When determining the most important resonance structure, we consider
full octets above other preferences. In neither of these
resonance
structures do all atoms have full octets. When looking to rank
structures,
we must look for differences. In each structure, the positive
charge
resides on the exactly the same type of atom, each has the same number
of covalent bonds, and each has the same number of formal
charges.
Thus, there is no preference between these two structures.
Resonance
structures which are deemed to be of equal importance (equal energy)
are
termed degenerate. Structures A and B contribute
equally
to the resonance hybrid structure.
Considering full octets first, we see that all atoms of structures
C
and D have full octets. The have no formal charge, and
the
same number of covalent bonds. These structures are of equal
importance.
Each of these structures has one carbon atom with an open octet (the
carbon with the positive formal charge). The charge always
resides
on a carbon, and there is an equal number of formal charges and
covalent
bonds for each structure. At first glance, we would conclude that
all five structures are of equal importance. (We will learn in
section
6.3 of the text that a carbon bearing a positive charge, called a
carbocation,
is most stable when the carbon bearing the charge has the greatest
number
of other carbons attached to it. We will also learn in sections
9.1
and 9.2 of the special stability associated with having three
alternating
carbon-carbon double bonds in a six-membered ring, called
aromaticity.
In this case, aromaticity is the most important factor, so degenerate
structures
E
and F are the most important.)
The carbon of resonance structure J has an open octet.
All atoms of structure K have complete octets. Although
structure
K
has formal charges, and the positive charge is on the more
electronegative
atom (oxygen vs. carbon), the full octet preference dominates.
Structure
K
is therefore the most important resonance structure for carbon
monoxide.
The carbon bearing the positive charge of structure M has an
open octet. All of the atoms of structures L and N
have full octets, so structure M is less important.
Structure
M
also has more formal charges than either of the other two
structures.
Structures L and N have the same number of formal
charges
and covalent bonds, so the difference is the position of the formal
charge.
A negative charge is best accommodated by a more electronegative atom,
so structure N is more important than structure L.
The carbon bearing the positive charge of structure P has an
open octet. All of the atoms of structures O and Q
have full octets, so structure P is less important.
Structures
O
and Q have the same number of covalent bonds. Structure
Q
has two formal charges whereas structure O has no formal
charges.
Thus structure O is more important than structures
P or Q.
All atoms in all of these resonance structures have a complete
octet.
(Recall that while phosphorus can expand its octet to ten or twelve
valence
electrons, it still needs only eight electrons to fill its
octet.)
The negative charges always reside on oxygen and the positive charges
always
reside on phosphorus, so we cannot rank importance of these structures
using Preference 2. Structure R has four covalent bonds,
whereas
structures S - V each have five covalent bonds (four
single
and one double), so structure R is less important than
structures
S
- V. Structures S - V all have three
oxygens
with formal charges. Thus we conclude that structures S -
V
are degenerate, and are equally important.