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Uploaded byShri Shankaracharya College, Bhilai,Junwani
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Mesomeric effect

Mesomeric effect is the permanent shifting of π-electrons from multiple bonds to single bonds or lone pairs, which mainly operates in conjugated double bond systems. There are two types of mesomeric effects: -m effect from electron withdrawing groups containing double or triple bonds between heteroatoms, and +m effect from electron donating groups where the first atom has a negative charge or lone pair. The effects influence the electron density on benzene rings, with -m decreasing and +m increasing density at the ortho and para positions.

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Presented by Dr. Neelam Department of chemistry
Mesomeric effect Mesomeric effect is defined as permanent effect of π-electron shifting from multiple bond to atom or from multiple bond to single bond or from lone pair to single bond. CH2= CH̶ CH2= CH ̶ CH2= CH ̶ (G) This effect mainly operates in conjugated system of double bond. So that this effect is also known as conjugate effect. Mesomeric effect is distance independent.
Types of Mesomeric effect (i) –m effect (ii) +m effect (i) – m group (Electron withdrawing group) A group that contains double bond or triple bond between hetero atoms will show –m effect. Due to – m the electron density on benzene ring is decreased particularly on ortho and para positions.
Relative order of –m group
(ii) + m group (Electron releasing group) A group, first atom of which bears -ve charge or lone pair always shows +m effect. Due to +m effect the electron density of benzene ring is increased particularly on ortho and para positions. Relative order of +m group :
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