Question

Consider the reaction, $C{H}_{3}C{H}_{2}C{H}_{2}Br+NaCN\to C{H}_{3}C{H}_{2}C{H}_{2}CN+NaBr$.
This reaction will be the fastest in :

• A. N, N' - dimethyl formamide (DMF)
• B. water
• C. ethanol
• D. methanol

Answer 1

The correct option is A N, N' - dimethyl formamide (DMF)

The solvent affects the rate of reaction because solvents may or may not surround a nucleophile, thus hindering or not hindering its approach to the carbon atom. Polar aprotic solvents, like tetrahydrofuran, are better solvents for this reaction than polar protic solvents because polar protic solvents will form a hydrogen bond to the nucleophile, hindering it from attacking the carbon with the leaving group.
A polar aprotic solvent with low dielectric constant or a hindered dipole end will favour $S_{N}2$ manner of nucleophillic substitution reaction. Examples: DMSO, DMF, acetone etc. In polar aprotic solvent, nucleophilicity parallels basicity.

The above reaction is carried out by $S_{N}2$ mechanism which is favored by polar aprotic solvents like DMF but remaining all the options are protic solvents in which the $S_{N}2$ mechanism is carried out slowly.

Hence, option A is correct.