Two very important bases in organic chemistry are pyrrole and pyridine. These are heterocyclic bases.
Firstly, what is a heterocycle? A heterocycle is a cyclic compound with a heteratom in the ring. A heteroatom in this context is any atom other than carbon.
Pyrrole is a 5 membered ring containing a nitrogen atom:
Pyrrole has some signs of aromaticity, as we would expect from Huckel’s rule; in pyrrole, there are (4n + 2) pi electrons. There are two electrons contributed from the nitrogen atom, and two each from each double bond.
However, pyrrole is not as aromatic as benzen; the delocalised electron cloud is polarized towards nitrogen.
Pyrrole reluctantly protonates at the carbon next to the nitrogen:
|It is important to note that pyrrole does NOT protonate at the nitrogen. This is because the nitrogen bears a slight positive charge due to the incorporation of it’s lone pair in the aromatic system. Pyrrole is barely basic at all; it has a pKa of around 0.|
Another very important heterocyclic base is pyridine:
This molecule is also aromatic. The nitrogen (like all the carbons) is sp2 hybridised, and contributes 1 electron to the pi system. Pyridine therefore has 6 pi electrons.
Pyridine has a pKa of approximately 5.2, which makes it much weaker than tertiary amines. This is due to the increasing s character of the nitrogen. In pyridine it is sp2 hybridised. As the nitrogen orbital has more s character, the electrons in the hybrid orbital are held more tightly, and therefore they are less likely to be able to forma bond with a proton, hence the compound is less basic.
Hence the general trend for nitrogenous bases is sp3 > sp2 > sp.
cf. increasing acidity of alkanes < alkenes < alkynes.
Unlike pyrrole, pyridine protonates on the nitrogen atom, as the nitrogen lone pair is NOT delocalised into the pi system.