Furan - Structure, Synthesis and Reactions

In this article we will learn about Furan. Here we will discuss structure, synthesis, its reactions and aromaticity of furan. 

Key words: Furan, Heterocyclic compounds, Aromatic compound, Huckel rule.

Introduction

Furan is a five member oxygen atom containing heterocyclic compound. It is aromatic compound with the Chemical formula C4H4O and Molecular weight 68.08 g/mol. Furan is colorless volatile liquid with boiling point 31.3 °C. Furan is used as a key starting material for the synthesis of many important compounds.

Table of Content

1. Structure of Furan
2. Synthesis of Furan
3. Reactions of Furan
4. Conclusion

1. Structure of Furan

Furan consists of five member planar ring with oxygen as heteroatom. Numbering of the position of atoms is given starting from oxygen atom. Furan molecule has two conjugated pi-bonds. Here oxygen atom has lone pair of electrons which also involved in pi-bond conjugation. Therefore total number of electrons in conjugation is 6 electrons. Hence furan qualify the Huckel rule 4n + 2 pi electrons (here n = 1). Thus furan is classified as Aromatic compound. 

Figure 1: Structure of Furan

The Huckel rule

A molecule is considered as aromatic if it qualify following criteria;

1. Cyclic molecule
2. Planar structure
3. Conjugated pi system (double bond)
4. 4n+2 number of pi electrons (n = 0, 1, 3 etc)

2. Synthesis of Furan

There are many methods available in the literature for 11 synthesis of furan. Some of the representative reactions are shown below;

2.1 Decarbonylation of furfural

Furfura2h is derived from biomass. In chemical industries furfural is used for the synthesis of furan. Here palladium catalyzed de-carbonylation of furfural at high temperature provides furan compound as product.

Figure 2: Decarbonylation of Furan

2.2 Oxidation of 1,3-butadiene 

Copper catalyzed oxidation of 1,3-butadiene provides furan compound. This method also used in chemical industries for the production of furan.

Figure 3: Copper catalyzed oxidation of 1,3-butadiene

2.3 Paal-Knorr Furan Synthesis

This method is used for the synthesis of substituted furans. In this reaction acid-catalyzed cyclization of 1,4-dicarbonyl compounds is achieved to form furan compounds. 

Figure 4: Paal-Knorr Furan Synthesis

2.4 Feist-Benary Synthesis

It is a chemical reaction of α-halogen ketones and β-dicarbonyl compounds under mild basic condition to produce substituted furan compounds. This reaction is an example of condensation reaction. It is a two steps reaction. The first step is similar to Knoevenagel condensation (Link) followed by nucleophilic substitution of halogen. The second step is dehydration to produce substituted furan compound.

Figure 5: Feist-Benary Synthesis
 

These are few important methods known for the synthesis of furan. There are many other related methods available in current literature which are being used by organic chemists for the synthesis of substituted furan derivatives. (Link)

3. Reactions of Furan

Since furan is aromatic compound it favors electrophilic aromatic substitution reactions (Link). In this context canonical structures of furan are shown below; 

Figure 6: Canonical / Resonance Structures of Furan

Due to availability of oxygen atom lone pair in the conjugation system, furan is more reactive than benzene in electrophilic aromatic substitution reactions. 
Typical reactions of furan compounds are shown below;

Nitration: Furan reacts with nitrating reagent such as H2SO4 / HNO3 to provide 2-nitrofuran as major product.

Halogention: Furan reacts with bromine at lower temperature to produce 2-bromo furan as major product.

Sulphonation :  Furan provides sulphonated product by treatment with regents like Sulfur trioxide pyridine complex.

Acylation : Acylating reagent like acetic anhydride reacts with furan to give 2-acetyl furan. This reaction requires Lewis acid catalyst like H3PO4 or BF3. 

Reaction with Diazonium salt : Furan reacts with benzene diazonium salt to provide 2-phenyl furan. 

Alkylation: Alkene compounds in presence of phosphoric acid (H3PO4) reacts with furan to give 2-alkyl furan product.

Diels Alder Reaction : Furan acts as diene in Diels Alder reaction to provide addition product. We have discussed Diels Alder reaction and other pericyclic reactions in previous article. (Link)

Hydrolysis : Furan undergo hydrolysis reaction in presence of aqueous acid to provide 1,4-carbonyl compound.

Reaction with Ammonia : In chemical industries, reaction of furan with ammonia is used for the production of pyrrole compounds.

Mercuration : Furan reacts with mercuric chloride (HgCl2) to form mercuration product. This intermediate product further used for halogenation of furan.

Figure 7: Reactions of Furan

4. Conclusion

• In this article we have discussed structure of furan and its aromatic nature. 
• Huckel rule for identification of aromatic compounds.
• We have learned some important methods which are available for the synthesis of furan. This includes Decarbonylation of furfural, Copper catalyzed oxidation of 1,3-butadiene, Paal-Knorr furan synthesis and Feist-Benary Synthesis.
• Finally we have seen some common reactions of furan compounds. This includes reactions such as Nitration, Halogention, Suphonation, Acylation, Reactions with Diazonium salts, Alkylation, Diels Alder reaction, Hydrolysis, Reaction with ammonia, and Mercuration reaction.

 That's all for this topic, see you in the next blog. Thank you.