Question

what is + R effect?​

Answer 1

RESONANCE EFFECT OR MESOMERIC EFFECT

The electron withdrawing or releasing effect attributed to a substituent through delocalization of p or π electrons, which can be visualized by drawing various canonical forms, is known asmesomeric effect or resonance effect. It is symbolized by M or R.

Negative resonance or mesomeric effect (-M or -R): It is shown by substituents or groups that withdraw electrons by delocalization mechanism from rest of the molecule and are denoted by -M or -R. The electron density on rest of the molecular entity is decreased due to this effect.

E.g. -NO2, Carbony group (C=O), -C≡N, -COOH, -SO3H etc.

Positive resonance or mesomeric effect (+M or +R):  The groups show  positive mesomeric effect when they release electrons to the rest of the molecule by delocalization. These groups are denoted by +M or +R. Due to this effect, the electron density on rest of the molecular entity is increased.

 E.g. -OH, -OR, -SH, -SR, -NH2, -NR2 etc.