Question 1

Which of the following statements best explains why IR spectroscopy is useful for identifying functional groups in organic molecules?

Accepted Answer

IR spectroscopy measures bond vibration frequencies, which are characteristic of specific functional groups.

Question 2

A molecule absorbs IR radiation at a wavenumber of $$3300\ \mathrm{cm}^{-1}. $$Which bond is most likely responsible for this absorption?

Accepted Answer

sp C-H stretch

Question 3

Which equation correctly relates the energy of a photon to its frequency?

Accepted Answer

$$E = h v$$

Question 4

A sample shows a strong, broad IR absorption from $$3200 to$$ $$3400\ \mathrm{cm}^{-1}$$ and a band at $$1050-1260\ \mathrm{cm}^{-1}. $$What functional group is most likely present?

Accepted Answer

Alcohol

Question 5

Which of the following changes would increase the stretching frequency ($$	ilde{
u}) of a $$bond according to Hooke's law?

Accepted Answer

Increasing the bond strength

Question 6

A compound with the formula $$C_5H_{10}O$$ shows a strong IR absorption at $$1710\ \mathrm{cm}^{-1}$$ and two peaks at $$2700$$ and $$2800\ \mathrm{cm}^{-1}. $$What is the most likely functional group?

Accepted Answer

Aldehyde

Question 7

Which region of the IR spectrum is known as the 'fingerprint region' and why is it important?

Accepted Answer

$$600-1400\ \mathrm{cm}^{-1}$$; it contains complex vibrations unique to each molecule.

Question 8

Which of the following best describes why a non-polar bond in a symmetrical molecule is often IR-inactive?

Accepted Answer

It does not produce a change in dipole moment during vibration.

Question 9

A molecule displays a strong IR absorption at $$2200\ \mathrm{cm}^{-1}$$ but no significant absorption at $$3300\ \mathrm{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Nitrile

Question 10

Which of the following statements about the y-axis of an IR spectrum is correct?

Accepted Answer

It is plotted as percent transmittance, which is the ratio of transmitted to incident IR radiation.

Question 11

Which of the following would cause the C=O stretching frequency in an aldehyde to decrease from its typical value?

Accepted Answer

Conjugation with a C=C bond

Question 12

A student observes a strong, broad IR absorption from $$2500 to$$ $$3500\ \mathrm{cm}^{-1}$$ and a strong absorption at $$1715\ \mathrm{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Carboxylic acid

Question 13

Which of the following best explains why the IR spectrum of an unknown compound can be used to confirm its identity?

Accepted Answer

No two molecules (except enantiomers) have exactly the same IR spectrum.

Question 14

Which of the following bonds would absorb at the lowest wavenumber in an IR spectrum?

Accepted Answer

C-C

Question 15

A molecule with the formula $$C_3H_7NO_2$$ shows a strong IR absorption at $$1550\ \mathrm{cm}^{-1}$$ and a broad absorption at $$3300\ \mathrm{cm}^{-1}$$ with two sharp spikes. What functional group is most likely present?

Accepted Answer

Primary amine

Question 16

Which of the following is a limitation of IR spectroscopy?

Accepted Answer

It cannot determine the complete structure of a molecule by itself.

Question 17

Which of the following best describes the effect of increasing atomic mass on the stretching frequency of a bond?

Accepted Answer

Stretching frequency decreases with increasing atomic mass.

Question 18

Which of the following IR absorptions would you expect for a conjugated ketone?

Accepted Answer

$$\sim 1680\ \mathrm{cm}^{-1}$$

Question 19

Which of the following is true about the IR absorption of a tertiary amine ($$R_3N$$)?

Accepted Answer

It shows no significant absorption in the N-H stretching region.

Infrared Spectroscopy: Principles and Applications in Organic Chemistry

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