Chemistry BENZYNES AND ARENIUM IONS AND THEIR STABILITY

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BENZYNES AND ARENIUM IONS AND THEIR STABILITY

Benzyne :

It is a reactive intermediate in some nucleophilic aromatic substitutions. It is a benzene with two hydrogen atoms removed. It is usually drawn with a highly strained triple bond in the six membered ring. Benzyne intermediate has been observed spectroscopically and trapped.

Benzyne is very different from what can be expected from simple concepts of chemical bonding.

All six carbon atoms are `sp^2` hybridized. Three normal `pi` bonds are formed by `p` orbitals of each carbon. These `pi` bonds are completely delocalized in the benzene ring. However, `sp^2` hybrid orbitals of two carbon atoms overlap in sideways manner to form a special `pi` bond. This special `pi` bond is very weak because the `sp^2` hybrid orbitals were directed slightly away and the extent of overlapping is very poor. The presence of six completely delocalized `pi` electrons makes benzyne aromatic. On the other hand, presence of a very weak `pi` bond makes it highly unstable (it cannot be isolated and acts as intermediate only). See fig.

It is a reactive intermediate in some nucleophilic aromatic substitutions. It is a benzene with two hydrogen atoms removed. It is usually drawn with a highly strained triple bond in the six membered ring. Benzyne intermediate has been observed spectroscopically and trapped.

Benzyne is very different from what can be expected from simple concepts of chemical bonding.

All six carbon atoms are `sp^2` hybridized. Three normal `pi` bonds are formed by `p` orbitals of each carbon. These `pi` bonds are completely delocalized in the benzene ring. However, `sp^2` hybrid orbitals of two carbon atoms overlap in sideways manner to form a special `pi` bond. This special `pi` bond is very weak because the `sp^2` hybrid orbitals were directed slightly away and the extent of overlapping is very poor. The presence of six completely delocalized `pi` electrons makes benzyne aromatic. On the other hand, presence of a very weak `pi` bond makes it highly unstable (it cannot be isolated and acts as intermediate only). See fig.

Formation of Benzyne :

(i) By dehydrohalogenation of halobenzene : See fig.1.

(ii) By dehalogenation of o-dihalobenzene : See fig.2.

(i) By dehydrohalogenation of halobenzene : See fig.1.

(ii) By dehalogenation of o-dihalobenzene : See fig.2.

Behaviour of Benzyne :

lnspite of high `pi` electron density, benzyne does not react with electrophile because it will generate a cation having `sp` hybrid atom in six membered ring (not generated due to very large strain). See fig.1.

In spite of high `pi` electron density, benzyne can react with nucleophile because it will generate `sp^2` hybrid carbanion that is stable. See fig.2.

In absence of nucleophile, benzyne can undergo dimerization although it will generate anti aromatic ring. See fig.3.

lnspite of high `pi` electron density, benzyne does not react with electrophile because it will generate a cation having `sp` hybrid atom in six membered ring (not generated due to very large strain). See fig.1.

In spite of high `pi` electron density, benzyne can react with nucleophile because it will generate `sp^2` hybrid carbanion that is stable. See fig.2.

In absence of nucleophile, benzyne can undergo dimerization although it will generate anti aromatic ring. See fig.3.

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