Q: Which of the following is a correct formula for calculating the degree of unsaturation (hydrogen deficiency index, HDI) for a molecule with the formula C$$_xH$_yN$_zO$_wX$_q ($$where X = halogen)?

$$\text{HDI} = \dfrac{2x + 2 + z - y - q}{2}$$

Question 1

Which of the following statements best explains why the IR stretching frequency of a C≡C bond is higher than that of a C=C bond?

Accepted Answer

A triple bond is stronger, leading to a higher stretching frequency according to $$
u = \dfrac{1}{2\pi c} \sqrt{\dfrac{k}{\mu}}$$, where $$k is $$the bond force constant.

Question 2

A compound shows a strong, broad IR absorption centered at approximately 3300 cm$$^{-1}$$ and a sharp absorption at 1700 cm$$^{-1}. $$Which functional groups are most likely present?

Accepted Answer

Carboxylic acid and carbonyl

Question 3

Which of the following best describes the 'fingerprint region' in an IR spectrum?

Accepted Answer

The region below 1500 cm$$^{-1}$$, which contains many unique absorption patterns for individual molecules

Question 4

A mass spectrum of an unknown compound shows a molecular ion (M$$^+) $$peak at $$m/z = 86$$ and a significant (M+2)$$^+$$ peak with nearly equal intensity. What does this suggest about the compound?

Accepted Answer

It contains a bromine atom.

Question 5

Which of the following IR absorptions would you expect to be absent in the spectrum of a symmetrical alkyne such as 2-butyne?

Accepted Answer

A strong absorption near 3300 cm$$^{-1}$$

Question 6

A compound with the formula C$$_5H$_{10}O $$shows a strong IR absorption at 1715 cm$$^{-1}$$ and no significant absorption above 3000 cm$$^{-1}. $$Which structure is most consistent with this data?

Accepted Answer

A ketone

Question 7

Which of the following best explains why the IR absorption for the O–H stretch in carboxylic acids is broader and shifted to lower wavenumber compared to alcohols?

Accepted Answer

Carboxylic acids have more extensive hydrogen bonding, which weakens the O–H bond and lowers the stretching frequency.

Question 8

A mass spectrum of an unknown organic compound shows a molecular ion (M$$^+) $$peak at $$m/z = 58$$ and a base peak at $$m/z = 43. $$Which of the following is the most likely explanation for the base peak?

Accepted Answer

Loss of an ethyl group to form a stabilized acylium ion (common in ketones)

Question 9

Which of the following statements about the relationship between the number of nitrogen atoms and the molecular ion (M$$^+) $$peak in mass spectrometry is correct?

Accepted Answer

An odd-massed M$$^+$$ peak indicates an odd number of nitrogen atoms.

Question 10

Which of the following best describes the effect of resonance on the IR stretching frequency of a carbonyl group?

Accepted Answer

Resonance delocalization weakens the C=O bond, decreasing the stretching frequency.

Question 11

A compound with the formula C$$_4H$_8O $$shows a strong IR absorption at 1050 cm$$^{-1}$$ and no absorption near 1700 cm$$^{-1}. $$Which functional group is most likely present?

Accepted Answer

Ether

Question 12

Which of the following is a correct formula for calculating the degree of unsaturation (hydrogen deficiency index, HDI) for a molecule with the formula C$$_xH$_yN$_zO$_wX$_q ($$where X = halogen)?

Accepted Answer

$$	ext{HDI} = \dfrac{2x + 2 + z - y - q}{2}$$

Question 13

Which of the following best explains why the IR absorption for a C–H bond in an sp-hybridized carbon appears at a higher wavenumber than that of an sp$$^3-$$hybridized carbon?

Accepted Answer

The sp-hybridized C–H bond is stronger due to greater s-character, resulting in a higher stretching frequency.

Question 14

A mass spectrum of an unknown compound shows a molecular ion (M$$^+) $$peak at $$m/z = 72$$ and a significant (M+1)$$^+$$ peak at $$m/z = 73$$ that is about 6% the height of the M$$^+$$ peak. What can you deduce about the number of carbon atoms in the molecule?

Accepted Answer

There are approximately 6 carbon atoms, since the (M+1)$$^+$$ peak is about 1% per carbon.

Question 15

Which of the following is a common fragmentation pattern for alcohols in mass spectrometry?

Accepted Answer

Alpha cleavage and dehydration (loss of H$$_2O)$$

Question 16

Which of the following best describes the base peak in a mass spectrum?

Accepted Answer

The tallest peak, representing the most abundant ion fragment

Question 17

Which of the following is a key advantage of high-resolution mass spectrometry compared to low-resolution mass spectrometry?

Accepted Answer

It can distinguish between compounds with the same nominal mass but different exact masses.

Question 18

Which of the following best explains why a completely symmetrical bond (such as in O$$_2 or N$_2) $$does not show an IR absorption?

Accepted Answer

Symmetrical bonds do not have a changing dipole moment during vibration, so they do not interact with IR radiation.

Question 19

A sample is analyzed by gas chromatography-mass spectrometry (GC-MS). What is the primary advantage of using GC-MS for analyzing a mixture of compounds?

Accepted Answer

GC-MS can separate the mixture and provide mass spectra for each component, allowing identification of individual compounds.

Chapter 14: Infrared Spectroscopy and Mass Spectrometry – Study Notes

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