A sharp C=O stretch at $$1710\ \text{cm}^{-1}$$ and a broad O-H stretch from $$2500 to$$ $$3500\ \text{cm}^{-1}$$

The compound contains a mixture of $$^{79}\text{Br}$$ and $$^{81}\text{Br}$$ isotopes.

It is found between $$600$$ and $$1400\ \text{cm}^{-1}$$ and contains complex vibrations unique to each molecule.

A sharp absorption at $$3300\ \text{cm}^{-1}$$

Question 1

Which of the following statements best describes the main purpose of infrared (IR) spectroscopy in organic chemistry?

Accepted Answer

It measures bond vibration frequencies to identify functional groups.

Question 2

A molecule shows a strong, sharp absorption at $$1718\ 	ext{cm}^{-1} in $$its IR spectrum. Which functional group is most likely present?

Accepted Answer

Ketone

Question 3

Which equation correctly relates the speed of light ($$c$$), wavelength ($$\lambda$$), and frequency ($$
u$$)?

Accepted Answer

$$c = \lambda 
u$$

Question 4

An IR spectrum shows a broad absorption centered around $$3300\ 	ext{cm}^{-1}$$ with a rounded tip. What is the most likely functional group?

Accepted Answer

Alcohol

Question 5

Which of the following IR absorptions would you expect for a carboxylic acid?

Accepted Answer

A sharp C=O stretch at $$1710\ 	ext{cm}^{-1}$$ and a broad O-H stretch from $$2500 to$$ $$3500\ 	ext{cm}^{-1}$$

Question 6

Which of the following statements about mass spectrometry (MS) is correct?

Accepted Answer

It fragments molecules and measures the mass of the fragments.

Question 7

A mass spectrum of a compound containing bromine shows two peaks of equal height at $$M^*$$ and $$M^*+2. $$What does this indicate?

Accepted Answer

The compound contains a mixture of $$^{79}	ext{Br}$$ and $$^{81}	ext{Br}$$ isotopes.

Question 8

Which of the following best explains why conjugation lowers the IR stretching frequency of a C=C bond?

Accepted Answer

Conjugation decreases the bond energy, making the bond easier to stretch.

Question 9

Which of the following is true regarding the fingerprint region in IR spectroscopy?

Accepted Answer

It is found between $$600$$ and $$1400\ 	ext{cm}^{-1}$$ and contains complex vibrations unique to each molecule.

Question 10

A molecule shows a strong IR absorption at $$2200\ 	ext{cm}^{-1}. $$Which functional group is most likely present?

Accepted Answer

Nitrile

Question 11

Which of the following is a correct statement about the effect of atomic mass and bond energy on IR stretching frequency?

Accepted Answer

Frequency decreases with increasing atomic mass and increases with increasing bond energy.

Question 12

Which IR absorption would you expect for an sp-hybridized C-H bond?

Accepted Answer

A sharp absorption at $$3300\ 	ext{cm}^{-1}$$

Question 13

Which of the following is a limitation of IR spectroscopy?

Accepted Answer

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

Question 14

A mass spectrum of a compound shows a base peak at $$m/z = 43$$ and a molecular ion peak at $$m/z = 86. $$What does the base peak represent?

Accepted Answer

The most abundant fragment ion.

Question 15

Which of the following best describes the fragmentation pattern of branched alkanes in mass spectrometry?

Accepted Answer

The most stable carbocation fragments form in greater abundance.

Question 16

Which of the following is true about the FT-IR spectrometer compared to a traditional IR spectrometer?

Accepted Answer

It has better sensitivity and completes scans more quickly.

Question 17

A primary amine shows which characteristic IR absorption?

Accepted Answer

Broad absorption with two sharp spikes around $$3300\ 	ext{cm}^{-1}$$

Question 18

Which of the following best explains why the O-H stretch in carboxylic acids is broader than in alcohols?

Accepted Answer

Carboxylic acids have stronger and more diverse hydrogen bonding interactions.

Question 19

Which of the following is a correct statement about the molecular ion (parent peak) in mass spectrometry?

Accepted Answer

It corresponds to the molecular weight of the original molecule.

Infrared Spectroscopy and Mass Spectrometry: Principles and Applications in Organic Chemistry

Study Guide - Practice Questions