WHAT IS BENZYNE [1]:
o-Benzyne is a highly reactive aryne intermedia
te. While most triple bonds consist of two pi-pi interactions, this is not the case with benzyne. I
nstead, the benzyne triple bond consists of one pi-pi bond, and one bond composed of interaction between two sp2 hybridised carbons. The latter bond creates enormous strain on the bond, leading to a very unstable intermediate. As a result of this reactiity, benzyne cannot be isolated and has been found to have a life-time of only 20ns!
The HOMO-LUMO gap about this triple bond is very small, allowing benzyne to function as both an electrophile and a nucleophile.
HOW WAS BENZYNE DISCOVERED [2]:
In the mid 19th century, reactions involving aromatic substitutions were yielding unexpected results. It was soon discovered by Wittig et al. the following:
In such a reaction, only two products were possible. This lead to the assumption of a zwitterionic intermediate; which was close, but not quite right...
Roberts then performed the classic '14C Labelling experiment'. In this experiment, one carbon, attached to the halogen, was a 14-C isotope. Upon reaction with sodium amide in ammonia, it was seen that only two products were possible, with the 14-carbon either at the substituted position, or ortho to it. It was reasoned that, if the zwitterionic intermediate were true, these two products would be produced in different ratios. This was not the case, however:
This lead to the proposed structure for benzyne:
References:
[1] Kit, H. (2005). The Chemistry of Benzyne: New Benzyne Precursor - Benzotrisoxadisilole. Hong Kong Baptist University. Available from: http://libproject.hkbu.edu.hk/trsimage/hp/02010305.pdf
[2] Tadross, P. (2006). Benzyne: The Adventures of a Reactive Intermediate. Stoltz Group Literature Presentation.
