Matrix isolation study benzyne

Organic stractures can be determined accurately and quickly by spectroscopic methods. Mass spectrometry determines mass of a molecule and its atomic composition. NMR spectroscopy reveals the carbon skeleton of the molecule, whereas IR spectroscopy determines functional groups in the molecules. UV-visible spectroscopy tells us about the conjugation present in a molecule. Spectroscopic methods have also provided valuable evidence for the intermediacy of transient species. Most of the common spectroscopic techniques are not appropriate for examining reactive intermediates. The exceptions are visible and ultraviolet spectroscopy, whose inherent sensitivity allows them to be used to detect very low concentrations for example, particularly where combined with flash photolysis when high concentrations of the intermediate can be built up for UV detection, or by using matrix isolation techniques when species such as ortho-benzyne can be detected and their IR spectra obtained. Unfortunately, UV and visible spectroscopy do not provide the rich structural detail afforded by IR and especially H and NMR spectroscopy. Current mechanistic studies use mostly stable isotopes such as H, and 0. Their presence and position in a molecule can... 

Sodamide also reacts with bromoarenes by elimination of HBr (Figure 10.37) to give benzyne, a very reactive and unstable intermediate. Mechanistically, the elimination may be E2 or ElcB or somewhere in between. Although it is common, and quite conventional, to write the structure of benzyne with a triple bond, this is not a triple bond of the same type as we find in alkynes. The extra bond is formed by weak overlap of two sp orbitals, each with one electron in it (10.20). It is probably more correct to think of it as a resonance hybrid of the triply bonded form, diradicals, and charge-separated species. The IR spectrum of isolated benzyne in an inert gas matrix at 8 K has been studied although assignments have been a matter of discussion, the value of 1846 cm for the C-C triple bond stretch is at least approximately correct. This suggests that the triple bond is considerably weaker than that in a conventional alkyne where a frequency of 2200 cm would be typical. 

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