why benzylchloride undergo sn1 reaction?

Hi,

S_N1 reaction takes place via the formation of the most stable carbocation in the intermediate step.

When benzyl chloride loses chlorine it forms benzyl carbocation, which is primary in nature but is highly stable because of resonance stabilization.

 

Thus, the formation of stable benzyl carbocation favours  S_N1 reaction.

Hope it is clear to you.

Best Wishes!