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SolutionsCarbonyl Addition

1. Products

  1. Benzaldehyde + CH3NH2
  2. Acetone + hydroxylamine (NH2OH)
  3. Cyclohexanone + pyrrolidine (secondary amine)
  4. Acetaldehyde + excess hydrazine
  1. N-Methylimine of benzaldehyde, C6H5CH=NCH3.
  2. Acetone oxime, (CH3)2C=NOH.
  3. An enamine: the secondary amine cannot form a neutral imine, and the iminium ion loses a proton from the α carbon.
  4. The hydrazone, CH3CH=NNH2.

2. The optimum pH

Imine formation is fastest at pH ≈ 4-5. Why is it slower in strongly acidic and in basic media?

In strongly acidic media almost all the amine is protonated (RNH3+) and is not nucleophilic. In basic media there is not enough acid to protonate the OH of the carbinolamine and turn it into a good leaving group (water). At intermediate pH there is enough free amine and enough acid catalysis.

3. Hydrolysis

What is obtained when the N-ethylimine of butanone is treated with water and dilute acid? Why?

Butanone and ethylamine (as the ethylammonium salt). Imine formation is reversible: with excess water and acid catalysis the equilibrium shifts towards the carbonyl compound and the amine, by the same mechanism run backwards.

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