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.
Bibliographical noteFunding Information:
Support from the National Science Foundation and the Minnesota Supercomputer Institute for Advanced Computational Research are gratefully acknowledged.
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- Bond dissociation energy
- Gas-phase energetics
- Hydrogen–deuterium exchange
- α-Styryl radical