Product isotope effects, given by the product ratio XC6H4CHs/XCBH4*CH,D, have been measured for cleavage a t ca. 21 ' of some compounds of the type XC6H4-CH,*MMe, ( M = Sn or Si) in 1 : 1 MeOH-MeOD containing sodium methoxide (2 mol 1-l). Valuesfor various substituents X and M = Sn are: p-Me, 2.4; H, 2.1 ; p-CI, 2-1 ; rn-CF,, 2.1 (all rt0.15) ; and for M = Si ; H, 1 .I 7 ; rn-OMe, 1.25 ; p-CI, 1.07 ; rn-CF,, 1 .I 9 (all k0.05). The corresponding value for C6FS*CH2.SiMe, is 1 .I 6. Overall solvent isotope effects, (ksH/ksD), given by the individual rates in non-deuteriated and deuteriated media at 50" have been determined for the compounds with X = m-CF,; the values are : for M = Si, 0-50 in MeOH or in MeOH containing 18.6 mole % H, O ; for M = Sn. 0.95 in MeOH and 0.97 in MeOH containing 20 mole % H, O.It is concluded that, in the cleavage of the tin compounds certainly, and probably also in that of the silicon compounds, a free carbanion is not formed. It is suggested that the rate-determining step involves proton transfer from the solvent to the carbon atom of the breaking M-CH2*C6H4-X bond, with the MeO-M bond fully or almost
