Answers and Explanations

DISCRETE PRACTICE QUESTIONS

1 . A

Infrared    spectroscopy    is  most    useful  for distinguishing  between different   functional  groups.

Almost  all organic compounds   have    C–H bonds,  choice  (B),    so  except  for fingerprinting  a

compound,   these   absorptions are not useful. Little  information about   the optical properties  of  a

compound,   such    as  choices (C) and (D),    can be  obtained    by  IR  spectroscopy.

2 . C
Because molecular oxygen is homonuclear (composed of only one element) and diatomic, there
is no net change in its dipole moment during vibration or rotation; in other words, the compound
does not absorb in a measurable way in the infrared region. IR spectroscopy is based on the
principle that, when the molecule vibrates or rotates, there is a change in dipole moment. Choice
(A) is incorrect because oxygen does have molecular motions; they are just not detectable in IR
spectroscopy. Choice (B) is incorrect because it is possible to record the IR of a gaseous molecule
as long as it shows a change in its dipole moment when it vibrates. Choice (D) is incorrect
because lone pairs do not have an effect on the ability to generate an IR spectrum of a
compound.

3 . D
In this reaction, the functional group is changing from a hydroxyl to an aldehyde. This means that
a sharp peak will appear around 1750 cm–1, which corresponds to the carbonyl functionality.
Choice (C) is the opposite of what occurs; the reaction will be characterized by the disappearance
of the O–H peak at 3100 to 3500 cm–1, not its appearance. Comparing the fingerprint regions, as
in choice (A), will provide evidence that a reaction is occurring, but is not as useful for knowing
that the reaction that occurred was indeed the one that was desired.

4 . A
The peak at 9.5 ppm corresponds to an aldehydic proton. This signal lies downfield because the