Bruice - Chimica Organica

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Chapter 13 441
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26. Ethanol dissolved in carbon disulfide shows an oxygen–hydrogen stretch at a larger wavenumber. There is
extensive hydrogen bonding in the undiluted alcohol and an oxygen–hydrogen bond is easier to stretch if it
is hydrogen bonded. Therefore, the O ¬ H stretch will be at a smaller wavenumber.
27. The absorption band at 1100 cm- 1 would be less intense if it were due to a C ¬ N bond because a smaller
change in dipole moment is associated with the stretch of a C ¬ N bond compared to the change in dipole
moment associated with the stretch of a C ¬ O bond.
28. a. The absorption band at 1700 cm - 1 indicates that the compound has a carbonyl group.
The absence of an absorption band at 3300 cm -1 indicates that the compound is not a carboxylic acid.
The absence of an absorption band at 2700 cm -1 indicates that the compound is not an aldehyde.
The absence of an absorption band at 1100 cm - 1 indicates that the compound is not an ester or an
amide. The compound, therefore, must be a ketone.
b. The absence of an absorption band at 3400 cm- 1 indicates that the compound does not have an
N ¬ H bond.
The absence of absorption between 2260 - 2220 cm - 1 indicates that the compound does not have
a C ‚ N bond.
The absence of a carbonyl absorption band between 1700 cm -1 and 1600 cm- 1 indicates that the compound
is not an amide. The compound, therefore, must be a tertiary amine or a quaternary ammonium salt.
29. a. An aldehyde would show absorption bands at 2820 and 2720 cm - 1. A ketone would not have these
absorption bands.
b. An open-chain ketone would have a methyl substituent and, therefore, an absorption band at
1385–1365 cm - 1 that a cyclic ketone would not have.
c. Cyclohexene would show an sp3 C ¬ H stretch slightly to the right of 3000 cm- 1. Benzene would not
show an absorption band in this region.
d. The cis isomer would show a carbon–hydrogen bending vibration at 730–675 cm- 1, whereas the trans
isomer would show a carbon–hydrogen bending vibration at 960–980 cm - 1.
e. Cyclohexene would show a carbon–carbon double-bond stretching vibration at 1680–1600 cm - 1 and
an sp2 carbon–hydrogen stretching vibration at 3100–3020 cm- 1. Cyclohexane would not show these
absorption bands.
f. A primary amine would show a nitrogen–hydrogen stretch at 3500–3300 cm - 1, and a tertiary amine
would not have this absorption band.
30. a. An absorption band at 1150–1050 cm -1 due to a C ¬ O stretching vibration would be present for the
ether and absent for the alkane.
b. An absorption band at 3300–2500 cm -1 due to an O ¬ H stretching vibration would be present for the
carboxylic acid and absent for the ester.
c. An absorption band at 1385–1365 cm - 1 due to C ¬ H bending vibrations of a methyl group would be
present for methylcyclohexane and absent for cyclohexane.
d. Only the terminal alkyne would show an absorption band at 3300 cm- 1 due to an sp C ¬ H stretching
vibration.