Mesomeric effect
From Academic Kids
The Mesomeric effect is the effect of the overlap of a p-orbital with the p-orbital on an adjacent atom in the chemical compound to form a delocalised cloud of electrons over both the atoms called a π (pi) bond. P-orbitals on many atoms may all overlap, to form an extensive π bond but only as long as the number of electrons on each atom in the conjugated system does not exceed the maximum allowed and the chain of atoms is continuous. The effect is symbolized by M. The mesomeric effect of delocalised electrons occurs in unsaturated organic compounds, where the double bond is separated by a single bond. Examples butadi-1,3-ene and aromatic compounds.The effect is responsible for stability greater than that predicted. It affects the stability of compounds by resonating the electrons/charge over the molecule. Thus it can stabilise the anion formed after the loss of a proton, making the compound more acidic. Additionally, the effect of delocalising the electrons and therefore the charge, will reduce the probability of the anion being reprotonated as it is now a "soft nucleophile" (disperse charge) compared to the "hard electrophile" of the proton (small localised charge).
It's important to note that the mesomeric affect as a result of p- orbital overlap has absolutely no affect on the inductional effect, as this is effect is purely to do with the electronegativity of the atoms and their structural chemistry (which atoms are connected to which).
