Investigation of the Substituent Effects of the Azide Functional Group Using the Gas-Phase Acidities of 3-and 4-Azidophenols

The electronic effect of the azide functional group on an aromatic system has been investigated using Hammett-Taft parameters obtained from the effect of azide substitution on the gas-phase acidity of phenol. Gas-phase acidities of 3- and 4azidophenol have been measured using mass spectrometry and the kinetic method and found to be 340.8 +/- 2.2 and 340.3 +/- 2.0 kcal/mol, respectively. The relative electronic effects of the azide substituent on an aromatic system have been measured using Hammett-Taft parameters. The o-F and o-R values are determined to be 0.38 and 0.02, respectively, consistent with predictions based on electronic structure calculations. The values of o-F and o-R demonstrate that azide acts as an inductively withdrawing group but has negligible resonance contribution on the phenol. In contrast, acidity values calculated for azide-substituted benzoic acids give values of o-F = 0.69 and o-R = -0.39, indicating that the azide is a strong pi donor, comparable to that of a hydroxyl group. The difference is explained as being the result of "chimeric", or, alternatively, "chameleonic" electronic behavior of the azide, similar to that observed previously for the N-oxide moiety, which can be more or less resonance donating in response to the environment.