Physical and Chemical Properties – Amines

00:01 Right. Physical and chemical properties. Let’s have a quick look here at the electronegativity of nitrogen and relate it to its reactivity. As we’ve seen before, where we have a species which is more electronegative than carbon, it polarises the carbon heteroatom bond.

00:18 In this case, on the Pauling scale, the electronegativity of carbon is 2,550 and the electronegativity of nitrogen is 3,066. What this results in is a dipole moment, as you can see here, where the bond is polarised. And a partial negative charge resides on the nitrogen and a partial positive charge resides on the carbon on the alkyl or aryl group.

00:44 Amines form intermolecular hydrogen bonds, not unlike alcohols and also carboxylic acids.

00:50 They can also form hydrogen bonds with water and as a result, amines with a linear chain shorter than three carbons are soluble in water. Again, the common odours that you will find which directly result from amines are fishlike odours, for example, things like C5 and C6 containing amines, methylamine and ethylamine, which smell similar to ammonia, in general. But, it is also important to aware there are a lot of long-chain mono and diamine compounds, compounds such as putrescine and cadaverine. And obviously, they correlate to the smells of decomposing bodies. Stereochemistry of nitrogen.

01:36 If you recall, when we looked at carbon, we had four electrons which could be hybridised or found in hybrid orbitals. Here, in case of the nitrogen stereochemistry, we could also have sp3 hybridisation. However, unlike in the case of the carbon, we have a lone pair which occupies one of those… which occupies a… one of those hybridised orbitals.

02:00 As a consequence, what we see here is a pyramidal structure, which is common to both ammonia and all of the amines that you see. The nitrogen is at the apex of the pyramid and the other groups are at each apex of a triangular base. The end is sp3 hybridised, like a sp3 hybridised carbon. But, one of the sp3 positions is occupied by the lone pair, which is shown at the top of that pyramid structure here. What’s also worthy of note is that theoretically, if we have different substituents attached to each of those posts, we would then have a nitrogen to which is attached four different things: a lone pair and then three substituents.

02:43 And the mistake that can be made sometimes is thinking that, therefore, it’s possible to achieve chirality or optical isomerism, as we observe in the case of a carbon with four different substituents on it. The reality in the case of amines and ammonia is that you actually get a flipping of that molecule. And so, you never actually see one particular enantiomeric form of a nitrogen appearing. It should be also be stressed that the bond angles between the substituents are 108 degrees.