SolutionsAlcohol Addition
1. Mechanism of acetal formation
Write the mechanism of the formation of the diethyl acetal of acetone (2,2-diethoxypropane) in acid.
- Protonation of the carbonyl oxygen.
- Attack of ethanol and loss of a proton: hemiacetal.
- Protonation of the hemiacetal OH and loss of water: oxocarbenium ion, stabilized by the lone pair of the ethoxy oxygen.
- Attack of a second ethanol molecule and loss of a proton: acetal.
All the steps are reversible: the same mechanism, run backwards in excess water, is the hydrolysis of the acetal.
2. Cyclic acetals
Why does ethylene glycol form acetals more easily than two molecules of ethanol?
With ethylene glycol the second addition is intramolecular: the second OH is already in the molecule and close to the carbon. Forming the cyclic acetal (a 1,3-dioxolane) does not involve the loss of translational freedom of a second molecule, so its entropy is more favorable.