Question

(S)-2-Butanol slowly racemizes on standing in dilute sulfuric acid. Explain.

Short answer

Mechanism for the reaction

Step-by-step solution

Secondary alcohol

Since 2-butanol is a secondary alcohol, substitution can occur by either an ${\mathbf{SN}}_1$or${\mathbf{SN}}_2$route, depending on reaction conditions. Two factors favor a${\mathbf{SN}}_1$mechanism in this case. The reaction is run under solvolysis (solvent as a nucleophile) conditions in a polar, protic solvent. Dilute acid converts a poor leaving group${\mathbf{OH}}^{\mathbf{-}}$ into a good leaving groupdata-custom-editor="chemistry" ${\mathbf{OH}}_{\mathbf{2}}$which dissociates easily.

Explanation

Initially, there is protonation of the hydroxyl oxygen, which is then followed by a loss of water to form a planar carbocation. Finally, an attack of water from either side of the planar carbocation yields a racemic product.

Mechanism for the reaction