Furans are organic compounds with a five-membered ring consisting of four carbon atoms and one oxygen atom. The ring’s planar structure generates a circular “ring current” due to its six “pi electrons,” satisfying Huckel’s law. Furan synthesis methods include the Paal-Knorr and Feist-Benary syntheses, and furans are important starting materials for chemical synthesis, with tetrahydrofurans being a major derivative.
The term “furan” refers to a class of aromatic organic compounds characterized by a five-membered ring. A furan ring consists of four carbon atoms plus one oxygen atom. Having 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 satisfies the requirement of Huckel’s law that organic compounds must have 4n+2 conjugated pi electrons in a closed loop, n being a small positive integer, to be aromatic.
Structurally the simplest compound to possess a furan ring is, in turn, called furan – C4H4O. For identification purposes, the ring is numbered starting with oxygen, counterclockwise. If a methyl group replaces the hydrogen atom on ring two atom, the compound is called 2-methylfuran. However, when the methyl group is found on ring atom three, the compound is called 3-methylfuran. You don’t get a separate compound if a methyl is placed on ring atom number four, as that would be the same thing as 2-methylfuran, as seen 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-dicarbonyl structure such as a diketone into a furan ring using an appropriate acidic reagent, such as phosphorus pentoxide. Side branches on the resulting furan ring can be introduced prior to cyclization in some cases. 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 halogens to produce 3-halofurans which can then be modified to produce important derivatives.
Furans are important starting materials in chemical synthesis. For example, saturation of the two carbon-carbon double bonds by catalytic hydrogenation produces molecules with only single bonds. These saturated “addition products” are cyclic ethers called tetrahydrofurans. The simplest of the tetrahydrofurans is, in turn, called tetrahydrofuran (THF), and is used as a solvent to replace the ethyl ether once used in many organometallic reactions. Other major synthetics are derived through a different mechanism called “electrophilic substitution,” in which one or more hydrogen atoms of a furan ring are or are replaced with one or more atoms or molecular fragments.
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