Ketones and aldehydes have cousins. If you replace the oxygen of those groups with nitrogen, you get imines. Imines are similar to aldehydes and ketones in a lot of ways.
Imines are made when you treat aldehydes or ketones with an amine. The byproduct of this reaction is water.
This reaction is usually helped by the addition of a little bit of acid. Remember: acid speeds up addition to carbonyls, and it also speeds up elimination reactions by making groups like OH into better leaving groups (like OH2). And if you look at the mechanism of this reaction, it usually goes:
- protonation (of carbonyl oxygen)
- addition (of N to carbon)
- proton transfer (from N to O)
- elimination (of water)
- deprotonation (of nitrogen)
Five steps…. P A P E D.
However it’s possible to have too much of a good thing. If you add too much acid, the reaction shuts down, and no imine forms at all! Why might that be? (Answer at bottom).
The type of amine you use is important: NH3 or primary amines will give you imines. However, when you use secondary amines, there’s a little problem with the last step.
It still goes P A P E D … but there aren’t any hydrogens on the nitrogen to remove!
However there is still an acidic proton that can bet taken away… but it’s on the carbon adjacent to what used to be the carbonyl carbon. So the base breaks the C-H bond, and we form a new C-C pi bond.
These are called enamines. Like a “spork” is part spoon, part fork, these molecules are part alkene, part amine. As you’ll see later, their chemistry is NOT that similar to that of aldehydes and ketones. They’re more similar to the isomers of ketones and aldehydes we call “enols”. More about them in subsequent chapters.
Tomorrow: let’s wrap up the week.
Thanks for reading! James
P.S. For full mechanisms and walkthroughs, see the Reaction Guide entries for formation of imines and formation of enamines.
P.P.S. Answer to the question: amines are bases! So amines plus strong acid…. make ammonium salts (the conjugate acid). Since there are no lone pairs on nitrogen, it can’t add to the carbonyl carbon!