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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