Heterocyclic Aromatic Compounds and Huckels Rule

Hiickel s rule can be extended to heterocyclic aromatic compounds. A heteroatom such as oxygen or nitrogen can contribute either zero or two of its unshared electrons as needed to the IT system so as to satisfy the (An + 2) electron requirement. 

The unshared pair in pyridine, for example, is not needed to satisfy the six it electron requirement for aromaticity, so is associated entirely with nitrogen and is not delocalized into the aromatic it system. 

The unshared pair in the Lewis structure for pyrrole, on the other hand, must be added to the four it electrons of the two double bonds in order to meet the six it electron requirement. 

In both pyridine and pyrrole the unshared electron pair occupies that orbital which provides the most stable structure. It is a different orbital in each case. In pyridine it is an p -hybridized orbital localized on nitrogen. In pyrrole it is a orbital of nitrogen that overlaps with the p orbitals of the ring carbons to give a delocalized 7T system. 

Write two different resonance fornns for pyrrole in which nitrogen has a formal charge of + 1. Are comparable resonance forms possible for pyridine  

Pyridine and pyrrole are both weak bases, but pyridine is much more basic than pyrrole. When pyridine is protonated, its unshared pair is used to bond to a proton and, since the unshared pair is not involved in the tt system, the aromatic character of the ring is little affected. When pyrrole acts as a base, the two electrons used to form a bond to hydrogen must come from the tt system, and the aromaticity of the molecule is sacrificed on protonation. 

FIGURE 11.15 (a) Pyridine has six TT electrons plus an unshared pair in a nitrogen sp orbital, (b) Pyrrole has six TT electrons, (c) Furan has six TT electrons plus an unshared pair in an oxygen sp orbital, which is perpendicular to the TT system and does not interact with it. 

PROBLEM 11.21 Imidazole Is a much stronger base than pyrrole. Predict which nitrogen Is protonated when imidazole reacts with an acid, and write a structural formula for the species formed. 

Benzene is the parent of a class of hydrocarbons called arenes, or aromatic hydrocarbons. 

An important property of aromatic hydrocarbons is that they are much more stable and less reactive than other unsaturated compounds. Benzene, for example, does not react with many of the reagents that react rapidly with alkenes. When reaction does take place, substimtion rather than addition is observed. The Kekuld formulas for benzene seem inconsistent with its low reactivity and with the fact that all of the C—C bonds in benzene are the same length (140 pm). 

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