Example 5.7
OCO
OCO
AB
+
Figure 5.12 Structure A is the prefer
red resonance form of CO
because there 2
is no charge separation. The non-zero formal charges on the oxygen atoms in structure B make it less favorable.
O C
HF
O CH
F
O CF
H
Example 5.7b
a) Which of the two resonance forms of CO
shown in Example 5.4 is preferred? 2
Refer to Figure 5.12 for the Lewis structures of the two resonance forms. Structure A: Each oxygen has four nonbondi
ng electrons and one-half of the four bonding
electrons, for a total of six valence electr
ons. Thus, both oxygen atoms have zero formal
charge. Alternatively, each oxygen has zero
formal charge because each is involved in
two bonds. The carbon has one-half of the eight
bonding electrons for a total of four
valence electrons, which is the same as th
e free atom. Consequently, the carbon atom
also has a zero formal charge. Alternatively, each carbon has zero formal charge because each is involved in four bonds. Structure B: Neither oxygen has two bond
s, so they each have nonzero formal charge.
The triple bonded oxygen has two nonbonding electrons and one-half of six bonding electrons, for a total of five valence electr
ons, one less than the free atom. Thus, this
oxygen carries a +1 formal charge. The single bonded oxygen contains six nonbonding and one-half of two bonding electrons, for a
total of seven valence electrons, one more
than the free atom. It, therefore, carries a -1
formal charge. The carbon has one-half of
eight bonding electrons and therefore a zero formal charge. Structure A is preferred because it has no charge separation.
b) The Lewis structure of FCHO requires four shared pairs. Use formal charge to
decide which of the structures in the margin is preferred. Structure A
:
FC
= 7-6-1 = 0; F
FC
= 6-4-2= 0; O
FC
= 4-0-4 = 0 C
Structure B
:
FC
= 7-4-2 = +1; F
FC
= 6-6-1= -1; O
FC
= 4-0-4 = 0C
Structure C
:
FC
= 7-4-2 = +1; F
FC
= 6-4-2= 0; O
FC
= 4-2-3 = -1 C
Structures B and C both place two bonds to
F, which puts positive formal charge on the
most electronegative atom. There is only a single
shared pair to F in Structure A, so it has
no formal charge. Structure A is the preferr
ed structure because it contains no formal
charge – only one bond to a terminal halogen and four bonds to carbon. Example 5.7 demonstrates that
placing more than one bonding pair to a halogen
should be avoided
because it places positive formal
on these electronegative atoms.
Placing positive formal charge on a halogen is
unavoidable in compounds or ions in which
it is the central atom. However, the other
atoms are highly electronegative in these cases,
so the positive formal charge is more reasonable. For example, the formal charge on Cl in ClO
1- 4
is +3. Recall from Chapter 4, that the oxidation state of Cl in ClO
1- 4
is +7, so a
formal charge of +3 is still much less than the charge the chlorine atom would have if the bonds were ionic.
Chapter 5 The Covalent Bond
© by
North
Carolina
State
University