Nucleophilic substitution at a saturated carbon atom

A type of reaction that has probably received more detailed study than any other—largely due to the monumental work of Ingold and his school—is nucleophilic substitution at a saturated carbon atom the classical displacement reaction exemplified by the conversion of an alkyl halide into an alcohol by the action of aqueous base  

Kinetic measurements on reactions in which alkyl halides are attacked by a wide variety of different nucleophiles, Nu , have revealed two, essentially extreme, types one in which, 

Hydrolysis of the primary halide bromomethane (methyl bromide) in aqueous base has been shown to proceed according to equation [1] 

By contrast, hydrolysis of the tertiary halide 2-chloro-2-methyl-propane (3,t-butyl chloride) in base is found kinetically to follow equation [2], i.e. as the rate is independent of [eOH], this can play no part in the rate-limiting step. This has been interpreted as indicating that the halide undergoes slow ionisation (in fact, completion of the R- -Cl polarisation that has already been shown to be present in such a molecule) as the rate-limiting step to yield the ion pair R Cle (4) followed by rapid, non rate-limiting attack by eOH or, if that is suitable, by solvent, the latter often predominating because of its very high concentration  

Thus the salient difference between reaction by the SN2 and SN1 pathways is that SN2 proceeds in one step only, via a transition state while SN1 proceeds in two steps, via an actual (carbocation) intermediate. 

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C. A. Bunion, Nucleophilic Substitution at a Saturated Carbon Atom, Elsevier, New Vbrk, 1963. 

Nucleophilic substitution at a saturated carbon atom 4.3 EFFECT OF STRUCTURE... 

He is, in contrast to H , a very poor leaving group indeed, with the result that in simple aromatic nucleophilic substitution ipso attack (cf. p. 161) is the rule rather than the exception. Cl , Bre, N2, S03, NR2, etc., are found to be among the more effective leaving groups and, with them, certain analogies to nucleophilic substitution at a saturated carbon atom (p. 77) may now be observed. 

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