1. The document discusses whether lone pairs should be considered part of aromatic π systems. In pyridine the lone pair is not included, in pyrrole it is included, and in furan one lone pair is included but the other is not. 2. The hybridization of the atom with the lone pair must be considered. Only electrons that can shift via resonance and contribute p-orbitals to the π system are included. 3. In pyrrole, all six π electrons shift together via resonance, but in pyridine the lone pair does not shift with the double bonds. In furan, one lone pair shifts with the double bonds while the other does not.

1. General consideration in organic
chemistry
Dr. Taj Khan
Dept. of Pharmaceutical Chemistry, Oriental college of pharmacy
Sanpada, New Mumbai.

2. 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.

3. All three contains lone pair of electron on hetero atom
In 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.

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

5. 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.

6. 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.

7. Give reasons

8. • 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?

9. 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.

10. 3) In pyridine nucleophilic substitution takes place
preferentially at the 2 & 4 positions
The intermediate anion is stabilized by the electronegative N.

11. 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