Q11-70E - Organic Chemistry (Mcmurry)

Question

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

Step-by-Step Solution

Answer

Mechanism for the reaction

Step 1: 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 group data-custom-editor="chemistry" ${\mathbf{OH}}_{\mathbf{2}}$ which dissociates easily.

Step 2: 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