8 Heterocyclic Chemistry cyclohexatriene were to exist in are slightly shorter than the C-C bonds (0.134 nm versus 0.139-0.140 a local ed form and was a planar nm).This ie ha aus nitrogen is more electronegative than carbon,and this fact also affects the nture of theystem.In pyridine the elo density is no longer uniformly distributed around the ring and is con- the C-C bond length is 0.134 centrated at the N atom. 018 Another difference between the molecules is that whereas in benzene each carbon is bonded to a hydrogen atom,in pyridine the nitrogen pos sesses a lone (unshared)pair of electrons.This lone pair occupies an sp orbital and is orientated in the same plane as the ring;moreover,it is bond separating the two chlorine available to capture a proton so that pyridine is a base In five-membered,formally derived from by the replacement of a CH=CH unit by a heteroatom,aromaticity is achieved by sharing four p-electrons,one from each ring carbon,with two elec- trons from the heteroaton n Thus in where the heteroatom isN. all the ring atoms are sp2 hybridized,and one sp2 orbital on each is bonded to hydrogen.To complete the six n-electron system the non- hybridized p-orbital of N contributes two electrons(Box 1.9).It follows that the nitrogen atom of pyrrole no longer posses ses a lone pair of elec- trons,and the compound cannot function as a base without losing its aromatic character. Box 1.9 Pyrrole Pyrrole 1.4.2 Non-aromaticity and Anti-aromaticity Cyclic polyenes and their heterocyclic counterparts which contain 4n o not show aromaticiy,since shoud thes moleculs bo a planar array the orbitals used to accommodate the electrons within the closed loop are no longer just bonding in nature, but a mixture of both bonding and non-bonding types.For a fully unsat- urated planar polyene containing four ring atoms,the number of bond- ing ene levels is one and there are two degenerate non-bonding levels (Box 1.10);in the case of an eight-membered ring,there are three bond ing sub-levels and two degenerate non-bonding levels. Consider a fully delocalized symmetrical 'cyclobutadiene':here each carbon atom is equivalent and sp2 hybridized;this leaves four p-electrons to overlap and to form a t-system.Two electrons would then a Heterocyclic Chemistry If cyclohexatriene were to exist in a localized form and was a planar molecule it would contain three long single bonds and three short double bonds (in buta-l,3-diene the C,-C,, bond length is 0.134 nm and the C,-C, bond length is 0.148 nm). The result would be an irregular hexagon and there would be two isomers for, say, a hypothetical 1,2-dichlorocyclo￾hexatriene: one with a single C-C bond separating the two chlorine atoms, and the other with a double C=C bond. are slightly shorter than the C-C bonds (0.134 nm versus 0.139-0.140 nm). This is because nitrogen is more electronegative than carbon, and this fact also affects the nature of the n-system. In pyridine the electron density is no longer uniformly distributed around the ring and is con￾centrated at the N atom. Another difference between the molecules is that whereas in benzene each carbon is bonded to a hydrogen atom, in pyridine the nitrogen pos￾sesses a lone (unshared) pair of electrons. This lone pair occupies an sp2 orbital and is orientated in the same plane as the ring; moreover, it is available to capture.a proton so that pyridine is a base. In five-membered heterocycles, formally derived from benzene by the replacement of a CH=CH unit by a heteroatom, aromaticity is achieved by sharing four p-electrons, one from each ring carbon, with two elec￾trons from the heteroatom. Thus in pyrrole, where the heteroatom is N, all the ring atoms are sp2 hybridized, and one sp2 orbital on each is bonded to hydrogen. To complete the six n-electron system the non￾hybridized p-orbital of N contributes two electrons (Box 1.9). It follows that the nitrogen atom of pyrrole no longer possesses a lone pair of elec￾trons, and the compound cannot function as a base without losing its aromatic character. I .4.2 Non-aromaticity and Anti-aromaticity Cyclic polyenes and their heterocyclic counterparts which contain 4n p-electrons do not show aromaticity, since should these molecules be forced to form a planar array the orbitals used to accommodate the electrons within the closed loop are no longer just bonding in nature, but a mixture of both bonding and non-bonding types. For a fully unsat￾urated planar polyene containing four ring atoms, the number of bond￾ing energy levels is one and there are two degenerate non-bonding levels (Box 1.10); in the case of an eight-membered ring, there are three bond￾ing sub-levels and two degenerate non-bonding levels. Consider a fully delocalized symmetrical ‘cyclobutadiene’; here each carbon atom is equivalent and sp2 hybridized; this leaves four p-electrons to overlap and to form a n-system. Two electrons would then