Benzyne elimination-addition mechanism

A MECHANISM FOR THE REACTION ] The Benzyne Elimination-Addition Mechanism 962... 

The Elimination-Addition Mechanism of Nucleophilic Aromatic Substitution Benzyne... 

THE ELIMINATION-ADDITION MECHANISM OF NUCLEOPHILIC AROMATIC SUBSTITUTION BENZYNE... 

Nucleophilic aromatic substitution can also occur by an elimination-addition mechanism This pathway is followed when the nucleophile is an exceptionally strong base such as amide ion m the form of sodium amide (NaNH2) or potassium amide (KNH2) Benzyne and related arynes are intermediates m nucleophilic aromatic substitutions that pro ceed by the elimination-addition mechanism... 

On the other hand labeling studies have shown that the base promoted hydro lysis of chlorobenzene (second entry m Table 24 3) proceeds by the elimination-addition mechanism and involves benzyne as an intermediate... 

The elimination-addition mechanism involves a highly unstable intermediate, which is referred to as dehydrobenzene or benzyne. ... 

Although nucleophilic aromatic substitution by the elimination-addition mechanism is most commonly seen with very strong amide bases, it also occurs with bases such as hydroxide ion at high temperatures. A " C-labeling study revealed that hydrolysis of chlorobenzene proceeds by way of a benzyne intennediate. 

An elimination-addition mechanism has been suggested for the substitution of amino for chloro in chlorobenzene.95 The neutral and symmetrical intermediate is called benzyne although it can not contain an ordinary linear bond system like that of acetylene. It is not known whether it is a distorted acetylene, a triplet (diradical), or a zwitterion. 

Roberts elucidated a second mechanism for nuclophilic aromatic substitution. Treatment of iodo-, bromo-, or chlorobenzene with potassium amide yields aniline. In 1953 Roberts observed that when chlorobenzene- 1-14C is the substrate, approximately 50 percent of the 14C in the product is found in the 1- and approximately 50 percent in the 2-position. The overall substitution then must go by an elimination-addition mechanism in which the highly strained intermediate, benzyne, is formed as shown in Equation 7.93.184... 

The elimination-addition mechanism for the formation of aniline proceeds via a benzyne intermediate. A benzyne is a benzene molecule that contains a theoretical triple bond. Thus, the following structure represents benzyne ... 

These two products can be explained by an elimination-addition mechanism, called the benzyne mechanism because of the unusual intermediate. Sodium amide (or sodium hydroxide in the Dow process) reacts as a base, abstracting a proton. The product is a carbanion with a negative charge and a nonbonding pair of electrons localized in the sp2 orbital that once formed the C—H bond. 

Notice the symmetry in this mechanism. Benzyne is formed from an ortho carbanion and it gives an ortho carbanion when it reacts with nucleophiles. The whole mechanism from bromobenzene to aniline involves an elimination to give benzyne followed by an addition of the nucleophile to the triple bond of benzyne. In many ways, this mechanism is the reverse of the normal addition-elimination mechanism for nucleophilic aromatic substitution and it is sometimes called the elimination-addition mechanism, the elimination step... 

Elimination-addition mechanism for nucleophilic aromatic substitution. Benzyne... 

We have seen that electron-withdrawing groups activate aryl halides toward nucleophilic substitution. In the absence of such activation, substitution can be made to take place, by use of very strong bases, for example. But when this is done, substitution does not take place by the mechanism we have just discussed (the so-called bimolecular mechanism), but by an entirely different mechanism the benzyne (or elimination-addition) mechanism. Let us f rst see what this mechanism is, and then examine some of the evidence for it. 

This phenol synthesis is different from the nucleophilic aromatic substitutions discussed in the previous section because it takes place by an elimination addition mechanism rather than an addition/elimination. Strong base first causes the elimination of HX from halobenzene in an E2 reaction, yielding a highly reactive benzyne intermediate, and a nucleophile then adds to benzyne in a second step to give the product. The two steps are similar to those in other nucleophilic aromatic substitutions, but their order is reversed elimination before addition for the benzyne reaction rather than addition before elimination for the usual reaction. 

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