Acetal formation template

From the hemiacetal of ethanal and methanol: protonation of OH, loss of water, attack by methanol and loss of H⁺ give 1,1-dimethoxyethane. Acetals protect aldehydes and ketones.

Mechanism step by step

The hemiacetal loses water to an oxocarbenium ion, which a second alcohol traps

1. Protonation – The hemiacetal OH takes H⁺ (hemiacetal from the first MeOH)
2. Loss of water – The OMe lone pair pushes water out: oxocarbenium ion
3. Methanol adds – The second alcohol attacks the cation
4. Deprotonation – The acetal; H₃O⁺ is regenerated

Key points

• Every step is reversible: removing water (Dean–Stark, molecular sieves) drives acetal formation; aqueous acid reverses it.
• Acetals survive bases, nucleophiles, Grignard reagents and LiAlH₄, so they protect C=O.
• Ethane-1,2-diol gives a cyclic acetal (1,3-dioxolane), favoured by entropy.
• Open-chain hemiacetals are unstable; cyclic ones (sugars) are stable.

Drawn with 62 atoms, 13 arrows, 8 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 11 Textbook
• Smith, March's Advanced Organic Chemistry, 8th ed., Wiley 2020, ch. 16 Textbook