Bondline / Reactions

Imine formation

A primary amine adds to an aldehyde or ketone, then water is lost to give C=N. Fastest at about pH 5: enough acid to protonate the OH, but not so much that the amine is all protonated.

Carbonyl addition · 5 steps

Conditions

RNH₂ with a trace of acid, removing water. With secondary amines the same route gives an enamine.

Mechanism in 5 steps

Red curly arrows show where each pair of electrons moves.

OH2NNH+HO−+
Step 1The amine nitrogen attacks the carbonyl carbon.
NH+HO−NHOHHOH2++
Step 2A proton moves from nitrogen to oxygen (in practice via the solvent), giving a neutral carbinolamine.
NHOHHOH2+NHOH2+H₃O⁺+
Step 3The OH group is protonated so it can leave as water.
NHOH2+N+HH2O+
Step 4The nitrogen lone pair pushes out water, forming an iminium ion.
N+HH2ONH3O+H₂O++
Step 5Water removes the N–H proton to give the neutral imine.

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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