General consideration in organic chemistry

Dr. Taj Khan Dept. of Pharmaceutical Chemistry, Oriental college of pharmacy Sanpada, New Mumbai.

 Aromaticity & Lone Pairs Difficult thing: Whether to include lone pairs as part of a cyclic π system or not?If a lone pair is included, then the number of π electrons increases by 2.

All three contains lone pair of electron on hetero atomIn Pyridine lone pair in pyridine is not included in the π system, In pyrrole lone pair in pyridine is included in the π system.In furan, one of the lone pairs on O is included but the other is not included in π system.

To understand this we need to consider hybridizations for the atom with the lone pair

Electrons can be shifted via resonance are the ones that contribute p orbitals to the same π system

Number of curved arrows = number of pairs of electrons that π system.

In pyrrole, 6 electrons can be shifted via resonance, involving all 5 atoms of the ring, so there is a single, cyclic π system containing six π electrons.

In pyridine, the double bonds are shifted via resonance without involving the lone pair, so the lone pair is not part of the same π system as the double bonds.

In furan, just one of the lone pairs on oxygen is involved in resonance with the double bonds, so the second lone pair is not part of that cyclic π system.

Give reasons

• Resonance puts partial negative charge at those positions.• It also puts negative charge at the 3 and 4 positions (see the far-right

structure). But the proximity of the positively charged, electronegative nitrogen atom to the 2 and 4 positions makes those resonance structures stronger contributors to the total electron density in the ring, so the 2 and 5 positions are a bit more negative than 3 and 4.

• You can get substitution at the 3 and 4 positions, but not as easily.

Why do electrophilic substitutions generally take place at the 2 and 5 positions in pyrrole?

2) Why is furan more reactive than thiophene towards electrophiles? E++

O more e -ve attract e strongly delocalization is prominent so aromatic stabilization is stronger in thiophene which makes it less reactive than furan.

O S

3) In pyridine nucleophilic substitution takes place preferentially at the 2 & 4 positions

The intermediate anion is stabilized by the electronegative N.

4) Protonation of N is easy in pyridine than pyrrole OR Protonation of N is difficult in pyrrole than pyridine

N2 of pyridine is sp2-hybridized and possesses a lone electron pair. This electron pair is located in an sp2 orbital that is parallel to the ring plane. (Not in same plane of π ring system)This lone electron pair of pyridine does not participate in the aromatic π electron system. So, pyridine can easily be protonated, to give pyridinium cation.In pyrrole lone pair in pyridine is included in the π system. So protonation is bit difficult as compared to pyridine