Work, Energy, and Power
Functional Groups in Organic Chemistry
Conjugated π Systems and Aromaticity
Reactions of Aromatic Compounds
|Let us take a moment to review one the of the big discussions within the topic of electrophilic aromatic substitution, which everyone remembers from their first year course and which definitely may be on the MCAT. This is the discussion concerning the effect of the original substituent upon the regioselectivity and rate of a second addition. Both the regioselectivity and the rate of electrophilic aromatic substitution depend on the substituent(s) already present on the ring. Concerning its effect on regioselectivity, a substituent is either an ORTHO, PARA director or it is a META director. Concerning its effect on the rate of reaction, a substituent is either ACTIVATING or DEACTIVATING.|
Whether an existing substituent is an ORTHO, PARA director or a META director depends on the effect it has on the stability of the cyclohexadienyl cation. With ORTHO, PARA directors, the cyclohexadienyl cation precursors to ORTHO and PARA products are more stable. When a new group is added ORTHO or PARA in relation to a substituent, the resonant cyclohexadienyl cation intermediate places positive charge on the carbon to which the original substituent is bonded. If the original substituent can donate negative charge, either by resonance or inductively, it will stabilize the carbocation, and the mechanism will be favored having the new group coming in ORTHO or PARA. However, if a group is electron withdrawing, the favored mechanism will avoid placing positive charge on the carbon to which it is bonded and META addition will be favored.
ORTHO, PARA directors tend to be ACTIVATING and META directors tend to be DEACTIVATING. The primary exceptions are the halogens, which are SLIGHTLY DEACTIVATING but are ORTHO, PARA directing. This is because they are electron withdrawing by induction but electron donating by resonance.