Write a note on mesomeric effect.

Explanation

Mesomeric effect:
"The electron pair displacement caused by an atom or group along a chain by a conjugative mechanism is called the mesomeric effect of that atom or group". Salient features of the mesomeric effect: .
i) Permanent effect operating in the ground state of the molecule.
ii) Lonė pairs and i electrons are involved and operate through conjugative mechanism of electron displacement.
iii) It influences the physical properties, reaction rates etc. Groups which tend to increase the electron density of the rest of the molecule are said to have (+M) effect: Such groups tend to posses lone pairs of electrons.
e.g. `: H _(2) overset (* *) (N) - underset(|) (C) = underset(|) (C)`- Hence `- NH _(2)` group has + M effect
Groups that decrease the electron density of the rest of the molecule are side to have (-M) effect. Unsaturated groups having polar character have - M effect.
`- underset(|) (C ) - underset(|) (C) - C =O.` Here C = O group decreases the electron density of the remaining molecule. It
has - M , effect.
+ M effect is `-F gt - Cl gt - Br gt - I:`
`- NH _(2) gt OR gt F,`
`- NH _(2) gt - OH gt - F,`
`- OR gt - SR gt SeR`
`- O gt - OR`
- M effect is `= O gt = NR gt = CR _(2)`
`=overset (+) ( N)R _(2) lt = NR`
`-= N gt -= CR`