# How does a halogen react in an electrophilic substitution reaction

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?

• The double bond in D is also electron rich – Waylander Dec 30 '19 at 8:40

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$$_2$$.