3- -2-thiophene Open-Chain Compound

Hydroxy derivatives. 2-Hydroxy derivatives usually exist as the oxo tautomers, unless the hydroxy tautomer is appreciably stabilized by electron-withdrawing or chelating substituents. The tendency for enolic hydroxy compounds to revert to the oxo form can be understood by reference to simple aliphatic ketones where the keto-enol equilibrium constants are of the order of 108. In the five-membered heterocycles under consideration, this tendency will be in opposition to the loss of aromatic resonance energy that increases in the order furan << thiophene < pyrrole. For the 2-hydroxy compounds 217 some extra stabilization of the oxo tautomers 218 and 219 is derived from the resonance energy of the X-C=0 group, which by analogy with open-chain compounds should increase in the sequence thiolester, ester << amide. 

Certain substituted pyrroles, furans, and thiophenes can be prepared from the parent heterocycles by substitution (see Sec. 31.4) most, however, are prepared from open-chain compounds by ring closure. For example ... 

Catalytic hydrogenation converts pyrrole and furan into the corresponding saturated heterocycles, pyrrolidine and tetrahydrofuran. Since thiophene poisons most catalysts, tetrahydrothiophene is synthesized instead from open-chain compounds. 

A heterocycle is a compound with a ring that has more than one kind of atom. Nitrogen, oxygen, and sulfur are often found along with carbon in heterocyclic rings. Saturated heterocyclic amines, ethers, and sulfides usually have the same chemistry as their open-chain analogs, but unsaturated heterocycies such as pyrrole, furan, and thiophene are aromatic. All three are unusually stable, and all three undergo aromatic substitution on reaction with electrophiles. 

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