Comparison of the nucleophilicities of alcohols and alkoxides

The kinetics of the reactions of benzhydrylium ions with some alcohols and alkoxides dissolved in the corresponding alcohols were photometrically investigated. Using the correlation equation log k (20 degrees C) = s(E + N), the N and s parameters of methoxide, ethoxide, n-propoxide, and isopropoxide in alcohol-acetonitrile (91:9 nu/nu) were determined. The cosolvent acetonitrile has only a little influence on the rate constants of the reactions of alcohols and alkoxides. The order of N values (OH- << MeO- < EtO- < n-PrO- < i-PrO-) shows that alkoxides differ only moderately in reactivity but are considerably more nucleophilic than hydroxide. As a consequence, the nucleophilic reactivity of a 0.5 mmol/L aqueous hydroxide solution increases by a factor of 13 when 10% (nu/nu) methanol is added. In 1-10 mmol/L alkoxide Solutions in alcohols, weak electrophiles react considerably faster with alkoxides than with the cot-responding alcohols. With increasing electrophilicity, the preference for alkoxides decreases, and electrophiles of -3 < E < 3 react with alkoxides (1-10 mmol/L) and alcohols with comparable rates. Stronger electrophiles will react with alcohols exclusively when alkoxides are present in concentrations <= 10 mmol/L.