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The ${\mathrm{S}}_{\mathrm{N}}2$reaction is a nucleophilic substitution reaction where bond is broken and other is form simultaneously. Two reacting species involved in the rate determining step of the reaction. The term ${\mathrm{S}}_{\mathrm{N}}2$stands for substitution nucleophilic bimolecular.

${\mathrm{S}}_{\mathrm{N}}2$REACTION

The${\mathrm{S}}_{\mathrm{N}}2$substitution of hydroxide ion with${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CHBr}}_2$ , yields an unstable bromoalcohol intermediate, which loses Br^-to give benzaldehyde.

Dibromomethyl benzenebenzaldehyde

The${\mathrm{S}}_{\mathrm{N}}2$substitutionreaction of (dibromomethyl) benzene ${\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CHBr}}_2$,with NaOH yields benzaldehyde rather than (dihydroxymethyl) benzene, ${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{CH}{\left(\mathrm{OH}\right)}_2$, because the presence of two hydroxyl group on one carbon makes the molecule unstable.This happens because two hydroxyl groups increase electron density on carbon. Because intramolecular dehydration takes place and water molecule is separated and we are left with an oxygen attached with double bond to carbon.