Consider the electrophilic addition of hydrogen bromide () to 1-phenylpropene () in a dark environme — Haloalkanes and Haloarenes Chemistry Question
Question
Consider the electrophilic addition of hydrogen bromide ($HBr$) to 1-phenylpropene ($C_6H_5-CH=CH-CH_3$) in a dark environment without any peroxides. After the reaction is complete, what is the absolute total number of $\pi$ -electrons remaining in the structure of the major organic product formed?
💡 Solution & Explanation
The reaction follows Markovnikov's rule via electrophilic addition. The $H^+$ adds to the alkene to form the most stable carbocation intermediate. Adding $H^+$ to C2 yields a benzylic carbocation ($C_6H_5-C^+H-CH_2-CH_3$), which is highly stabilized by resonance with the adjacent phenyl ring. Adding $H^+$ to C1 yields a $2^\circ$ carbocation. Thus, the benzylic cation dominates. The $Br^-$ then attacks to form 1-bromo-1-phenylpropane. While the alkene pi-bond has been saturated and destroyed, the aromatic phenyl ring remains completely intact. A standard benzene ring contains exactly 6 delocalized $\pi$ -electrons.