I Substituent Effects in Electrophilic Aromatic Substitution

I Substituents affect the reactivity of the aromatic ring. Substituents make the ring more reactive than benzene or less reactive than benzene in reaction with electrophiles (E + ), depending on how they interact with the carbocation intermediate (stabilizing it or destabilizing it relative to H). Substituents affect the orientation of the incoming electrophile. The electrophile attacks the ortho/para positions or the meta position depending on how the substituent stabilizes or destabilizes the intermediate.

Reactivity of Aromatic Rings n Substituents can affect the orientation of the reaction (o-, m-, or p-) n Substituents also affect the reactivity (rate of reaction). n The substituent ALREADY ON the aromatic ring determines the position and rate of substitution of the INCOMING electrophile. n We use the reactivity of BENZENE (no substituent) as a reference compound.

Electrophilic Aromatic Substitution (Overall Reaction) Although they resemble alkenes, aromatic compounds do not undergo addition reactions. Rather, they undergo substitution reactions in order to preserve the stable aromatic ring.

Electrophilic Aromatic Substitution (mechanism) -H + The electrophile is attacked by the electrons of the aromatic ring. A new sigma bond is formed in the resonance stabilized arenium ion intermediate. A proton is lost and the aromaticity is restored.

General Substituent (Electrophile attacks para to the substituent, G) Depending on what G is, it can either stabilize or destabilize the arenium ion. Notice the three resonance contributors to the arenium ion intermediate.

Stabilization of the Carbocation Intermediate by a Substituent (faster rate) The resonance contributor where the + charge is on the ring carbon bonded to the –CH3 group is especially stable. It resembles a 3° carbocation; the other contributors resemble a 2° carbocation. CH 3

Destabilization of the Carbocation Intermediate by a Substituent (slower rate)

Substituent Classification n Activating Substituents (EDG) NH2 OH OCH3 NHCOCH3 CH3 Phenyl H n Deactivating Substituents (EWG) H F, Cl, Br, I HC=O etc... less more Reactivity See your text for more groups

Equation for the Bromination Reaction Br 2 red G = CH 3 Toluene The o-, m-, p- product %’s will be determined by IR