Kinetics and mechanism of some aliphatic and aromatic electrophilic substitutions.

The kinetics of the reactions between tetra-alkyltins, R4Sn, (R = Me, Et) and mercury(II) halides, HgX2,(X = Cl, I) in solvent ethyl acetate have been studied kinetically at various temperatures. The stoichiometry of the reactions is expressed by equation (1): R4Sn + HgX2→RHgX + R3SnX . (1) The reactions in ethyl acetate follow second-order kinetics, first-order in each reactant. From the kinetic salt effect studies on reaction (1) (R = Et and X = Cl) in ethyl acetate, it is shown that the reaction proceeds through a polar transition state. The sequence of reactivity among the alkyl groups for reaction (1) (R = Me, Et and X = Cl, I) is shown to be Me > Et. This sequence has been used by previous workers as evidence for mechanism S E2(open). By analogy, this mechanism is suggested for the reactions in ethyl acetate; the salt effect studies, above, provide confirmation for this suggestion. Activation parameters for reaction (1) (R = Me, Et and X = Cl, I) in ethyl acetate were shown to be consistent with mechanism S E2 (open) . The solvent effects on the variation of the activation parameters DeltaG+, DeltaH+ and DeltaS+ for reaction (1) (R = Me, Et and X = Cl, I) when solvent methanol or acetonitrile is replaced by ethyl acetate have been dissected into initial- and transition-state contributions. It is shown that in nearly all cases both initial- and transition-state effects are important, but the major solvent effect is on the transition state. Reaction (2) (R = Et and R' = Me, Et, Bu t, ClCH2CH2) and (2) (R = Me and R' = Me, Et) in ethyl acetate has been studied at 25C. R4Sn +Hg(OCOR')2 →RHgOCOR' + R3SnOCOR' (2) From the effect of the substituents (R') on the rate of reaction, mechanism SE2(open) is postulated. Reactions of phenyltriethyltin with mercury(II) halides have been studied kinetically in methanol at various temperatures. The stoichiometry of the reaction is represented by equation (3) (X = Cl, I): PhSnEt3 + HgX2 → PhHgX + Et3SnX (3) Reaction (3) follows second-order kinetics, first-order in each reactant. When X = I the reaction is followed by a fast equilibrium, equation (4): Et3SnI + Hgl2 ↔ Et3Sn + Hgl3 (4) It is shown that mercury(II) chloride reacts only via the covalent species HgCl2. The order of reactivity among the mercury(II) salts for reaction (3) in methanol was found to be: Hg(OAc)2 > HgCl2 > Hgl2 > Hgl3 (5) Added water mildly accelerates reaction (3) (X = Cl) whilst inert salts tend to retard it; the activation enthalpy of reaction (3) (X = Cl, I) is quite low, and it is suggested that the transition state of the reaction resembles a pi-complex rather than a sigma-complex. Reaction (3) (X = Cl, I) has also been investigated in solvents acetone and ethyl acetate at various temperatures. Results are similar to those found in methanol and similar mechanistic conclusions are drawn. It is concluded that reaction (3) is a suitable electrophilic aromatic substitution for studies of salt effects and solvent effects