Chemistry of furan

Session Objectives

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

• Discuss the chemistry, reactivity, properties and method
of synthesis of furan

Chemistry of furan

• Occurs in secondary plant metabolites of terpenoids.

• Furan-2-carboxylic acid was first obtained by Scheele in
1780 from the dry distillation of mucic acid

• Furan was obtained much later in 1870 by heating barium
fureate with soda lime

• The group derived from furan is designated as furyl. Fully
saturated- tetrahydrofuran

• Two dihydrofurans, namely 2,3-dihydrofuran and  2,5-dihydrofuran  are possible.

Physical properties of furan

• Colorless liquid with boiling point 31.5 ⁰C

• Possess a chloroform like odor

• Soluble in most organic solvents but slightly miscible
with water

Molecular properties of furan

• Aromatic properties of a molecule is dependent on the
availability of the lone pair of electrons for resonance

• More electronegativity of heteroatom will have a greater
hold on the lone pair which will therefore be localized.

Resonance
structures of furan

• Furan is less aromatic than pyrrole or thiophene

• Aromatic sextet in furan is made up of one pz orbital
contributed by each of four carbon atoms and lone pair provided by heteroatom

• Similar to pyrrole, there are six electrons in five
orbitals 

• From the resonance structures, 2^nd position is
most favorable for electrophilic attack

Synthetic methods of furan

1)      from
carbohydrates:

• Most important source of furan is furfural which is
available by acid hydrolysis of polysaccharides present in oat, husks and corn
cobs

• These are degraded to pentoses which are subsequently
converted to furfural on further treatment with acid

• Furan is obtained by passing vapor of aldehyde over nickel
catalyst

• Also by decarboxylation of 2-furoic acid which is obtained
by air oxidation of furfural in presence of cuprous and silver salts 

2) Ring closure method- Fiest-Benary Synthesis:

• Synthesis involves an aldol condensation of α-haloketone or α-haloaldehyde with a β-ketoester in the presence of sodium
hydroxide

• Resulting furan contains an ester substituent at the β-position

• The ester anion attacks carbonyl group of α-chloroketone followed by formation of an
intermediate and cyclization takes place by intramolecular displacement of
chloride ion and finally loss of water.

Chemical properties of furan

• furan ring behaves chemically as
typical diene ether and resonance stabilized

1) Reaction
with acids: 

• furan is readily hydrolysed by acids, but acidification of
furan to yield 1,4-diketo derivatives is not preferred

• Mild conditions should be employed for isolation of
succinaldehyde

• Furans containing electron-withdrawing groups are more
stable to acids

2) Electrophilic
substitution:

• Analogous to pyrrole, undergoes electrophilic substitution
and to be carried out in controlled conditions

• 2^nd position is more favorable than 3^rd
position of furan

• Increasing the proportion of chlorine yields
tetrachlorofuran

• Yield of 2-chlorofuran may be increased by using less
quantity of chlorine

• Bromination (Br2/dioxane)
at -5 ⁰C gives 2-bromofuran in good yield

• Nitration:
achieved with mild nitrating agents such as acetyl nitrate (mixture of acetic
anhydride and nitric acid) at -5 to -30 ⁰C

• It is an advantage in special cases to use a mixture of
acetyl nitrate and pyridine

• Friedel-Crafts reaction: similar to pyrrole and
yields 2-acetylfuran

Summary

• Furan is less aromatic than pyrrole or thiophene.

• From the resonance structures, 2^nd position is
most favorable for electrophilic attack.

• Furan ring behaves chemically as typical diene ether and
resonance stabilized.

• 2^nd position is more favorable than 3^rd
position of furan for electrophilic substitution.