Write salient features of SN2 mechanism.(b) What is the action of following reagents on bromomethane :(i) bromobenzene(ii) mercurous fluoride

Answer: (a) Features below; (b)(i) toluene (Wurtz-Fittig), (ii) fluoromethane (Swarts).

(a) Salient features of S_N2:

1. It is a one-step (concerted) bimolecular reaction: rate = k[R−X][Nu⁻], second order.
2. The nucleophile attacks from the side opposite to the leaving group (back-side attack).
3. It passes through a transition state in which carbon is pentacoordinated (bonds to Nu and X partly formed/broken).
4. It gives inversion of configuration (Walden inversion); an optically active substrate gives the inverted product.
5. Reactivity order: CH₃X > 1° > 2° > 3° (steric hindrance).

(b)(i) The paper names only bromobenzene, but bromobenzene by itself does not react with bromomethane; the reaction intended is the Wurtz-Fittig reaction, in which bromomethane and bromobenzene are heated with sodium metal in dry ether to give toluene (methylbenzene):

\(\mathrm{CH_3Br + 2Na + C_6H_5Br \xrightarrow{dry\ ether} C_6H_5CH_3 + 2NaBr}\)

(b)(ii) With mercurous fluoride (Swarts reaction), fluoromethane is formed:

\(\mathrm{2CH_3Br + Hg_2F_2 \rightarrow 2CH_3F + Hg_2Br_2}\)