Why is pyridine very basic in aqueous solution? and pyrrole only slightly?

Question

Why is pyridine very basic in aqueous solution? and pyrrole only slightly?

Answer 1

In pyridine, the lone pair on the nitrogen is delocalized, hence basic.  Pyrrole is different because the lone pair isn't taken into the system so the nitrogen doesn't make a very good proton acceptor.

Look at the structures if you're confused.

Answer 2

Pyridine has 2 sigma bonds to each of the neighbouring carbon atoms and one pi bond which is delocallised in the ring giving it aromatic stability. The remaining sp electrons point out of the ring and are not involved in bonding. This pair of electrons act as Lewis donors to a proton, the resulting nitrogen cation can share the positive charge with the aromatic system thereby making it very basic. Pyrrole has 2 sigma bonds to the neighbouring carbons and one sigma bond to the proton, the remaining p electrons are involved in the aromatic system and if the were to act as Lewis donors would require the aromatic system to be broken which would lead to a higher energy state and so it is not very basic.

Answer 3

There is much confusion in here.

In pyridine the lone pair of the nitrogen is NOT localised in the aromatic ring. The nitrogen is sp2 therefore the lone is the plane of the ring and cannot achieve the lateral overlapping to be localised. And as the lone pair is not localised it is more basic (more available to perform nucleophilic attacks).

In pyrrole, the delocalisation exits because the lone pair is conjugated to the pi bond (conjugation N-sigma-pi). Therefore this lone pair is less basic (available to perform nucleophilic attack).

Pyridine is more basic than pyrrole and not only in water.

Answer 4

Pyrrole is in a 4N+2 aromatic system and is very stable. The addition of a proton to the system will remove the aromaticity from the molecule. Aromatic = stable. Stable is good

Pyridine is non-aromatc but can be made so with the addition of a proton. Hence why it is very basic.