- linear plots fell off badly. The rate constants are pared because of the unavailability (until recently4) corrected for temperature expansion of solvent, when necessary. The rate constants are probably of such cis-dibromides. I t has been shown,.l for example, that both cis- good to within 5-10%, except for those with the nitrate ester, where the errors appeared to be someand trans-1 1,12-dichloro-9,10-dihydro-9,10-ethanoanthracene react with zinc t o give the corresponding what greater. For this reason, differences in the olefin (9,lO-dihy&o-9,10-ethenoanthracene),but it activation energy for isomers recorded in Table I did not seem wise to make the generalization from probably are not significant. Reaction rate constants a t 70.0" read off activathis example that stereospecific trans elimination is not required with dihalides and zinc, as this is the tion energy plots of the data of Table I are given in system where cis alkaline dehydrochlorination is Table 11. A comparison of reactivities may be made from the data in this table. It is to be noted favored over For these reasons it was decided to undertake a that in the p-toluenesnlfonate series, the trans isostudy of elimination from derivatives of cis- and mer eliminates 62 times as fast as the cis isomer; tram-2-bromocpclohexanol with zinc and w+th io- an analogous ratio OF 48 is obser'ved in the p-bromodide ion. The zinc-promoted elimination studies benzenesulfonate series; these latter compounds will be reported in a separate communication6; eliminate about 4 times as rapidly as the p-toluenethe work with iodide ion is described in this paper. sulfonates and the trans isomer about 90 times as After some preliminary experiments with acetone fast as the tvans dibromide. The cis and trans and methanol, n-propyl alcohol was chosen as sol- bromo nitrates react a t substantially the same rate as fast as the cis-bromo p-bromobenvent, and sodium iodide as reagent. Studies were and about zenesulfonate. tocalibrate made with trans-l,2-dibromocyclohexane Although elimination of t r a m groups is definitely this procedure with that used previously. No subfavored over cis groups with the arenesulfonates, stantial difference was observed.' Data on all of the rate ratios appear to be several orders of mag(1) Previous paper i n series 'i J C r i s t o l and R F Helmreich Tins nitude too small when compared with those often J O U R \ A L 7 7 , 5034 (1955 observed in base-promoted dehydrohalogenation,l" 12) (a) W G Young a n d S U i n i t e i n tb7 1 , 5 8 , 102 ( I c >~ ( I J , G Young, Z Jasaitis and L L e i a n a s i b z d 6 9 , 403 (1c117), ( c I and even this slight advantage vanishes with the W G Young, D Pressman and C D Lijriell z b t Z , 61, 18-10 ( l Q 3 c J J , nitrates. Accordingly one must assume that deid) S. Winstem, D Pres4man and TI' G I o r i n g i b t d , 61, 1645 (19'3q), bromination with iodide represents a non-stereo( e ) W G Young, S J Criatol and T 5 bkei t b t d 6 5 , 20'1'1 (l'J47) specific elimination, or else that the elimination (3) W .\I Schubert B S R a b i n ~ i ~ i t c A h- , R Larsun aud V A Sims, rbrd , 74, 4.590 (1952) process may be stereospecific, but, in certain cases (4) H L G a e n n g P I Ahell and B F Aycock, tbzd , 7 4 , 3588 a t least, is not the rate-determining step of the over(1952) all reaction. The literature teaches that the first ( 5 ) S J Cristol and S L Hause, zbzd 74, 2193 (1952) alternative must be rejected. *Ismentioned above, (6) S J Cristol and L I3 Rademacher unpublished 7 ) I lie r a t e C--CC-C
C=C< + Br-
>C-C
C-C
