From Wikipedia,
the free encyclopedia.
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
inductive effect, as this
effect is purely to do with the
electronegativity of the atoms
and their structural chemistry
(which atoms are connected to
which).
