Typical reactivity of pyrroles, thiophenes and furans

In this chapter are gathered the most important generalisations that can be made, and the general lessons that can be learned about the reactivity, and relative reactivities, one with the other, of the prototypical flve-membered aromatic heterocycles pyrroles, furans and thiophenes. 

The chemistry of pyrrole and thiophene is dominated by a readiness to undergo electrophilic substitution, preferentially at an a-position but, with only slightly less alacrity, also at a P-position, should the a positions be blocked. It is worth re-emphasising the stark contrast between the five- and six-membered heterocycles - the hve-membered systems considered in this chapter react much more readily with electrophiles than does benzene, but the azines react much less readily (cf. Chapters 7-9 and 13, 14). 

An example of easy a-electrophilic substitution of pyrrole with a weak electrophile 

Heterocyclic Chemistry 5th Edition John Joule and Keith Mills 2010 Blackwell F iblishing Ltd 

Positional selectivity in these five-membered systems, and their high reactivity to electrophitic attack, are well explained by a consideration of the Wheland intermediates (and by implication, the transition states that lead to them) for electrophilic substitution. Intermediate cations from both a- and P-attack are stabilised (shown for attack on pyrrole). The delocalisation, involving donation of electron density from the heteroatom, is greater in the intermediate from a-attack, as illustrated by the number of low-energy resonance contributors. Note that the C-C double bond in the intermediate for p-attack is not and cannot become involved in delocalisation of the charge. 

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