Electron Availability - Mesomeric Effects
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A mesomeric effect is an electron redistribution that occurs via a pi orbital, quite often via conjugated systems. A good example of this effect is seen in the carbonyl group:

The properties of the carbonyl are not properly explained by the classical image (far left), nor by the dipole (centre), which represents the total relocation of the pi electrons; the "real" structure is a combination of these, shown on the far right.  There is an inductive effect (indicated by the arrow) in the "real" structure, but it is smaller than the mesomeric effect. This is because the sigma electrons are less readily polarizable than pi electrons, hence they are not shifted as far.

The mesomeric effect can be transmitted along conjugated systems if, for example, a carbonyl group is conjugated with a C=C bond:

Therefore, as above, there is an electron deficiency at the carbon next to oxygen, and also at the one indicated in the C3 position.  Mesomeric effects are much better transmitted through bonds than the inductive effect;in the C3 position, the effect of the the carbonyl mesomeric influence is still noticeable.

The stabilisation that results from delocalisation of charge through a mesomeric effect, can be an important influencing factor in the formation of the ion itself:

Hence we note that phenol is much more acidic than a straight-chain alcohol (i.e. ethanol pKa = 15.9).  It is the stabilisation of the negative charge of the phenoxide ion that is a very important factor in the acidity of phenol.

Mesomeric (and inductive) effects are permanent effects that are present in the ground state of the molecule.  They are therefore present in the physical properties of the molecule. 

It is important to remember that inductive effects involve only electrons in sigma bonds, and mesomeric effects involve only electrons in pi bonds.

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