Why do certain groups withdraw/ donate electron to double bond or conjugated system in resonance effect?

Why for the Resonance effect certain group of substituents/functional group donate electron from the double bond or to a conjugated system while some withdraw electron from the double bond or to a conjugated system? What causes the groups/substituent to withdraw or donate electron causing +R and -R effect?




Generally, it is observed that if there are lone pair(s) present on the atom of function group directly attached to to the conjugated system, then $+R$ effect is observed.


If the same atom has double bonds or dative bonds with another atom, then $-R$ effect is observed.


**Example:** –$\ce{OH}$ shows +R due to 2 lone pairs on Oxygen.
— $\ce{NO2}$ shows -R due to one double bonds between Nitrogen and Oxygen, and one dative bond from Nitrogen to another Oxygen.


 2

Withdrawing an electron from a conjugated system is usually favoured when the withdrawing group has a higher electronegativity than the group from which the electron is withdrawn This is also termed as -M effect. For example : double bond O attached to carbon withdraws electrons


[![enter image description here](https://i.stack.imgur.com/ooeNV.png)](https://i.stack.imgur.com/ooeNV.png)


resonates to


[![enter image description here](https://i.stack.imgur.com/fmFGG.png)](https://i.stack.imgur.com/fmFGG.png)


But an atom or group containing lone pairs may also donate it's lone pair to form a coordinate (dative) bond and here elctronegativity does not matter (if the atom can accomodate another bond (like double bond O does not form a dative bond with carbon) (to help conjugation) This is also termed as +M effect. For example : OH attached to carbon donates it's lone pair to form a coordinate bond


[![enter image description here](https://i.stack.imgur.com/OX7e1.png)](https://i.stack.imgur.com/OX7e1.png)


resonates to


[![enter image description here](https://i.stack.imgur.com/YOkAG.png)](https://i.stack.imgur.com/YOkAG.png)


some groups show both +M,-M effect (which have both a double bond (to show -M effect) and a lone pair (to show +M effect)), like NH=NH, phenyl, et cetera.


 -2