Chapter 6 Molecular Structure & Bonding
112
angles (
) of ~109α
o. Thus, the carbon atom is in the center of a tetrahedron with the three
H’s and another C at the apices. The carbon atom at position 2 is surrounded by three electron regions, so it lies in the center of a trigonal planar arrangement with
~120β
o. The
oxygen atom at position 3 has four electron
regions (two bonds and two lone pairs), so
(^) γ
~109
o. The resulting three-dimensional representation of acetic acid is shown as a ball-
and-stick model in Figure 6.7b. The COOH group is called a carboxylic acid group, which is what makes acetic acid acidic.
Figure 6.8 Lewis structure of benzene (a) Two resonance forms of benzene and (b) the structure frequently used to show that all of the bonds between carbon atoms are identical. The circle shows the double bonds are shared between all C-C bonding regions.
C C
C
C C
C
H
H
HHH
H
C C
C
C C
C
H
H
HHH
H
C C
C
C C
C
H
H
HHH
H
(a)
(b)
Benzene
has the formula C
H 6
and is a six-membered ring (the six carbon atoms bond 6
so as to form a hexagon). The Lewis structur
es of its two resonance forms are shown in
Figure 6.8a. Each carbon has three regions, so
each is trigonal planar and all bond angles
are 120
o. In addition, the double bonds require that the molecule is planar. As a result of
resonance, all of the carbon atoms are identi
cal as are all six of the bonds between them.
The bond lengths between carbon atoms (1.4 Å) lie between those of a single C-C bond (1.5 Å) and those of a C=C double bond (1.3 Å)
, which is consistent with a bond order of
1.5. Thus, the three double bonds are shared equally between the six bonding regions in the ring. Indeed, the double bonds are frequently
represented as a circle rather than three
lines (Figure 6.8b) to emphasize the equi
valence of the carbon-carbon bonds. We will
revisit this important bonding characteristic
of benzene in our discussion of molecular
orbital theory at the end of the chapter.
The Lewis structure of aspirin, C
H 9
O 8
, is shown in Figure 6.9a. Note that lone pairs 4
are not drawn in this figure, but carbon and ox
ygen both obey the octet rule. Thus, the two
oxygen atoms numbered 3 and 4 each have four
electron regions because each must have
two lone pairs in addition to the regions shown. The structure of aspirin combines the structural features described for acetic acid and benzene. The C at position 1 is part of a benzene ring, and the three regions are 120
o apart. As in benzene, the bonds between the
carbon atoms in the ring have the same lengths
due to resonance. The C and O at positions
2 and 3 are part of a carboxylic acid group (COOH) and have the same geometry as the C and O atoms at positions 2 and 3 in the structure of acetic acid. The carboxylic acid group makes aspirin acidic, which is why it can upset your stomach. The O at position 4 has four electron regions, so it is tetrahedral with ~109
o bond angles. The C at position 5 is in the
center of a plane with ~120
o separating the three electron groups. The carbon atom at
position 6 is in the center of a tetrahedron with ~109
o separating its four electron groups.
The ball-and-stick model in Figure 6.9b summarizes these conclusions.
C C
C
C C
C
H
H
H H
C
O
O
O
H
O C
H C
H
H
1
3 2
4
5
6
(a)
(b)
Figure 6.9 Lewis structure (a) and ball-and-stick (b) re
presentations of as
pirin
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North
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State
University