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
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.