Q: Given the IR spectrum of hexane, which region would you expect to see the strongest absorption due to saturated C–H stretches?

Below $$3000\ \text{cm}^{-1}$$, specifically $$2850\ \text{cm}^{-1} to$$ $$2990\ \text{cm}^{-1}$$

Q: Which IR absorption is characteristic of a carbonyl (C=O) stretch in ketones and aldehydes?

$$1710\ \text{cm}^{-1} to$$ $$1750\ \text{cm}^{-1}$$

Q: Which of the following is true about the IR spectrum of an aromatic compound such as benzene?

It shows unsaturated C–H stretches above $$3000\ \text{cm}^{-1}.$$

Q: Which of the following best describes the IR absorption for a terminal alkyne?

Strong absorption at $$2100\ \text{cm}^{-1}$$ and a C–H stretch above $$3000\ \text{cm}^{-1}$$

Q: Which of the following is a correct statement about the IR spectrum of a tertiary amine ($$3^{\circ}$$)?

It shows no absorption above $$3000\ \text{cm}^{-1}$$ due to the absence of N–H bonds.

Question 1

Which of the following statements best explains why a molecule must have a change in dipole moment to absorb infrared (IR) radiation?

Accepted Answer

A change in dipole moment during vibration allows the molecule to interact with the oscillating electric field of IR light, leading to absorption.

Question 2

Given the IR spectrum of hexane, which region would you expect to see the strongest absorption due to saturated C–H stretches?

Accepted Answer

Below $$3000\ 	ext{cm}^{-1}$$, specifically $$2850\ 	ext{cm}^{-1} to$$ $$2990\ 	ext{cm}^{-1}$$

Question 3

A compound shows a strong absorption at $$2100\ 	ext{cm}^{-1} in $$its IR spectrum but no absorption above $$3000\ 	ext{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Internal alkyne

Question 4

Which IR absorption is characteristic of a carbonyl (C=O) stretch in ketones and aldehydes?

Accepted Answer

$$1710\ 	ext{cm}^{-1} to$$ $$1750\ 	ext{cm}^{-1}$$

Question 5

An unknown compound with formula $$\mathrm{C_8H_{16}}$$ shows a strong absorption just above $$3000\ 	ext{cm}^{-1}$$ and a peak at $$1600\ 	ext{cm}^{-1}. $$What is the most likely structure?

Accepted Answer

An alkene

Question 6

Which of the following functional groups would show a very broad absorption between $$2500\ 	ext{cm}^{-1}$$ and $$3600\ 	ext{cm}^{-1} in $$its IR spectrum?

Accepted Answer

Carboxylic acid

Question 7

A molecule displays two absorptions above $$3000\ 	ext{cm}^{-1} in $$its IR spectrum. Which type of amine is most likely present?

Accepted Answer

Primary amine ($$1^{\circ})$$

Question 8

Which equation correctly relates the frequency $$
u of $$electromagnetic radiation to its wavelength $$\lambda$$ and the speed of light $$c$$?

Accepted Answer

$$
u = \dfrac{c}{\lambda}$$

Question 9

An IR spectrum shows a strong absorption at $$1730\ 	ext{cm}^{-1}$$ and another at $$1710\ 	ext{cm}^{-1}. $$Which compound is most likely to show both these peaks?

Accepted Answer

Methyl p-acetylbenzoate (contains both ester and ketone groups)

Question 10

Which of the following is true about the IR spectrum of an aromatic compound such as benzene?

Accepted Answer

It shows unsaturated C–H stretches above $$3000\ 	ext{cm}^{-1}.$$

Question 11

A student analyzes an IR spectrum and observes a strong, sharp absorption at $$2250\ 	ext{cm}^{-1}. $$Which functional group is most likely responsible?

Accepted Answer

Nitrile ($$\mathrm{C\equiv N})$$

Question 12

Which of the following would NOT be expected to show a peak above $$3000\ 	ext{cm}^{-1} in $$its IR spectrum?

Accepted Answer

Internal alkyne

Question 13

A compound with the formula $$\mathrm{C_6H_{12}}$$ shows no absorption at $$1600\ 	ext{cm}^{-1}$$ but does show a peak above $$3000\ 	ext{cm}^{-1}. $$What does this suggest about the structure?

Accepted Answer

It is a symmetric alkene, such as trans-3-hexene.

Question 14

Which of the following best describes the IR absorption for a terminal alkyne?

Accepted Answer

Strong absorption at $$2100\ 	ext{cm}^{-1}$$ and a C–H stretch above $$3000\ 	ext{cm}^{-1}$$

Question 15

Which functional group is most likely present if an IR spectrum shows a very broad absorption above $$3000\ 	ext{cm}^{-1}$$ and no C=O stretch?

Accepted Answer

Alcohol

Question 16

A molecule shows a strong absorption at $$1470\ 	ext{cm}^{-1}$$ and $$1350\ 	ext{cm}^{-1} in $$its IR spectrum. Which type of vibration do these peaks most likely correspond to?

Accepted Answer

C–H scissoring and bending

Question 17

Which of the following compounds would show both saturated and unsaturated C–H stretches in its IR spectrum?

Accepted Answer

1-Heptene

Question 18

If an IR spectrum shows a strong absorption at $$1738\ 	ext{cm}^{-1}$$, which functional group is most likely present?

Accepted Answer

Ester

Question 19

Which of the following is a correct statement about the IR spectrum of a tertiary amine ($$3^{\circ}$$)?

Accepted Answer

It shows no absorption above $$3000\ 	ext{cm}^{-1}$$ due to the absence of N–H bonds.

Question 20

A sample of caffeine shows two strong C=O stretches at $$1700\ 	ext{cm}^{-1}$$ and $$1659\ 	ext{cm}^{-1}. $$What does this indicate about its structure?

Accepted Answer

It contains two different types of carbonyl groups.

Infrared (IR) Spectroscopy: Principles and Applications in Organic Chemistry

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