The base-catalyzed isomerization of α-cyano-cis-stilbenes

Abstract

The rates of the base-catalyzed isomerization of a-cyano-cis-stilbenes were measured in methanolic and ethanolic dimethyl sulphoxide (DMSO) containing the appropriate 0.01 M-alkoxide as base ; the logarithms of the first-order rate constants were found to correlate with the H,acidity function. The slopes range from 0.426 to 0.665 depending on the substituent present in the a-phenyl ring and the solvent system and were shown to be related to the structure of the transition state.The effect of substituents on the rate of isomerization has been examined. Substituents in the a-phenyl ring produce effects on the rates which correlate with U-values and yield p-values of +3.07 in DMSO-ethanol and +2.82 in DMSO-methanol.The activation parameters for the isomerization reaction of three substituted a-cyano-cis-stilbenes were determined in DMSO-methanol solutions. The enthalpies of activation were in the range 14.3-1 6.7 kcal. mole-l and the entropies of activation ranged from -9.3 to -1 3.7 e.u.The mechanism of isomerization is believed to involve rate-controlling attack of alkoxide ion on the P-carbon atom of the cis-stilbene to form a carbanion in which free rotation can occur. Elimination of alkoxide ion gives the trans isomer.

Kinetic equations for reactions in concentrated aqueous acids based on the concept of "excess acidity"

“…[9] = nz*in"X + log (k,/KsH+) For case 2, a,,, = Cs,+,lr,,+ can be substituted into eq. [ 6 ] ; taking logs and rearranging produces eq. [I I ] .…”

“…The observed rate (pseudo-firstorder rate constant k*) is then given by eq. [ 6 ] , where the substrate concentration is written as the sum of protonated and unprotonated forms (since the extent of protonation is also medium-dependent) ; a is activity and f , is the activity coefficient of the transition state.…”