Question

How do chemists make imines?

Answer 1

Easily, if the amine is alkyl and the carbonyl is an aldehyde. Not so easily in other circumstances.

Explanation:

Imine formation is a condensation reaction, and the reaction proceeds with the elimination of water:

`RC(=O)H + H_2NR' rarr RC=NR' + H_2O`

If electron rich (alkyl) reagents are used, sometimes these reactions may be performed WITHOUT a solvent, and the water product may be removed with a Pasteur pipette. Such imines (formally aldimines) tend to be a little unstable, and should be used directly.

With ketones, the reaction tends to be a little bit more sluggish, and often the reaction is performed at reflux under Dean Stark conditions, so that the water is progresively removed:

`R^1C(=O)R^2 + H_2NR'_2 +Deltararr "R^1R^2C(=NR'_2)+H_2O`

Dean Stark conditions are a classic technique in which the reactants are heated at reflux, and the condensate is collected over a glass bridge in a trap. Because water is denser than say benzene or toluene, the typical solvents, the water produced in the reaction collects in the trap. Other methods of imine formation use dehydrating agents.