This reaction goes through an SN1 mechanism because when the bromine leaves, it forms a really stable benzylic carbocation. That positive charge can spread into the benzene ring through resonance, which makes the intermediate much more stable. An SN2 reaction would not work well here since the carbon attached to bromine is too crowded for a nucleophile to attack directly. Because of that, the reaction naturally follows the SN1 path, where the bond to bromine breaks first and the nucleophile comes in afterward to form the final product.

The reaction follows the SN1 mechanism instead of SN2 due to the substrate structure and the stability provided by the phenyl ring. Although the bromine is attached to a primary carbon, the phenyl ring can stabilize the intermediate carbocation through resonance, which favors the SN1 mechanism, where a carbocation is formed before the nucleophile attacks. In an SN2 reaction, the direct attack of the nucleophile on a primary carbon would be more typical, but in this case, the formation of the carbocation is more likely. Additionally, if the solvent is polar protic, such as water or alcohol, it can help stabilize the carbocation, further favoring the SN1 mechanism.