Mesomeric effect • It is simply the resonance occuring in a molecule when any atom or group of atoms is in conjugation with organic molecule. In general there is no difference between resonance and mesomeric effect. Heinsenberg (1926) called it as resonance and Ingold (1933) called it as mesomeric effect. • A very minute difference is there between resonance and mesomeric effect i.e. when a molecule which do not have any polarity, such as CH2=CH-CH=CH2 ,is considered then pi electrons may move in either direction (i.e. from left to right or from right to left), in such case resonance is operating in the molecule but when the same compound is attached with CHO group i.e. CH2=CH-CH=CH-CH=O then electrons will always moves from left to right, in such case due to CHO group carbon aquires positive charge and oxygen aquires negative charge. Therefore in such case resonance is called as mesomeric effect. www.OrganicChemistry.co.in
Types of Mesomeric effect • Similar to I effect, mesomeric effect is also of two types: + M effect: • Those atoms or groups which donate electron density towards the conjugated system are called as + M groups and the effect is called as +M effect. + M groups in decreasing order of power: -O- >-NH2 >-NHR >-NR2 >-OH > -OR > NHCOR > OCOR > Ph > CH=CH2 > -F > -Cl > -Br > -I – M effect: • Those atoms or groups which withdraw electron density from the conjugated system are called as - M groups and the effect is called as -M effect. - M groups in decreasing order of power: • -NO2 > -SO3H > -CN > -CHO > -COR > -COX > -COOCOR > COOR > -COOH > -CONH2 > -CONHR > -CONR2 www.OrganicChemistry.co.in
Capsule of Mesomeric effect • Atom directly attach to the conjugated system have at least one lone pair of electron or a negative charge with complete octet are said to be + M groups. • Atom directly attach to the conjugated system have multiple bond (double or triple) with more electronegative atom without lone pair of electrons are said to be – M groups. www.OrganicChemistry.co.in
