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In page Mesomeric effect:

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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.[citation needed]