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Home , Leaving group

AROMATIC NUCLEOPHILIC SUBSTITUTION (SNAr)

Although less common, nucleophilic substitution reactions are possible for aromatic compounds. Neither SN2 nor SN1 are viable routes for this reaction, for different reasons. SN2 requires a BACKSIDE attack, which is impossible, because of the planar ring structure. SN1 requires the formation of a phenyl , which is extremely unstable and of very high energy. Instead, there are two other general mechanisms: 1. The Addition – Elimination Mechanism. A. It is a two-step process, the first step (rate-determining!!) being the addition of the to the carbon atom bearing the leaving group. The intermediate so formed bears resemblance (structurally, not electronically!!) to the

s-complex in SEAr reactions. It is known as the Meisenheimer Complex. B. There is a negative charge, which is best stabilized by the presence of one or more strongly electron-withdrawing groups, such as nitro groups. In fact, the addition-elimination mechanism operates ONLY with comp ounds that have one or more electron-withdrawing groups. C. The leaving group is usually (but not exclusively) a halogen: F, Cl, Br or I. Its electronegativity further facilitates the addition of the nucleophile. Thus the order of reactivity is: > > > . In practice, only fluorides and chlorides are useful as substrates. 2. The Elimination – Addition Mechanism. A. If the ring does not have any electron-withdrawing substituents, then the addition-elimination mechanism is not possible. Instead, if the nucleophile is a very strong base as well, it can effect a substitution via elimination – ¯ ¯ addition mechanism. Typical are amide ( :NH2) or (under extreme conditions) ( :OH). Halides are typical leaving groups. B. The mechanism involves a rate-determining elimination of hydrogen halide (HX) by the action of the strong base. It leads to the formation of an unstable intermediate, with a triple bond (formally!!) known as benzyne. C. The formal triple bond is very labile and is easily attacked by the nucleophile/base in the subsequent addition step. The attack can (and does) occur at either carbon atom of the triple bond. This leads to mixtures of isomers. D. Since the breaking of the carbon – halogen bond is in the rate-determining step, the reactivity of the halides follows the usual order depending on their leaving group ability: > > > (NOTE: The order is opposite to that in the Addition – Elimination mechanism!!).