N-bromosuccinimide (NBS) is used for brominating alkene at allylic position (H atom attached to a carbon atom next to a double bond). Grignard formation occurs preferntially with the more reactive benzylic halide
${\text{C}}_{\text{6}}{\text{H}}_{\text{5}}{\text{CH}}_{\text{3}}\stackrel{{\text{Cl}}_{\text{2}}}{\to }\text{p}-{\text{ClC}}_{\text{6}}{\text{H}}_{\text{4}}{\text{CH}}_{\text{3}}\stackrel{\text{NBS}}{\to }\text{p}-{\text{ClC}}_{\text{6}}{\text{H}}_{\text{4}}{\text{CH}}_{\text{2}}\text{Br}\stackrel{\text{Mg}}{\to }\text{p}-{\text{ClC}}_{\text{6}}{\text{H}}_{\text{4}}{\text{CH}}_{\text{2}}\text{MgBr}$
$\stackrel{\text{(i)}\text{ethylene}\text{oxide}}{\to }\text{p}-{\text{ClC}}_{\text{6}}{\text{H}}_{\text{4}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{OH}$