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Wednesday, November 13, 2019

Nucleophilic Aromatic Substitution Reactions : (SN-Ar):

(1) SN-Ar-(Substitution of hydrogen of benzene):

(3) Substitution of unactivated hydrogen: Benzyne Intermediate Mechanism:

SN-Ar-Elimination-Addition:  Benzyne Intermediate Mechanism:

An aryl halide can undergo a nucleophilic substitutoin reaction in the presence of a very strongbase such as NH2 When chlorobenzene – that has the carbon to which chlorine is attached isotopically labeled with Cabon-14 –is treated with amide ion in liquid ammonia,  aniline is obtained as a product. Half of the product has the amino group attached to the isotopically labelled carbon (14) as expected, but the other half has the amino group attached to the carbon adjacent to the labelled carbon.

The mechanisms that accounts for the experimental observations involves formation of a benzene intermediate which has two equivalent carbon atoms to which amino group can be attached. Benzyne has an extra (Pi) bond between two adjacent carbon atoms of benzene and can be formed as

Step-(1): Strong base NH2- removes a proton from the position ortho to halogen:

Step-(2): Anion formed in step (1) eliminates the halide ion, thereby forming Benzyne:

The incoming nucleophile can attack either of the carbons of the “triple bond” of benzyne. Protonation of the resulting anion form the substitution product. The overall reaction is an elimination-addition reaction; benzyne is formed in an elimination reaction and immediately undergoes an addition reaction.

Substitution at the carbon (C-14) that was attached to the leaving group is called direct substitution product (DSP). Substitution at the adjacent labeled carbon (C-14) of is called cine substitution product (CSP).

Characteristic of Benzyne reaction:

(1) It is a SN-EA reaction  proceed via elimination –Addition Nucleophilic Substitution reaction.
(2) Aryl halide + strong base required
(3) Presence of beta hydrogen must be needed.
(4) NaNH2 and KNH2 can be considered to be essentially the same for our purposes
(5) For symmetrical intermediate which is attacked equally on either side. so that the roughly   50:50 ratio of products are obtained .

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