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Chapter 13 445
Copyright © 2017 Pearson Education, Inc.
45.
a.
An absorption band at 1250 cm
-
1
due to a C
¬
O stretching vibration will be present for the ester
and absent for the ketone.
b.
An absorption band at 720 cm
-
1
due to in-phase rocking of the five adjacent methylene groups will be
present for heptane and absent for methylcyclohexane.
c.
An absorption band at 3650–3200 cm
-
1
due to an O
¬
H stretching vibration will be present for the
alcohol and absent for the ether.
d.
An absorption band at 3500–3300 cm
-
1
due to an N
¬
H stretching vibration will be present for the
amide and absent for the ester.
e.
The secondary alcohol will have an absorption band at 1385–1365 cm
-
1
for the methyl group. The
primary alcohol does not have a methyl group, so it will not have this absorption band.
f.
The trans isomer will have a C
¬
H bending absorption band at 980–960 cm
-
1
, whereas the cis isomer
will have the absorption band at 730–675 cm
-
1
. In addition, a weak C
“
C absorption band at
1680–1600 cm
-
1
will be present for the cis isomer and absent for the trans isomer because only the
trans isomer has no dipole moment.
g.
The
C C
absorption band will be at a larger wavenumber for the ester (1740 cm
-
1
) than for the
ketone (1720 cm
-
1
).
h.
The
C O
absorption band will be at a larger wavenumber for the
b
,
g
-unsaturated ketone (1720 cm
-
1
)
than for the
a
,
b
-unsaturated ketone (1680 cm
-
1
), since the double bonds in the latter are conjugated.
i.
The alkene will have absorption bands at 1680–1600 cm
-
1
due to a C
“
C stretching vibration and at
3100–3020 cm
-
1
due to an
sp
2
C
¬
H stretching vibration that the alkyne will not have. The alkyne
will have an absorption band at 2260–2100 cm
-
1
that the alkene will not have.
j.
An absorption band at 2820 and 2720 cm
-
1
due to the aldehyde C
¬
H stretching vibration will be
present for the aldehyde and absent for the ketone.
k.
Absorption bands at 1600 cm
-
1
and 1500 cm
-
1
(aromatic ring stretching vibrations) and at 3100–
3020 cm
-
1
(
sp
2
C
¬
H stretching vibration) will be present for the compound with the benzene ring
and absent for the compound with the cyclohexane ring. An absorption band at 2960–2850 cm
-
1
due
to an
sp
3
C
¬
H stretching vibration will be present for the compound with the cyclohexane ring and
absent for the compound with the benzene ring.
l.
Absorption bands at 990 cm
-
1
and 910 cm
-
1
due to an
sp
2
C
¬
H bending vibration will be present for
the terminal alkene and absent for the internal alkene.
46.
a.
If the reaction had occurred, the intensity of the absorption bands at 1700 cm
-
1
(due to the carbonyl
group) and at 2700 cm
-
1
(due to the aldehyde C
¬
H bond) of the reactant would have decreased. If
all the aldehyde had reacted, these absorption bands would have disappeared.
b.
If all the NH
2
NH
2
had been removed, there would be no N
¬
H absorption at 3400 cm
-
1
.
47.
If the force constants are approximately the same, the lighter atoms absorb at higher frequencies.
C
¬
C
7
C
¬
N
7
C
¬
O