Bondline / Reactions

Acetal formation

A hemiacetal loses water (helped by the neighbouring oxygen) to give an oxonium ion, which a second alcohol traps. Acetals are stable to base, so they protect aldehydes and ketones.

Carbonyl addition · 4 steps

Conditions

Excess alcohol, acid catalyst, water removed (e.g. Dean–Stark). Reversed by aqueous acid.

Mechanism in 4 steps

Red curly arrows show where each pair of electrons moves.

OOHHOH2+OOH2++
Step 1The hemiacetal OH is protonated.
OOH2+O+HO+
Step 2The OMe lone pair pushes out water, giving an oxonium ion.
O+HOHO+OH2OMeOH++
Step 3A second methanol attacks the oxonium carbon.
HO+OH2OOOH3O+++
Step 4Loss of a proton gives the acetal and regenerates the acid.

About carbonyl addition reactions

The carbon of a C=O is δ+, so nucleophiles attack it and the π electrons move onto oxygen. Weak nucleophiles often need acid to activate the carbonyl first.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.

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