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An Elimination Reaction with Kinetics Characteristic of the ElcB Mechanism’ Thomas I. Crowell, Robert T. Kemp, Robert E. Lutz, and Arthur A. Wall Contribution from the Cobb Chemical Laboratory, University of Virginia, Charlottemille, Virginia 22901. Received February 23, 1968 Abstract: The dehydrochlorination of erythro-4,4’-dichlorochalconedichloride, in absolute ethanol at 25 O, is subject to general base catalysis. The rate is a nonlinear function of the buffer concentration. The reaction is inhibited by acid. The trans-elimination product is predominantly formed from the erythro as well as from the more reactive threo substrate. A two-step mechanism, involving ionization followed by uncatalyzed cleavage of the carbanion, is consistent with the kinetics. Unless the carbanion is extremely short lived, however, some other explanation of the observed acid inhibition is required, since extensive deuterium exchange in the substrate does not take place.
I
n 1933, Hughes, Ingold, and Pate12 stated that the elimination reaction of a tetraalkylammonium ion SHX in basic solution (B = OH-, X = R3N) might,
sults with this method indicate that the carbanion is present and very probablylo is an intermediate in the elimination. Negative results, though admittedly usually due to the concerted or E2 mechanism, do not rule kl out the ElcB nor necessarily prove that it is experimen>C-C< B >C-C< + BH’ (1) I1 11 k I I tally indistinguishable from the E2 type; lack of exH X X change only places a lower limit, depending on the exSHX sxperimental uncertainty, upon the ratio dissociation k2 rate/protonation rate. >_c-c< >c=c< + x(2) Stereochemical f a c t o r ~ *are * ~ ~most evident when the 1 X S impossibility of trans elimination results in very greatly reduced dehydrohalogenation rates. However, the in certain cases, proceed aia the carbanion SX-. This mechanism of the slow elimination may be carbanion or mechanism involving the conjugate base of the sub12,13 concerted. strate was later designated E I C B . ~ , At ~ that early date, Isotope effects* and ingenious studies of elimination/ it was recognized that this type of reaction would show substitution product ratios as influenced by structure l 4 a dependence of rate on hydrogen ion concentration difhave been used to estimate the amount of “carbanion ferent from that expected of a concerted, E2 elimination. character” in concerted but nonsynchronous baseIn fact, retardation by acid had already been observed in the case of N,N,N-trimethyl-N-(p-nitrophenethy1)- promoted elimination mechanisms, especially with /3phenylethyl bromide and related compounds. ammonium iodide,j though the reason for the effect There is much more information to be gleaned from was not ~ n a m b i g u o u sand , ~ ~ later studies have failed to studies of an ElcB reaction than mere second-order support the carbanion mechanism for the Hofmann kinetics. If the rate constants kl, k+ and k2 are in the elimination in strong base.6 proper range, the complex dependence of rate both on The most frequently employed criterion of the ElcB on buffer (general acid and base) concentration pH and mechanism has been deuterium exchange in the ungiven by the steady-state equation (eq 3) should be obreacted substrate, suggested by Skell and Hauser’ and served. Here klB is the second-order rate constant of observed in a few cases8 Its users have been well aware 1 when the base is B. The sum in the numerator is step of the fact that appreciable exchange will not take place if the carbanion dissociates to products much faster (3) than it is reprotonated to s ~ b s t r a t e . Positive ~ ~ ~ ~ ~re-
+ e -1
e
(1) Preliminary report: T. I. Crowell, A. A. Wall, R. T. Kemp, and R. E. Lutz, J . Amer. Chem. Soc., 85, 2521 (1963). (2) E. D. Hughes, C. I
