Q: Which of the following best describes the McLafferty rearrangement observed in mass spectrometry?

A rearrangement in which a $$eta-$$hydrogen is transferred to a carbonyl oxygen, resulting in cleavage and formation of an alkene and an enol ion.

Question 1

Which of the following statements best explains why tetramethylsilane (TMS) is used as a reference compound in $$^1H $$NMR spectroscopy?

Accepted Answer

TMS has a single, sharp peak upfield from most organic protons and is chemically inert.

Question 2

A compound with the formula $$C_3H_8O$$ shows two signals in its $$^1H $$NMR spectrum: a triplet at $$1.0$$ ppm and a quartet at $$3.5$$ ppm. What is the most likely structure of the compound?

Accepted Answer

Ethanol

Question 3

Which of the following nuclei is NOT NMR-active?

Accepted Answer

$$^{12}C$$

Question 4

In $$^1H $$NMR, what is the effect of an electronegative atom (such as oxygen or chlorine) attached to a carbon bearing a proton?

Accepted Answer

It deshields the proton, causing its signal to move downfield (higher ppm).

Question 5

A $$^1H $$NMR spectrum of a compound shows a singlet integrating to 9 hydrogens at $$1.2$$ ppm and a singlet integrating to 1 hydrogen at $$2.1$$ ppm. Which structure is most consistent with this data?

Accepted Answer

tert-Butyl acetate

Question 6

Which of the following best describes the N+1 rule in $$^1H $$NMR spectroscopy?

Accepted Answer

A proton coupled to N equivalent protons will appear as N+1 peaks.

Question 7

Which of the following splitting patterns would you expect for the methylene ($$CH_2) $$protons in ethyl bromide ($$CH_3CH_2Br) in $$its $$^1H $$NMR spectrum?

Accepted Answer

Quartet

Question 8

A mass spectrum of an unknown compound shows a molecular ion 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 $$C_3H_7^+$$ ion.

Question 9

Which of the following best explains why the molecular ion peak in the mass spectrum of alcohols is often weak or absent?

Accepted Answer

Alcohols readily lose water ($$H_2O) $$during ionization, resulting in a lower $$m/z$$ fragment.

Question 10

Which of the following statements about the Fourier transform in NMR spectroscopy is correct?

Accepted Answer

It converts the free induction decay (FID) from the time domain to the frequency domain.

Question 11

A compound with the formula $$C_6H_5CH_2CH_3 ($$ethylbenzene) shows a triplet at $$1.2$$ ppm, a quartet at $$2.6$$ ppm, and a multiplet at $$7.2$$ ppm in its $$^1H $$NMR spectrum. Which protons correspond to the quartet?

Accepted Answer

Methylene protons adjacent to the aromatic ring

Question 12

Which of the following best describes the McLafferty rearrangement observed in mass spectrometry?

Accepted Answer

A rearrangement in which a $$eta-$$hydrogen is transferred to a carbonyl oxygen, resulting in cleavage and formation of an alkene and an enol ion.

Question 13

Which of the following is a likely fragmentation pattern for a straight-chain alkane in mass spectrometry?

Accepted Answer

Cleavage at the branch point to form a more stable secondary or tertiary carbocation.

Question 14

Which of the following statements about coupling constants ($$J$$ values) in $$^1H $$NMR is correct?

Accepted Answer

The coupling constant for trans protons on a double bond is typically larger than for cis protons.

Question 15

A $$^1H $$NMR spectrum of a compound shows two doublets, each integrating to one proton, at $$6.5$$ ppm and $$7.5$$ ppm. Which of the following best explains this observation?

Accepted Answer

The compound contains two nonequivalent, mutually coupled protons, such as in a disubstituted alkene.

Question 16

Which of the following best describes the difference between homotopic, enantiotopic, and diastereotopic hydrogens in NMR spectroscopy?

Accepted Answer

Homotopic hydrogens are always chemically equivalent, enantiotopic hydrogens are equivalent in achiral environments, and diastereotopic hydrogens are always chemically non-equivalent.

Question 17

Which of the following is a common reason for observing broad $$^1H $$NMR signals for hydrogens attached to heteroatoms (such as $$OH or$$ $$NH$$)?

Accepted Answer

Rapid exchange of protons between molecules leads to signal broadening.

Question 18

A mass spectrum of a compound containing chlorine shows two molecular ion peaks at $$m/z = 78$$ and $$m/z = 80 in a 3$$:1 ratio. What does this indicate about the isotopic composition of chlorine?

Accepted Answer

Chlorine has two major isotopes, $$^{35}Cl$$ and $$^{37}Cl$$, in a 3:1 natural abundance ratio.

Question 19

Which of the following best describes the process of integration in $$^1H $$NMR spectroscopy?

Accepted Answer

Integration measures the area under each signal, corresponding to the relative number of protons producing that signal.

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

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