Explain the factors favouring `S_(N)2` reaction.

Explanation

The `S_(N)2` reaction requires the approach of the nuclephile to the carbon bearing the leaving group, and the presence of bulky substituents on or near the carbon atom retards the attack of nuclephile. Thus, the methyl halides react more rapidly in `S_(N)2` reactions because there are only three small hydrogen atoms.
Tertiary halides are the least reactive because bulky groups hinder the approach of nucleophiles. Thus, order of reactivity is

The `S_(N)2` reaction is also favoured by strong nucleophiles and polar aprotic solvents such as acetone, dimethylsulpoxide (DMSO) etc. Also the presence of strong electron withdrawing group favours `S_(N)2`.