Mesomeric effect

By Dr. Neelam

1. Presented by
Dr. Neelam
Department of chemistry

2. Mesomericeffect
Mesomericeffectisdefinedaspermanenteffectofπ-electronshifting
frommultiplebondtoatomorfrommultiplebondtosinglebondorfrom
lonepairtosinglebond.
CH
2=CH̶CH
2=CH̶CH
2=CH̶(G)
Thiseffectmainlyoperatesinconjugatedsystemofdoublebond.Sothat
thiseffectisalsoknownasconjugateeffect.Mesomericeffectisdistance
independent.

3. TypesofMesomericeffect
(i)–meffect
(ii)+meffect
(i)–mgroup(Electronwithdrawinggroup)
Agroupthatcontainsdoublebondortriplebondbetweenheteroatoms
willshow–meffect.Dueto–mtheelectrondensityonbenzenering
isdecreasedparticularlyonorthoandparapositions.

4. Relativeorderof–mgroup

5. (ii)+mgroup(Electronreleasinggroup)
Agroup,firstatomofwhichbears-vechargeorlonepairalwaysshows
+meffect.Dueto+meffecttheelectrondensityofbenzeneringis
increasedparticularlyonorthoandparapositions.
Relativeorderof+mgroup:

7. THANK YOU