Relative Aromaticity and Reactivity of Pyrrole, Furan and Thiophene

Aromaticity

These properties must be present in aromatic compounds

• Cyclic molecules must exist.
• A planar molecule is one that has been hybridized with two sp2 atoms.
• A complete conjugation occurs when the p-orbital of the atom in the ring overlaps with the p-orbital of the atom on either side.

Aromaticity of Pyrrole

• Cyclic molecules make it up.
• Hybridized planar molecules with Sp2.
• A double bond that is conjugated.

Huckel's rule -

• When these p orbitals overlap, they produce a π orbital containing six electrons.
• Because the π molecular orbital in pyrrole conforms to Huckel's rule, it is Aromatic.
• One electron resides in the p orbitals of carbon, and two electrons (lone pair) occupy the p orbitals of nitrogen.

Calculation

(4n + 2) = π electrons

4n +2 = 4

4n = 2

n = 2/4=½

Hence, it is not aromatic

(4n + 2) = π electrons

4n +2 = 6

4n = 4

n = 1

Thus, furan, pyrrole and thiophene are aromatic

Aromaticity of Furan

• Molecules like this are cyclic.
• A planar molecule hybridized with Sp2.
• Double bonds that are conjugated.

Huckel's rule

• An unhybridized p atomic orbital surrounds each carbon and oxygen atom, and one of these orbitals is perpendicular to the plane of the σ bonds.
• A sextet molecule containing six electrons is formed from the lateral overlap between these p orbitals.
• In the p orbitals of carbons, there is only one electron per orbital, while in the p orbitals of oxygen, there is one unshared pair of electrons (another lone pair).
• Since the molecular orbital resulting from the Furan reaction satisfies the Huckel's rule, Furan is Aromatic.

Aromaticity of Thiophene

• Cyclic molecules.
• A planar molecule hybridized with Sp2.
• A double bond that is conjugated.

Huckel’s rule

Because of the π molecular orbital resulting from the (4n + 2) = 3 electrons in thiophene, thiophene is Aromatic.

Relative Aromaticity

Aromaticity is listed in this order:

Thiophene > Pyrrole > Furan



Herero atoms are ordered by electronegativity

O > n > S

• Heteroatom electronegativity determines relative aromaticity.
• Atoms with a higher electronegativity hold their lone electron pair more tightly, reducing the ease of delocalization (aroma).
• Therefore, the most electronegative heteroatom in furan decreases aromaticity the most.
• Increasing the electronegative S in Thiophene will lead to the greatest increase in aromaticity.

Relative reactivity of pyrrole, furan and thiophene

Heterocycles with five members are excessive. Electrophilic substitution reactions are then conducted on carbon rings.

π Electron's delocalizing (Resonance):

Pyrrole



Furan



Thiophene

Relative Reactivity

Reactivity is in this order:

Pyrrole > Furan > Thiophene



Reactivity α 1/Stability

• Electron availability at the ring carbon or the release of electrons from heteroatoms determines the reactivity of these heterocycles towards electrophiles.

Pyrrole

• pyrrole has electron-rich ring carbons due to its trivalent nitrogen, which also has a greater mesomeric effect than inductive.
• In comparison to the other two, pyrrole is most reactive.

Furan

• Likewise, furane is also reactive (but not as much as pyrrole), due to the fact that O is more electronegative, which means there are fewer electrons on the carbon ring.

Thiophene

• Because S has a lower electronegativity than thiophene, it withdraws fewer electrons from carbon than furan.
• As a result of the lack of effective overlap among p-orbitals, a mesomeric effect (+M) is also smaller.

Reactivity is in the following order:

Pyrrole > Furan > Thiophene

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