THE ANALOGS OF BENZENE WITH TRIPLE BONDS - MESOMERISM OR ISOMERISM

The analogue of benzene with one CC triple bond (benzyne) can be prepa red by the photochemical decomposition of phthalic anhydride. Prelimin ary experimental results indicate that the analogues with two triple C C bonds (C6H2) and three triple bonds (C6) might be formed from the co rresponding di - and trianhydrides of benzene. Quantum chemical consid erations of a number of cyclic and aliphatic systems with conjugated t riple bonds suggest that structures that differ only in the electron d istribution in the molecule do not correspond to limiting mesomeric fo rms but rather to minima on the respective potential energy surface. C onsequently, these forms should be related by the symbol for equilibri um reactions (half-arrow-right-over-half-arrow-left) and not by a doub le-ended arrow (<-->).