Styrene energetics and characterization of its conjugate base: An example of isotopic labeling gone awry

The equilibrium acidity of styrene was measured (ΔH°_acid(PhCH[dbnd]CH₂) = 390.6 ± 0.5 kcal mol⁻¹) and its deprotonation site was revised from the ortho position on the aromatic ring to the α-hydrogen atom based upon deuterium-labeling studies and extensive computations. Somewhat surprisingly, the nature of the anionic base plays a critical role in properly determining the ionization site and avoiding misleading results due to extraordinary hydrogen–deuterium exchange. Bracketing the electron affinity of α-styryl radical (PhC[rad][dbnd]CH₂, 23.1 ± 3.4 kcal mol⁻¹) enabled the α-C[sbnd]H bond dissociation energy (100.1 ± 3.4 kcal mol⁻¹) of styrene and the effect of a phenyl substituent at an sp²-hybridized carbon to be determined. These results were compared to B3LYP, M06-2X, G3 and G4 computations.