Question

Explain the mechanism of $S_{N}2$ reaction with one example.

Answer 1

• The $S_{N}2$ reaction involves displacement of a leaving group (usually a halide or tosylate), by a nucleophile. It is a one-step reaction.
  
• This reaction works the best with methyl and primary halides because bulky alkyl groups block the backside attack of the nucleophile, but the reaction does work with secondary halides (although it is usually accompanied by elimination), and will not react at all with tertiary halides.
  
• In this reaction the inversion of configuration takes place. Thus here backside attack of nucleophile takes place to avoid hindrance due to the high molecular mass groups.
• In this mechanism, the bond breaking between $C-X$ and bond formation between $C-Nu$ takes place simultaneously. During this stage energy of the species increases and this stage is known as 'transition stage'.