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442 Chapter 13
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e.
An absorption band at 1780–1650 cm
-
1
due to a
C O
stretching vibration would be present for the
carboxylic acid and absent for the alcohol.
f.
An absorption band at 2960–2850 cm
-
1
due to a
sp
3
C
¬
H stretching vibration would be present for
the compound with the methyl group and absent for benzene.
31.
2-butyne, H
2
, Cl
2
, and ethene because they are symmetrical molecules.
32.
The absorption bands in the vicinity of 3000 cm
-
1
indicate that the compound has hydrogens attached to
both
sp
2
and
sp
3
carbons. The absence of absorptions at 1600–1800 cm
-
1
and the absence of broad absorp-
tions between 2500 and 3650 cm
-
1
rules out compounds containing
C O
, N
¬
H, and O
¬
H groups.
The lack of absorption at 1600 cm
-
1
and 1500 cm
-
1
indicates that the compound does not have a benzene
ring. The
sp
2
hydrogens, therefore, must be those of an
alkene
.
The lack of significant absorption at 1600 cm
-
1
indicates that the compound must be an alkene with a rela-
tively small (if any) dipole moment change when the vibration occurs. The absorption band at 965 cm
-
1
indicates that the compound is a
trans
-alkene
.
The molecular ion with
m
>
z
=
84 suggests that the compound has a molecular formula of C
6
H
12
. The
base peak with
m
>
z
=
55 indicates that the group that the molecular ion most easily loses most easily is
an ethyl radical (84
-
29
=
55). Therefore, the ethyl group must be attached to an allylic carbon. The
compound, therefore, is
trans
-2-hexene
.
C C
CH
3
H
CH
2
H
trans
-2-hexene
CH
2
CH
3
allylic carbon
33.
The absorption band at 1700 cm
-
1
indicates that the compound has a carbonyl group, and the absorption
band at 1600 cm
-
1
indicates that the compound has a carbon–carbon double bond. The absorption bands
in the vicinity of 3000 cm
-
1
indicate that the compound has hydrogens attached to both
sp
2
and
sp
3
carbons.
The absorption band at 1380 cm
-
1
indicates that the compound has a methyl group. Because the compound
has only four carbons and one oxygen, it must be
methyl vinyl ketone
. Notice that the carbonyl stretch is at a
lower frequency (1700 cm
-
1
) than expected for a ketone (1720 cm
-
1
) because the carbonyl group has partial
single-bond character due to electron delocalization.
O
C
H
3
C
CH CH
2
34.
A
=
c
l
e
c
=
A
l
e
c
=
0.52
12,600
=
4.1
*
10
-
5
M