Electrophilic Substitution in Furan, Pyrrole, and Thiophene

pyrrole, and thiophene are all much more reactive than benzene toward electrophilic substitution. Each reacts predominantly at the 2-position (and, if that position is already substituted, at the 5-position). Here are typical examples  

The reason for predominant attack at C-2 (instead of the other possibility, C-3) becomes clear if we examine the carbocation intermediate in each case  

Attack at C-2 is preferred because, in the carbocation intermediate, the positive charge can be delocalized over three atoms, whereas attack at C-3 allows delocalization of the charge over only two positions. 

PROBLEM 13.8 Write out the steps in the mechanism for bromination of furan (eq. 13.11). 

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