Chemistry Reference
In-Depth Information
nonbonding to bonding
•
Separation
of
charge
in
a
resonance
structure
decreases stability.
•
Structures with charge separation are more stable if
the negative charge is located on an electronegative
atom.
O
C
O
C
the two lone pair electrons are used to make a
π
bond
to carbon; this effectively assigns one electron to
each atom;
C
+
was electron deficient but now gains one electron
from the lone pair;
oxygen now loses one electron from the lone pair and
carries formal charge +1;
the new structure still has overall charge +1
•
Structures
with
adjacent
like
charges
are
dis-
favoured,
as
are
those
with
multiple
isolated
charges.
•
The
bond framework and steric factors must
permit a planar relationship between contributory
resonance structures.
σ
This is illustrated for a carbonyl compound and an
alkene.
bonding to new bonding
C
CO
CO
C
CC
CC
most favourable
resonance form
charge separation;
less favourable;
negative charge on more
electronegative atom
π
electrons from the double bond are used to make a
new double bond;
C
+
was electron deficient but now gains one electron
from the pair of
π
electrons;
the carbon at the donor end of the arrow is now
electron deficient and carries formal charge +1;
the new structure still has overall charge +1
CO
CO
least important;
negative charge on less
electronegative atom
•
All structures must be valid Lewis structures. An
atom may become electron deficient, but, on the
other hand, it must never be shown with more
valence electrons than it can accommodate. For
example, it is not possible to have pentavalent
carbon.
CC
CC
•
The overall charge must remain the same.
most favourable
resonance form
charge separation;
less favourable;
positive and negative charges
on same type of atom
•
There should be the same number of unpaired
electrons in each structure.
This redistribution of electrons provides us with
one or more new
resonance structures
(also
called
canonical structures, limiting structures
,or
mesomers
). However, some structures are more real-
istic than others.
CC
CC
also unfavourable
We then consider the potential relative importance
of the resonance structures we have drawn.
•
The more covalent bonds a structure has, the more
stable it is.
•
A structure in which all the atoms have the noble
gas structure is particularly stable.
•
Equivalent resonance structures contribute equally
to the hybrid.
