Substitution Électrophile Aliphatique. Mécanisme et Vitesse de la Réaction D'Iododémétallation des Tétraméthyl, Éthyl, nPROPYL, nBUTYL et Allylétains

The reactivity of a series of tetraalkyltins towards iodine has been determinated. The rate equation has been found to be v=k 2 . SnR 4 . I 2 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=CH 3 , k 2 =6,81.m −1 . sec −1 ; R=C 2 H 5 , k 2 =0,8; R= n C 3 H 7 , k 2 =0,1; R= n C 4 H 9 , k 2 =0,04. The CH 3 <C 2 H 5 < n C 3 H 7 < n C 4 H 9 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 I 2 . An S E 2′ mechanism, free from steric effects, is suggested to explain this very large difference.