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From what I understand, electrophiles are electron deficient and therefore would be attracted to the electron-rich benzene ring. So would the correct answer be A?

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  • $\begingroup$ The double bond in D is also electron rich $\endgroup$ – Waylander Dec 30 '19 at 8:40
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A is correct because the question specifically says an electrophilic substitution reaction (D will also react, but it's an electrophilic addition reaction). However, you also need a Lewis acid catalyst like FeCl$_3$. This is because, while the delocalised system of electrons in benzene is indeed electron rich, the fact that it's aromatic (there is a system of pi electrons delocalised throughout the ring) makes it very stable. The mechanism of the bromination of benzene is shown below (image from Wikipedia as I don't have ChemDraw on this computer). The mechanism is the same for Cl$_2$.

Mechanism of halogenation of benzene

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  • $\begingroup$ The OP did not ask about electrophilic aromatic substitution. $\endgroup$ – user55119 Dec 30 '19 at 14:56
  • $\begingroup$ No, but they asked which molecule reacts via an electrophilic substitution mechanism. In this case it happens to be an electrophilic aromatic substitution, and I thought providing the mecanism would help to explain how this happens. $\endgroup$ – atbm Dec 31 '19 at 7:20

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