chemistry

Several distinct mechanisms are possible for nucleophilic substitution reactions at saturated C atoms, depending on the substrate, nucleophile, leaving group and the reaction conditions. By far the most common are the S N 1 and S N 2 mechanisms. Simple alkyl groups, methyl and primary alkyl groups always react by the S N 2 mechanism and never by S N 1 . Why methyl and primary alkyl groups react by the S N 2 mechanism? What are the steps in an S N 2 mechanism? Partly because the corresponding cations are unstable and partly because it is easy for the nucleophile to attack the C atom since it is surrounded by H atoms – and not by bulky alkyl groups. Experimental results show the following in S N 2 reactions: In this mechanism (Fig. 1)  there is backside attack 1 . The nucleophile attacks the carbon atom on the opposite side from the leaving group – 180 away from the leaving group – and the carbon atom turns inside out as the reaction proceeds The reaction is a one-step process

Substitution in general is the replacement of one functional group by another. It has been observed that tertiary alcohols react rapidly with HBr to give tertiary alkyl bromides. Primary alcohols, on the other hand, react very slowly with HBr and are usually converted to primary alkyl bromides when they react with PBr3. The mechanisms proposed for these reactions are known as S N 1 and S N 2 respectively.   When do S N reactions at saturated C atoms take place according to the S N 1 mechanism and when do they take place according to the S N 2 mechanism? Substitution reactions take place with the S N 2 mechanism in the following cases: Almost always in sterically unhindered benzyl και allyl positions (except benzyl and allyl triflates that react according to S N 1) Always in substitutions in CH 3 X and RCH 2 X (primary C) In substitutions in R 2 CHX (secondary C) Never in substitutions in tertiary and primary substrates of the type R 3 CX or R 3 CCH 2 X  Substitution reactions take p