Why can't you have 2,2-dinitrobenzene ?

Im answering a question on naming the two isomers of 1,3-dinitrobenzene.I got 1,2-dinitrobenzene and 1,4-dinitrobenzene.
It seems you can't have two nitro groups on one carbon atom..Why ?

1 Answer
anor277
Sep 8, 2016

A benzene ring is defined by 4 degrees of unsaturation, with which 2,2-substitution is incompatible.

Explanation:

An fully saturated alkane molecule has a formula of C_nH_(2n+2). Chemists often speak of degrees of unsaturation where each so-called degree of unsaturation entails a double bond or a ring junction. By way of example, saturated ethane has formula C_2H_6; ethylene, with an olefin bond has formula C_2H_4 and 1""^@ of unsaturation. Propane has formula C_3H_8; propylene and cyclopropane have formula C_3H_6 and thus 1""^@ of unsaturation with respect to propane.

We can represent the benzene ring, C_6H_6, as "1,3,5-cyclohexatriene", and it thus has 4""^@ of unsaturation. While any hydrogen on the ring can be replaced by a heteratom or a functional group, double substitution on the SAME carbon is rendered unviable by the aromatic stability of the unsaturated ring. The stability of substitution on alternate carbons is much greater, and this is certainly observed experimentally.

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