Mesomeric effect It is special type of resonance of in which lone pair of electrons are delocalized over the alternative double bond and lone pair always present on same heteroatom in function group. Heteroatom=> These types of functional group are called +M Group .same function groups are present on which lone pairs are not present but these functional groups show mesomeric effect and these types of functional groups are called –M group +M group = O - >NR 2 >NHR,>NH 2 >HO>NHCOR>-OCOR and alky |& ary |groups. -M group = -NO 2 >-CN>-CHO>COR>COR>-COOH>-COOP>CONH 2 etc +M group =ortho-para directing and activating groups. In this example negative charge observed at ortho and para position to the –NH 2 group and negative charge means extra electron is present in the ring and ring is activated and electrophile attacks on ortho & p