E1cB eliminations. Substituent and solvent effects on the E1cB elimination of the second type from 2-aryl-1,1,2-tricyanopropanes

Tri-n-butylamine-promoted elimination of HCN from 2-aryl-1,1,2- tricyanopropanes shows 'catalysis' by the amine in chloroform and in acetonitrile. The reaction is of first order in the amine at low amine concentrations and zeroth order at high amine concentrations. In the latter region the reaction is faster in chloroform than in acetonitrile, the Hammett ρ⁺ values being -0.36 and k_H/k_D = 1.0 ± 0.08. The reaction is discussed in terms of non-steady-state formation of the conjugate base of 2-aryl-1,1,2-tricyanopropane, which leads to complete ionisation (E1cB of the second type) at the high amine concentrations with rate-determining C-CN bond cleavage. In chloroform (and in some cases in acetonitrile) the kinetics fit the ion-pair variant (E1cB)_ip.