Pyrrole has maximum electron density on 2 and 5 . It generally reacts with electrophiles at the C-2 or C- 5 due to the highest degree of stability of the  intermediate formed. Attack at position 3 or 4 yields a carbocation that is a hybrid of structures (I) and (II). Attack at position 2 or 5 yields a carbocation that is a hybrid not only of structures (III) and (IV) (analogous to I andII) but also of structure (V). The extra stabilization conferred by (V) makes this ion the more stable one. Also, attack at position 2 or 5 is faster because the developing positive charge is accommodated by three atoms of the ring instead of by only two.