Comparison of gas‐phase acidities of some carbon acids with their rates of hydron exchange in methanolic methoxide

Abstract

AbstractHydron exchange reaction rates, kexch M−1s−1, using methanolic sodium methoxide are compared with gas‐phase acidities, ΔG kcal/mol, for four 9‐YPhenylfluorenes‐9‐iH, seven YC6H4CiH(CF3)2, seven YC6H4CiHClCF3, and C6F5iH. Fourteen of the fluorinated benzylic compounds and pentafluorobenzene result in near unity experimental hydrogen isotope effects that suggest substantial amounts of internal return associated with the exchange process. Although the reactions of 9‐phenylfluorene have experimental isotope effects that appear to be normal in value, they do not obey the Swain–Schaad relationship. This suggests that they occur with small amounts of internal return. The entropies of activation, ΔS‡, are +12 to +14 eu, for the benzylic compounds and different significantly from those for the 9‐YPhenylfluorenes, ΔS‡ of −8 to −12 eu. The ΔS‡ ∼ 1 eu for the reactions of pentafluorobenzene falls between the other compounds. Density functional calculations using B3LYP/6‐31+G(d,p) are reported for the reactions of CH3O−(HOCH3)3 with C6F5H, C6H5CH(CF3)2, C6H5CHClCF3, and 9‐phenylfluorene. Copyright © 2006 John Wiley & Sons, Ltd.