+M effect The +M effect, also known as the positive mesomeric effect, occurs when the substituent is an electron donating group. The group must have one of two things: a lone pair of electrons, or a negative charge. In the +M effect, the pi electrons are transferred from the group towards the conjugate system, increasing the density of the system. Due to the increase in electron density, the conjugate system will develop a more negative charge. As a result, the system under the +M effect will be more reactive towards electrophiles, which can take away the negative charge, than a nucleophile.
+m effect the +m effect also known as the positive mesomeric effect occurs when the substituent is an electron donating group the group must have one of two things a lone pair of electrons or a negative charge in the +m effect the pi electrons are transferred from the group towards the conjugate system increasing the density of the system due to the increase in electron density the conjugate system will develop a more negative charge as a result the system under the +m effect will be more reactive towards electrophiles which can take away the negative charge than a nucleophile