The reactivity of a series of tetraalkyltins towards iodine has been determinated. The rate equation has been found to be Y = ka. S n h . l a and the rate constant is not affected by added I-. The reaction is an electrophilic substitution at a saturated carbon atom. The following rate constants have been observed : R = CHS, ks = 6,8 1.m-l .sec-l; R = CaHa, ka = 0,8; R = nCsH7, ka = 0,l; R = m e , ka = 0,04.
The CHs<CaHs<nCaH7<nC&e sequence is interpreted as being due to an increasing steric repulsion between the incoming and leaving groups.
Tetraallyltin has further been found to react 300 million times faster than tetrapropyltin towards 4. An S E ~' mechanism, free from steric effects, is suggested to explain this very large difference.
I. INTRODUCTION (192)
La plupart des travaux rtalists dans le domaine de la substitution Clectrophile aliphatique bimolkculaire concernent la dCmCtallation de dtrivts organomercuriques et sont plus particulikrement consacrds a la stdrdochimie des sE2 qui, dans tous les cas CtudiCs, donne lieu a une rttention de configuration. Citons, parmi les nombreux exemples d e la litttrature :
les riastions de dtrivts mercuriques du cyclohexane (3-6) avec (*) Titulaire d'une bourse de spkialisation de 1'1. R. S.I.A. (**) Associk au F. N. R. S.
(l) M.