Chemistry
of Thiophene

Session Objectives

By the end of this
session, students will be able to:

• Discuss the chemistry, reactivity of Thiophene

• Discuss the methods of synthesis and properties of
Thiophene

Chemistry of Thiophene

• Discovered by Victor Meyer in 1882 during his ingenious
experiments on investigation of aromatic compounds

• Occurs in coal tar and large number of thiophene
derivatives occurs in plants and animal metabolism

• Responds to indophenine test- treatment of 1ml of dilute
solution thiophene in benzene with a few mg of isatin and 1ml of conc. Sulfuric
acid gives intense blue color

• Reduced thiophenes are given trivial names

Physical properties of Thiophene:

• Colorless liquid with boiling point of 80 ⁰C and
freezing point -38.3 ⁰C

• Miscible with water but soluble in most organic solvents

Molecular properties of Thiophene:

• Resonance hybrids of thiophene

• Sulfur atom is less electronegative than oxygen and
nitrogen

• Thiophene is greater aromatic and planar molecule with sp2
hybridized carbon atoms

• Electrophilic attack takes place preferably at 2^nd
and 5^th positions

• Pyrrole is weak base while furan is extremely weak and
thiophene in contrast, seems to be devoid of basic properties

Synthetic methods of Thiophene:

1) From sodium
succinate: classical method by heating a mixture of sodium succinate and
phosphorus trisulfide

2) Ring
closure methods- from unsaturated compounds:

• Involves the use of methyl thioglycolate and its
condensation with dimethyl fumarate in the presence of a base

• From unsaturated compounds:

Chemical properties of Thiophene:

• Contains a sulfur atom which belongs to second row
elements of periodic table

• Can expand its valence shell by using empty d-orbitals in
hybridization

• One of the reason for extreme stability of thiophene-
resembles to benzene in physical and chemical properties

1) Reaction
with acids: very stable to action of acids, very strong acids may bring
about oligomerization

• Orthophosphoric acid under milder conditions has been
found to give trimer

2) Electrophilic
substitution: more reactive than furan and pyrrole

• Preferential attack takes place at 2^nd position

A) Halogenation:
with chlorine yields a mixture of substituted products

• Bromination occurs rapidly even in dark at -30 ⁰C
and gives exclusive product

• Iodination results in mono and diiodothiophenes

B) Sulfonation:

• With 95% H₂SO₄ gives
thiophene-2-sulfonic acid readily

• But chlorosulfonation gives very lower yield of product

C) Friedel-Crafts
reaction:

• Acetylation can be carried out in wide variety of
conditions

• With acetyl chloride in the presence of stannic chloride,
thiophene yields 2-acetylthiophene

• Also reacts with phthalic anhydride in the presence of
aluminium chloride to yield 2,2’-thienoylbenzoic acid

D) Vilsemeier
formylation:

• Formylation proceeds rapidly on thiophene with DMF and
POCl₃

Summary

• Sulfur atom is less electronegative than oxygen and
nitrogen

• Thiophene is greater aromatic and planar molecule with sp2
hybridized carbon atoms

• Electrophilic attack takes place preferably at 2nd and 5th
positions