Chemistry Reference
In-Depth Information
CHAPTER 4
Resonance and
Delocalization
4.1 WHAT IS RESONANCE?
In a molecular structure, there is often more than one way to draw the π-bonding
and/or non-bonding lone pair of electrons, without changing the atom connec-
tions. These are examples where a single Lewis structure does not give a com-
plete picture of the bonding in the molecule. The concept of
resonance
can be
used to describe the bonding in these situations.
55
As
Figure 4.1
shows, the bonding in a carbonate dianion is described by a
reso-
nance hybrid
(an average) of three equal
resonance forms
(alternative Lewis
structures). Rather than have the bonding electrons localized, the resonance
hybrid has the available electrons delocalized over the bonded atoms.
FIGURE 4.1
Resonance forms for the
CO
2
−
3
carbonate dianion.
The three oxygen atoms share the π and/or non-bonding lone pair of electrons
and the negative charges equally. This resonance model represents accurately
the experimental measurement of the three C-O bonds in the dianion. All C-O
bonds are:
the same in length;
n
a little shorter than a standard C-O single bond;
n
a little longer than a standard C
]
O double bond.
n
4.2 DRAWING USEFUL RESONANCE STRUCTURES
The following simple guidelines will help you to draw and understand reso-
nance structures:
Individual resonance forms are not real. The only real structure is a hybrid of
the resonance forms. As shown in
Figure 4.1
, a double-headed arrow ↔ is
used to link resonance forms. This type of arrow shows only a resonance situ-
ation and does not show an equilibrium between real structures.
n
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