Furan and Thiophene

Structure of Furan And Thiophene

The structure of thiophene and furan are closely similar to that discussed in detail for pyrrole above, except that the NH is replaced by S and O, respectively. 

A consequence is that the heteroatom in each has one lone pair as part of the aromatic sextet, as in pyrrole, but also has a second lone pair that is not involved, and is located in an sp2 hybrid orbital in the plane of the ring. Mesomeric forms exactly similar to those for pyrrole can be written for each, but the higher electronegativity of both sulfur and oxygen means that the polarised forms, with positive charges on the higher on the heteroatoms, make a smaller contribution. 

The larger bonding radius of sulfur is one of the influences making thiophene more stable (more aromatic) than pyrrole or furan - the bonding angles are larger and angle strain is somewhat relieved, but in addition, a contribution to the stabilisation involving sulfur d-orbital participation may be significant.

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