comparative study to understand easily

1. Relative Aromaticity and
Reactivity of Pyrrole,
Furan & Thiophene
Presented by
SOWMIYA PERINBARAJ
Assistant Professor
Dept. of Pharmaceutical Chemistry
SVCP

2. Aromaticity
The aromatic compounds must possess following properties
 Molecule must be cyclic.
 There must be a complete conjugation i.e each atom of ring must have
p-orbital so that they overlap with p-orbital of other atom on either side.
 Molecule must be planar i.e each atom of the ring is sp2 hybridised.
 The compound must follow Huckel rule (4n+2)π electrons.

3. Aromaticity of Pyrrole
 It is cyclic molecule.
 Consists of conjugated double bonds.
 Sp2 hybridisation- planar molecule.

4.  Huckel’s Rule:
❖ Each carbon atom and nitrogen atom possesses an
unhybridized p atomic orbital and these are
perpendicular to the plane of σ bonds.
❖ The p orbitals on carbons contain one electron each
and p orbital on nitrogen contains two electrons
(lone pair).
❖ The lateral overlap of these p orbital produces a π
molecular orbital containing six electrons, this
arrangement is referred to as aromatic sextet.
❖ Thus pyrrole is Aromatic because the resulting π
molecular orbital satisfies the Huckel’s rule.

5. CALCULATION
(4n + 2) = π electrons
4n +2 = 4
4n = 2
n = 2/4=1/2
Not aromatic
(4n + 2) = π electrons
4n +2 = 6
4n = 4
n = 1
Pyrrole
Furan Aromatic
Thiophene

6. Aromaticity of Furan
 It is cyclic molecule.
 Consists of conjugated double bonds.
 Sp2 hybridisation- planar molecule.

7.  Huckel’s Rule:
❖ Each carbon atom and oxygen atom possesses an
unhybridized p atomic orbital and these are
perpendicular to the plane of σ bonds.
❖ The p orbitals on carbons contain one electron
each and p orbitals on oxygen contains an unshared
pair of electrons (other lone pair).
❖ The lateral overlap of these p orbital produces a π
molecular orbital containing six electrons, this
arrangement is referred to as aromatic sextet.
❖ Thus Furan is Aromatic because the resulting π
molecular orbital satisfies the Huckel’s rule.

8. Aromaticity of Thiophene
 It is cyclic molecule.
 Consists of conjugated double bonds.
 Sp2 hybridisation- planar molecule.
 Huckel’s Rule:
❖ Thiophene is Aromatic because the
resulting π molecular orbital satisfies the
Huckel’s rule [(4n + 2) = π electrons].

9. Relative Aromaticity
 The order of aromaticity is as follows:
Thiophene > Pyrrole > Furan
Resonance energy
(kcal/mol)
 The order of electronegativity of Herero atoms
O > N > S
>
29 22 16

10.  Relative aromaticity depends on order of electronegativity of
heteroatoms.
 The more electronegativity is the atom the more tightly holds
its lone pair of electrons more reduce the ease of
delocalisation (aromaticity).
 Hence, most electronegative O hetero atom in furan leads to most
decreased aromaticity.
 Least electronegative S in Thiophene leads to most increased
aromaticity.

11. RELATIVE REACTIVITY OF PYRROLE,
FURAN & THIOPHENE
 Five membered heterocycles are π excessive. Thus undergoes electrophilic substitution
reaction on the ring carbons.
Delocalisation of π electrons (Resonance):
PYRROLE

12. FURAN
THIOPHENE

13. Relative Reactivity
 The order of Reactivity is as follows:
Pyrrole > Furan > Thiophene
Reactivity α 1/Stability
> >

14.  The reactivity of these heterocycles towards electrophile depends on the electron
availability on the ring carbon or mesomerically electron release from
heteroatom.
PYRROLE
 In pyrrole, due to trivalent nitrogen, the ring carbons are electron rich and also
N shows greater mesomeric effect than the inductive effect.
 So pyrrole is most reactive in comparison to other two.
FURAN
 Furan is also reactive (but less than Pyrrole) because O is more electronegative
and so less electron density on ring carbon.

15. Thiophene
 Thiophene is less reactive than even furan because S has less
electronegativity so withdraw less electrons from carbon.
 A mesomeric effect (+M) is also smaller due to non effective
overlapping of p-orbitals.
 The order of Reactivity is as follows:
Pyrrole > Furan > Thiophene

16. ✓ A Time Passes Fastly
Aromaticity: Thiophene > Pyrrole > Furan
✓ Rosy Perform Fabulous in Test
Reactivity: Pyrrole > Furan > Thiophene