Alkyl halides factors determining mechanism

Effect of the Substrate The structure of the substrate (the alkyl halide) is an important factor in determining which of these substitution mechanisms might operate. Most methyl halides and primary halides are poor substrates for SnI substitutions because they cannot easily ionize to high-energy methyl and primary carbocations. They are relatively unhindered, however, so they make good Sn2 substrates. 

The most important factor in determining whether a reaction follows the S l or S]>j2 mechanism is the identity of the alkyl halide. 

Alkyl halides (R2CHX) undergo both 8n1 and 8 2 reactions. Other factors determine the mechanism. 

Secondary alkyl halides occupy a borderline region in which the nature of the nucleophile is the main determining factor in respect to the mechanism. Secondary alkyl hahdes usuaUy react with good nucleophiles by the Sn2 mechanism, and with weak nucleophiles by SnI. 

For 2° alkyl halides, SnI and Sn2 will occur and other factors determine which mechanism operates. 

Other Approaches.— The determination of both kinetic and solubility data can, in favourable systems, permit the separation of solvent effects on reactivities into initial-state and transition-state components. Some interesting mechanistic conclusions have been drawn from such experiments on reactions of tetra-alkyltin compounds with mercury(n) halides and on Menschutkin reactions of primary amines with alkyl halides. In the former case solvent effects on both initial and transition states are important in determining reactivities, whereas in the latter case solvent effects on initial states are of predominant importance. This approach to the diagnosis of reaction mechanisms has been discussed and fully referenced in pp. 112—113 of Volume 1 and pp. Ill—112 of Volume 2 of this Report. A similar treatment of the reactions of tetra-alkyl-lead compounds with iodine, in a range of ten solvents, has recently indicated that solvent effects on both the initial and transition states are important factors in determining rates here, as for the tetra-alkyltin reactions mentioned above. ... 

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