Question 1

Which of the following m/z values is the base peak for benzyl alcohol?

Accepted Answer

The base peak is the most intense peak in the mass spectrum. In the case of benzyl alcohol, the most intense peak is at m/z 91, which corresponds to the benzyl cation (C7H7+). Therefore, the correct answer is C (91).

Question 2

The mass spectra of alcohols often fail to exhibit detectable M peaks but instead show relatively large ________ peaks.

Accepted Answer

The answer of The mass spectra of alcohols often fail...

Question 3

Provide the structure of the species which results when the molecular ion of 4-heptanone undergoes fragmentation via a McLafferty rearrangement.

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Question 4

In the mass spectrum of bromoform, CHBr₃, molecular ion peaks appear at m/z 250, 251, 252, and 253? What is the ratio of the intensities of these peaks?

Accepted Answer

The molecular ion peak corresponds to the ion with the highest mass-to-charge ratio (m/z) that results from the molecule's intact molecular ion breaking into smaller fragments. In this case, the molecular ion peak appears at m/z 251, which suggests that the molecule has lost a single electron. However, there are also peaks at m/z 250, 252, and 253, which suggest the presence of isotopes of bromine (Br) and carbon (C) in the molecule. The ratio of isotopes in naturally occurring organic molecules follows a predictable pattern based on their abundance, and thus we can use this to determine the expected ratio of these peaks. The isotopes of bromine that contribute to these peaks are 79Br and 81Br, which have natural abundances of 50.69% and 49.31%, respectively. Likewise, the isotopes of carbon that contribute to these peaks are 12C, 13C, and 14C, which have natural abundances of 98.93%, 1.07%, and trace amounts, respectively. Using these values, we can calculate the expected ratio of the peaks by multiplying the abundance of each isotope that contributes to each peak. 
For the peak at m/z 250, there is only one possible combination of isotopes that could contribute: 79Br and 12C. Therefore, the expected ratio is (0.5069)(0.9893) = 0.5007. 
For the peak at m/z 251, the possible combinations of isotopes that could contribute are 79Br and 13C or 81Br and 12C. Therefore, the expected ratio is [(0.5069)(0.0107)] + [(0.4931)(0.9893)] = 0.4973. 
For the peak at m/z 252, the possible combinations of isotopes that could contribute are 81Br and 13C or 80Br and 12C. However, 80Br is a radioactive isotope and only present in trace amounts, so we can ignore it. Therefore, the expected ratio is [(0.4931)(0.0107)] = 0.0053. 
For the peak at m/z 253, the only combination of isotopes that could contribute is 81Br and 14C. However, 14C is a radioactive isotope and only present in trace amounts, so we can ignore it. Therefore, the expected ratio is [(0.4931)(0.0001)] = 0.0000493. 
To simplify the ratios and make them easier to compare, we can multiply each value by 10000: 
Peak at m/z 250: 5007
Peak at m/z 251: 4973
Peak at m/z 252: 53
Peak at m/z 253: 0.49
Therefore, the ratio of intensities is approximately 1:3:3:1.

Question 5

Which of the following is the base peak for the compound below?

Accepted Answer

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Question 6

Describe the molecular ion region in the mass spectrum of CH₃CH₂Br.

Accepted Answer

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Question 7

Show the m/z values of the molecular ion and 5 likely fragments for the compound ethyl ether, CH₃CH₂OCH₂CH₃

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Question 8

Which of the following is/are true about the MS of 1-bromobutane?

Accepted Answer

A) Peaks of approximately equal intensity are observed at m/z 136 and 138, which correspond to the molecular ion peak (M) and a peak with one carbon-13 atom (M+2), respectively. This indicates that 1-bromobutane is a symmetrical compound with no significant isotopic distribution. 
B) The major fragmentation occurs by cleavage of the C-Br bond, which results in the formation of a butyl cation (C4H9+) and a bromide anion (Br-). This butyl cation can then undergo further fragmentation to produce peaks at m/z 57 and 43. 
C) The peak at m/z 43 corresponds to the loss of a methyl group from the butyl cation, which is a common fragmentation pattern for alkyl cations. However, this is not the most intense peak in the spectrum. The most intense peak is actually the peak at m/z 57, which corresponds to the butyl cation.

Question 9

In the mass spectrum of 2-chloropropane, what is the m/z of the molecular ion peak of greatest intensity?

Accepted Answer

The molecular weight of 2-chloropropane is 78 g/mol. The molecular ion peak in a mass spectrum represents the ion with the same mass as the molecular weight of the compound. Therefore, the m/z of the molecular ion peak in the mass spectrum of 2-chloropropane is 78.

Question 10

Which compound's mass spectrum shows peaks at M, M+2, and M+4 whose abundances are in a ratio of 1:2:1?

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Question 11

Provide the structure of the two ions which result when the molecular ion of 2-methoxypentane undergoes fragmentation by

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Question 12

Which of the following structures will give a base peak of 43 in mass spectrometry?

Accepted Answer

The only structure with a base peak of 43 is option C. None of the other structures have a base peak at 43.

Question 13

In the mass spectrum of 1,2-dichloroethane, what is the m/z of the molecular ion peak of greatest intensity?

Accepted Answer

The molecular weight of 1,2-dichloroethane is 98 g/mol (2 x 35.5 for chlorine atoms + 2 x 12.01 for carbon and 2 x 1.01 for hydrogen atoms). Therefore, the molecular ion peak of greatest intensity should correspond to the m/z ratio of 98. None of the other options are a possible molecular ion peak for this compound.

Question 14

Which of the following m/z values is least intense in the mass spectrum of 1-chloropropane?

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Question 15

What technique can be used to determine the molecular formula of a compound?

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Question 16

The mass spectrum of an unknown compound has a molecular ion peak with a relative abundance of 43.27% and an M + 1 peak with a relative abundance of 3.81%. How many carbon atoms are in the compound?

Accepted Answer

The answer of The mass spectrum of an unknown compound...

Question 17

A compound shows a molecular ion peak at m/z 167. The relative intensities of the M and M+1 peaks are 50.0 and 4.4 respectively. How many carbons are in each molecule of the compound?

Accepted Answer

The answer of A compound shows a molecular ion peak...

Question 18

Which of the following compounds exhibits the pattern of m/z values shown below? 41, 43, 57, 87, 101, 116

Accepted Answer

The pattern of m/z values suggests the presence of a propyl group (m/z = 43), an ethyl group (m/z = 57), and a tert-butyl group (m/z = 101). The only compound among the options that contains all three groups is sec-butyl isopropyl ether.

Question 19

An acylium ion is often produced as a fragment in the MS of which of the following class of organic compounds?

Accepted Answer

Acylium ions are often produced as a fragment in the MS of ketones due to the loss of a carbonyl group, leaving behind a carbocation.

Question 20

Is it possible to have an M + 2 peak in mass spectrometry? Explain.

Accepted Answer

The answer of Is it possible to have an M...

Quiz 14: Mass Spectrometry, Infrared Spectroscopy, and Uvvis Spectroscopy