Mish Mashamine

by Kiley Lynch

One thing about how amines are taught in organic chemistry 2: it’s generally a total mish-mash.

There’s aromatic chemistry, alkene chemistry, substitution reactions, and reactions of carboxylic acid derivatives – and that’s not all. 

It’s hard to find a common thread.

So maybe I’ll just lay out some general hints.  If this post is a little bit all over the map, well, so is the course material in this chapter. Sorry.

1. Diazonium salts: Diazonium salts are formed from aromatic amines (“anilines”) when they’re treated with nitrous acid and H2SO4. The mechanism is a long one and often tested. This is a good test for you at this stage. Can you try drawing the mechanism?

If you’re stuck, draw bonds formed, bonds broken for the product. Start by figuring out where the electron rich and electron poor parts of the molecule are. Negative goes to positive.

Remember: a mechanism is the story of how that product formed. Every step in the mechanism should involve one (or more) of the events in your bonds formed/bonds broken table.
For the full mechanism, you can see it here

Diazonium salts are useful intermediates for synthesis. Look at all the things that can be done with them!

2. The Hofmann, Wolff and Curtius rearrangments often give students trouble. But they each have the same key step. In each case we have a good leaving group, and there’s a rearrangement that involves migrating a carbon atom over one atom to form a new bond, while kicking off the good leaving group (e.g. Br or N2). Believe it or not, you’ve seen something like this before – think waaaay back to Org 1, and themechanism for the oxidation step of hydroboration. It’s the exact same key step. Move a bond over, displace a good leaving group.

 

3. Forming amides with DCC: N,N-dicyclohexylcarbodiimide (DCC) is a useful “dehydrating” reagent for taking amines and carboxylic acids, and obtaining amides. If you count the missing atoms here in formation of an amide from a carboxylic acid, you’ll see that water is missing. DCC takes these atoms and forms a urea.

 

4. The Cope Elimination: Possibly too obscure for your purposes, the Cope elimination is a way of forming an alkene you haven’t learned before. Treating an amine with H2O2 results in a species called an N-oxide, and when heated, this can remove a proton from the carbon adjacent to the nitrogen. So here, we have a concerted reaction where we break C-H, break C-N and form a new C-C (pi) bond.  The important thing here is that the hydrogen and nitrogen are on the same side of the (future) alkene, making this a “syn” elimination. 

Hope this helps you wrap your head around amines.

Tomorrow: let’s wrap up the week! 

Thanks for reading! James