Bondline / Reactions

Aldol reaction

Base removes an α-hydrogen to give an enolate, which attacks a second carbonyl molecule. Protonation gives a β-hydroxy carbonyl; heating can eliminate water to an enone.

Enolates · 3 steps

Conditions

Dilute NaOH, room temperature (heat for the aldol condensation).

Mechanism in 3 steps

Red curly arrows show where each pair of electrons moves.

HO−HOO−O++
Step 1Hydroxide removes an α-proton; the C–H electrons form C=C and push the C=O electrons onto oxygen (enolate).
O−OOO−HOHCH₃CHO++
Step 2The enolate carbon attacks the carbonyl carbon of a second ethanal molecule as the C=O reforms.
OO−HOHOOHHO−H₂O++
Step 3The alkoxide takes a proton from water, regenerating hydroxide.

About enolates reactions

A C–H next to a carbonyl is acidic, because the anion formed (an enolate) is stabilised by resonance. Enolates are carbon nucleophiles that make new C–C bonds.

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