The term “furan” refers to a class of aromatic organic compounds featuring a five-member ring. A furan ring consists of four carbon atoms plus one oxygen atom. Possessing a planar structure allows the ring, with its six “pi-electrons,” to generate a circular “ring current” above and below that plane. Two pairs of double-bonded carbon atoms donate four of those electrons, while the remaining two electrons come from a lone, unshared pair located on the oxygen atom. This fulfills the requirement of Huckel’s Law that organic compounds must have 4n+2 closed-loop, conjugated pi-electrons, n being a small positive integer, in order to be aromatic.
Structurally the simplest compound to possess a furan ring is, itself, called furan — C4H4O. For purposes of identification, the ring is numbered starting with oxygen, counterclockwise. If a methyl group replaces the hydrogen atom on ring atom two, the compound is called 2-methylfuran. When the methyl group is located on ring atom three instead, the compound is called 3-methylfuran. A separate compound does not result if a methyl is placed on ring atom number four, since it would be the same thing as 2-methylfuran, as is observed by simply flipping the structure 180 degrees.
There are a few common methods used to synthesize a furan ring structure. The Paal-Knorr synthesis converts a 1,4-di-carbonyl structure such as a di-ketone into a furan ring by employing an appropriate acid reactant, such as phosphorous pentoxide. Side branches on the resultant furan ring may be introduced prior to cyclization in some instances. Another, older method called the Feist-Benary synthesis reacts an α-halocarbonyl compound with a β-dicarbonyl in the presence of a base, the most popular being pyridine. Another more modern development is the “one pot” procedure developed in Germany, which uses sodium iodide in place of stronger halo acids to produce 3-halofurans that can then be modified to produce important derivatives.
Furans are important starting materials in chemical synthesis. For example, saturating the two carbon-carbon double bonds by catalytic hydrogenation produces molecules with single bonds only. These saturated “addition products” are cyclic ethers called tetrahydrofurans. The simplest of the tetrahydrofurans is, itself, called tetrahydrofuran (THF), and is used as a solvent substituting for the once-used diethyl ether in many organometallic reactions. Other important synthetics are derived through a different mechanism called “electrophilic substitution,” in which one or more hydrogen atoms of a furan ring is or are replaced with one or more atoms or molecular fragments.