Ketone from a nitrile and Grignard: practice and mechanism
The Grignard adds once to the C≡N group. The imine anion is not attacked a second time; acidic workup hydrolyses the imine to the ketone.
Example
The Grignard reagent adds once to the C≡N group, giving an imine anion. A second attack does not happen (the negative charge protects it). Acidic work-up hydrolyses the imine to the ketone.
Mechanism step by step
- 1
The Grignard carbon attacks the carbon of the C≡N group.
- 2
An imine anion (C=N⁻ MgBr⁺) forms. It is negatively charged and is not attacked again.
- 3
On acidic workup the imine is hydrolysed to the ketone and NH₄⁺ is released.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- After the first addition the nitrogen anion is negatively charged and is not attacked again. No amine with two new methyl groups forms; hydrolysis gives a ketone.
Often confused with: Reduction with lithium aluminium hydride · Acidic hydrolysis of amides and nitriles
Frequently asked questions
Why a ketone here but a tertiary alcohol with esters?
With esters the ketone forms in the reaction mixture and reacts further. With nitriles it only forms on workup, when no Grignard is left.
How many carbons does the ketone have?
The nitrile carbons plus those of the Grignard group. Propanenitrile and CH₃MgBr give butan-2-one.