The polarity produced in the molecule by the interaction of two pi bonds or between a pi bond and loan pair of electron present on an adjacent atom is known as RESONANCE EFFECT. There are two types of resonance effect:
Positive resonance effect ; (Transfer of electron is away from an atom or substituent group attached to the conjugated system) +R effect e.g. –OH, -OR, -NH2
Negative resonance ; (Transfer of electron is towards the atom or substituent group attached to the conjugated system) –R effect e.g. –COOH, -CHO, -CN
