Relative Aromaticity & Reactivity of Pyrrole, Furan & Thiophene

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
Thearomaticcompoundsmustpossessfollowingproperties
Moleculemustbecyclic.
Theremustbeacompleteconjugationi.eeachatomofringmusthave
p-orbitalsothattheyoverlapwithp-orbitalofotheratomoneitherside.
Moleculemustbeplanari.eeachatomoftheringissp
2
hybridised.
ThecompoundmustfollowHuckelrule(4n+2)πelectrons.

3. Aromaticity of Pyrrole
Itiscyclicmolecule.
Consistsofconjugateddoublebonds.
Sp
2
hybridisation-planarmolecule.

4. Huckel’sRule:
❖Eachcarbonatomandnitrogenatompossessesan
unhybridizedpatomicorbitalandtheseare
perpendiculartotheplaneofσbonds.
❖Theporbitalsoncarbonscontainoneelectroneach
andporbitalonnitrogencontainstwoelectrons
(lonepair).
❖Thelateraloverlapoftheseporbitalproducesaπ
molecularorbitalcontainingsixelectrons,this
arrangementisreferredtoasaromaticsextet.
❖ThuspyrroleisAromaticbecausetheresultingπ
molecularorbitalsatisfiestheHuckel’srule.

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

6. Aromaticity of Furan
Itiscyclicmolecule.
Consistsofconjugateddoublebonds.
Sp
2
hybridisation-planarmolecule.

7. Huckel’sRule:
❖Eachcarbonatomandoxygenatompossessesan
unhybridizedpatomicorbitalandtheseare
perpendiculartotheplaneofσbonds.
❖Theporbitalsoncarbonscontainoneelectron
eachandporbitalsonoxygencontainsanunshared
pairofelectrons(otherlonepair).
❖Thelateraloverlapoftheseporbitalproducesaπ
molecularorbitalcontainingsixelectrons,this
arrangementisreferredtoasaromaticsextet.
❖ThusFuranisAromaticbecausetheresultingπ
molecularorbitalsatisfiestheHuckel’srule.

8. Aromaticity of Thiophene
Itiscyclicmolecule.
Consistsofconjugateddoublebonds.
Sp
2
hybridisation-planarmolecule.
Huckel’sRule:
❖ThiopheneisAromaticbecausethe
resultingπmolecularorbitalsatisfiesthe
Huckel’srule[(4n+2)=πelectrons].

9. Relative Aromaticity
Theorderofaromaticityisasfollows:
Thiophene>Pyrrole>Furan
Resonanceenergy
(kcal/mol)
TheorderofelectronegativityofHereroatoms
O>N>S
>
29 22 16

10. Relativearomaticitydependsonorderofelectronegativityof
heteroatoms.
Themoreelectronegativityistheatom themoretightlyholds
itslonepairofelectrons morereducetheeaseof
delocalisation(aromaticity).
Hence,mostelectronegativeOheteroatominfuranleadstomost
decreasedaromaticity.
LeastelectronegativeSinThiopheneleadstomostincreased
aromaticity.

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

12. FURAN
THIOPHENE

13. Relative Reactivity
TheorderofReactivityisasfollows:
Pyrrole > Furan > Thiophene
Reactivity α1/Stability
> >

14. Thereactivityoftheseheterocyclestowardselectrophiledependsontheelectron
availabilityontheringcarbonormesomericallyelectronreleasefrom
heteroatom.
PYRROLE
Inpyrrole,duetotrivalentnitrogen,theringcarbonsareelectronrichandalso
Nshowsgreatermesomericeffectthantheinductiveeffect.
Sopyrroleismostreactiveincomparisontoothertwo.
FURAN
Furanisalsoreactive(butlessthanPyrrole)becauseOismoreelectronegative
andsolesselectrondensityonringcarbon.

15. Thiophene
ThiopheneislessreactivethanevenfuranbecauseShasless
electronegativitysowithdrawlesselectronsfromcarbon.
Amesomericeffect(+M)isalsosmallerduetononeffective
overlappingofp-orbitals.
TheorderofReactivityisasfollows:
Pyrrole > Furan > Thiophene

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