Bondline / Reactions

Cannizzaro reaction

With concentrated alkali, an aldehyde that has no α-hydrogen is both oxidised and reduced: hydroxide adds to one molecule, which then passes a hydride ion to a second.

Carbonyl addition · 3 steps

Conditions

Concentrated NaOH or KOH, for aldehydes with no α-hydrogen (methanal, benzaldehyde).

Mechanism in 2 steps

Red curly arrows show where each pair of electrons moves.

HO−OO−HOHO++
Step 1Hydroxide adds to the carbonyl carbon of one benzaldehyde.
O−HOHOOOHO−PhCHO++
Step 2The C=O reforms and pushes a hydride ion onto a second benzaldehyde.
OOHO−O−OHO++
Step 3The alkoxide takes the acidic proton: benzoate and benzyl alcohol.

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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