Chemistry, Properties and Synthesis of Thiophene

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 2nd and 5th 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 2nd 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

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