Imine formation: practice and mechanism
The amine adds to C=O and water is then lost: C=O becomes C=N–CH₃ (imine).
Example
The primary amine adds to the carbonyl group (hemiaminal), then water is eliminated under acid catalysis. An imine (Schiff base) with a C=N double bond forms.
Mechanism step by step
- 1
The amine attacks the carbonyl carbon with its lone pair.
- 2
After proton transfer the hemiaminal forms (carbon with OH and NHR).
- 3
Acid protonates the OH group, water leaves and the iminium ion loses a proton: imine (C=N).
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Background
The right pH
Imine formation is fastest at a slightly acidic pH (about 4–5). Too acidic and the amine is protonated and can no longer attack; too basic and the OH group of the hemiaminal is not protonated and does not leave as water.
Imine or enamine?
Primary amines (R–NH₂) give imines with a C=N double bond. Secondary amines have no H left on nitrogen after the attack, so a C=C bond to the neighbouring carbon forms instead: an enamine.
Where imines matter
With a reducing agent such as NaBH₃CN the imine is turned straight into an amine (reductive amination). In biochemistry enzymes often bind substrates as imines (Schiff bases), for example pyridoxal phosphate in transaminations.
Typical mistakes
- That is the hemiaminal, only the intermediate. Under acid catalysis it loses water and the imine with C=N forms.
Often confused with: Acetal formation (protecting group) · Grignard reaction
Frequently asked questions
Why only weakly acidic (pH ~4–5)?
Too acidic: the amine is protonated and no longer nucleophilic. Too basic: the OH is not protonated and does not leave.
What are imines used for?
Reduced with NaBH₃CN they give amines (reductive amination); in biochemistry enzymes bind substrates this way.
How does imine formation work?
In four steps: 1. The amine attacks the carbonyl carbon. 2. Proton transfer gives a hemiaminal (carbinolamine). 3. The OH group is protonated and leaves as water, giving an iminium ion. 4. Loss of a proton from nitrogen gives the imine. The reaction is reversible; removing water drives it to the imine.