The aluminum chloride catalyst in Friedel-Crafts alkylation facilitates carbocation formation from an alkyl halide, preparing the species as an alkyl electrophile for electrophilic aromatic substitution. Electrophilic aromatic substitution begins with the addition of the electrophile into the aromatic pi?system of the ring. A conjugated, carbocation intermediate is formed, a resonance combination of three forms, concentrating positive charge at three locations, the two ortho positions and one para. After this addition, a proton departs, completing the overall substitution with aromaticity restored.
Consider the case where electrophilic aromatic substitution is attempted on an aromatic ring that already contains an original alkyl substituent. If electrophilic aromatic substitution is attempted upon rings that already contain substituents, the location of the next substitution depends on the characteristics of the original substituent. Whether the new substitution occurs ortho, para or meta to the original substituent depends on whether the original substituent either stabilizes or destabilizes a concentration of positive charge upon its carbon at the carbocation intermediate stage. If the substituent already present is electron donating, it will stabilize the carbocation by donating negative charge. New substitutions will occur ortho or para to such electron donating substituents already present on the ring. Electron withdrawing substituents destabilize a carbocation, so the new substitution will most likely be meta. Alkyl groups are electron-donating by induction. Because carbon is only moderately electronegative, alkyl groups have negative charge to share with the positive carbon in the orthopara resonance form. An alkyl group, already present in the ring, is ring activating and ortho-para directing.