Mechanism of SN reaction

Table 10.3's reactions show that, in a nucleophilic substitution of an alkyl halide, the halogen atom detaches from carbon and a new bond forms between that (electrophilic) carbon and the incoming nucleophile: the covalently bonded halogen is converted into a halide ion, taking the two electrons of the original C-X bond away with it. Because of this, the halogen atom is called the 'leaving group' in the context of this reaction. Two changes therefore happen to the substrate during an SN reaction: heterolysis of the original C-X bond, and formation of a new bond between carbon and the nucleophile using the nucleophile's own electron pair; the sequence and timing of these two changes -- whether they happen together in one step, or one after the other in two steps -- is what is meant by the 'mechanism' of the SN reaction, and it is worked out from studying reaction kinetics. Two such mechanisms are observed: SN2 and SN1. (a) SN2 mechanism: methyl bromide's reaction with hydroxide to give methanol follows second-order kinetics, rate = k[CH3Br][OH-] -- the rate depends on the concentration of BOTH reacting species, which means the reaction happens in a single step, with bond-breaking (C-Br) and bond-forming (C-OH) occurring simultaneously; this is called substitution nucleophilic bimolecular, SN2. Its salient features (Fig. 10.4) are: a single-step mechanism with simultaneous bond breaking/forming; backside attack of the nucleophile (approaching from the side directly opposite the leaving group, to minimise steric and electrostatic repulsion with the leaving group); a transition state in which the carbon is briefly five-coordinate (three sigma bonds in a plane at 120 degrees, plus two partial bonds -- to the nucleophile and to the leaving group -- along a perpendicular axis), with the total negative charge spread (diffused) over the nucleophile and leaving group; and, critically, that when SN2 happens at a chiral carbon, the product's configuration comes out INVERTED relative to the substrate's -- an effect called Walden inversion, pictured as the three untouched groups flipping through the carbon like an umbrella turning inside out. (b) SN1 mechanism: tert-butyl bromide's reaction with hydroxide to give tert-butyl alcohol instead follows first-order kinetics, rate = k[(CH3)3CBr] -- the rate depends only on the substrate's concentration, meaning the nucleophile plays no part in the rate-determining (slow) step; this is called substitution nucleophilic unimolecular, SN1, and it proceeds in two steps (Fig. 10.5): Step I (slow, reversible) is heterolysis of the C-X bond on its own, forming a flat, sp2-hybridised, planar c …