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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?

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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.

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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is usually favoured when the withdrawing group has a higher electronegativity than the group from which the electron is withdrawn This is also termed as -M effect.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

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resonates to

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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

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resonates to

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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.

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    $\begingroup$ This answer unfortunately suffers from the same issues as the others you recently posted. The concept is called resonance, which has nothing to do with resonating. $\endgroup$ Apr 4, 2023 at 16:31

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