Ward, TA; Levin, G; Szwarc, M
We studied the kinetics of cis-stilbene isomerization induced in THF by electron transfer from Li+, Na+, K+, or Cs+ anthracenide. The reaction is first order in cis-stilbene (cis-St) and the pseudo-first-order rate constant, kobsd, is proportional to {[A-,Cat+]/[A]}2 provided the concentration of the unreduced anthracene [A] is sufficiently high. This implies the following mechanism of isomerization: A-,Cat++ cis-St (k1) â A + cis-St-,Cat+(k-1); A·-,Cat+ + cis-St (formula omitted), trans-St denoting trans-stilbene. At low concentration of A the concentration of cis-St2-,2Cat+ is determined by the stationary state, i.e., (formula omitted), and thus (formula omitted) can be obtained from the kinetic data. Assuming tentatively a constant value of â¼1010 M-1 sec-1 for k-2, we find kiso = 11 à 106, 5.6 à 106, 6.5 à 106, and 0.64 à 106 sec-1 for Li+, Na+, K+, and Cs+, respectively. We conclude that the association of the dianion with the cations is responsible for the hindrance of rotation causing the isomerization, the tighter the association the lower kiso. Literature evidence is quoted to show that the association becomes tighter in the series Li+ < Na+ < K+ < Cs+, justifying the observed trends of kiso with the nature of the counterion. © 1975, American Chemical Society. All rights reserved.