Furan - Structure, Synthesis and Reactions

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

Key words: Furan, heterocyclic compound, aromatic compound, Huckel rule.

Introduction

Furan is a five-member oxygen-containing heterocyclic compound. It is an aromatic compound with the chemical formula C₄H₄O and molecular weight 68.08 g/mol. Furan is a colorless volatile liquid with a boiling point of 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 a five-member planar ring with oxygen as a heteroatom. Numbering of the position of atoms is given starting from the oxygen atom. The furan molecule has two conjugated pi-bonds. Here the oxygen atom has a lone pair of electrons, which is also involved in pi-bond conjugation. Therefore, the total number of electrons in conjugation is 6 electrons. Hence, furan qualifies for the Huckel rule of 4n + 2 pi electrons (here n = 1). Thus, furan is classified as an aromatic compound. 

Figure 1: Structure of Furan

The furan molecule has sp² hybridized oxygen and sp² hybridized carbon atoms. One lone pair of electrons (p orbital) from oxygen and p orbitals of all four carbon atoms participate in the aromatic pi system. 

The Huckel rule

A molecule is considered aromatic if it qualifies for the 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 the synthesis of furan. Some of the representative reactions are shown below.

2.1 Decarbonylation of furfural

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

Figure 2: Decarbonylation of Furan

2.2 Oxidation of 1,3-butadiene 

Copper-catalyzed oxidation of 1,3-butadiene provides a furan compound. This method is 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 conditions to produce substituted furan compounds. This reaction is an example of a condensation reaction. It is a two-step reaction. The first step is similar to Knoevenagel condensation (Link) followed by nucleophilic substitution of halogen. The second step is dehydration to produce a substituted furan compound.

Figure 5: Feist-Benary Synthesis
 

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

3. Reactions of Furan

Since furan is an 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 the availability of the 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 a nitrating reagent such as H₂SO₄/HNO₃ to provide 2-nitrofuran as a major product.

Halogenation: Furan reacts with bromine at lower temperatures to produce 2-bromofuran as the major product.

Sulphonation: Furan provides a sulphonated product by treatment with reagents like the sulfur trioxide pyridine complex.

Acylation: An acylating reagent like acetic anhydride reacts with furan to give 2-acetyl furan. This reaction requires a 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 the presence of phosphoric acid (H₃PO₄) react with furan to give a 2-alkyl furan product.

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

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

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

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

Figure 7: Reactions of Furan

4. Conclusion

• In this article we have discussed the structure of furan and its aromatic nature. 
• Huckel's rule for identification of aromatic compounds.
• We have learned some important methods that 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, halogenation, sulphonation, acylation, reactions with diazonium salts, alkylation, the Diels-Alder reaction, hydrolysis, reaction with ammonia, and mercuration reaction.

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