Bondline / Reactions

Hemiacetal formation (acid)

Protonating the carbonyl makes it more electrophilic so a weak nucleophile, an alcohol, can add. Loss of H⁺ gives the hemiacetal.

Carbonyl addition · 3 steps

Conditions

Alcohol with a trace of acid. With excess alcohol and removal of water, the reaction continues to the acetal.

Mechanism in 3 steps

Red curly arrows show where each pair of electrons moves.

OHOH2+OH+HO++
Step 1The carbonyl oxygen is protonated, making the carbon much more electrophilic.
OH+HOO+HOHH2OMeOH++
Step 2Methanol attacks the activated carbonyl carbon.
O+HOHH2OOOHH3O+++
Step 3Water removes the proton from the new O⁺, giving the hemiacetal and regenerating the acid catalyst.

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.

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