1.7.6: Mass Spectrometry- Problems
- Page ID
- 358962
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)What is the m/z of the molecular ion in this mass spectrum?
- Hint
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Some compounds can lose a hydrogen easily, for example CH<sub>3</sub>OH (methanol) will lose an H to form CH<sub>2</sub>=OH<sup>+</sup>. In deciding whether 45 or 46 is the MW of this compound, ask whether the isotope ratio of peaks 45/46 makes sense? How about peaks 46/47?
- Answer
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The molecular ion peak has a m/z of 46.
The mass spectrum of which of the following compounds is in Exercise \(\PageIndex{1}\)?
- formic acid (aka methanoic acid)
- 1-propanol
- ethanol
- methanol
- 2-propanol
- Hint
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Alpha cleavage (the C-C bond next to the oxygen) occurs in alcohols. The fragment at 31 does not contain just carbon and hydrogen; it is part of a series (31, 45, 59) that indicates the presence of oxygen.
- Answer
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ethanol
Identify the molecular ion in the mass spectrum shown below.
- Hint
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Addition of an alcohol group to a saturated alkane reduces the intensity of the molecular ion (in fact, it may not be observed!).
- Answer
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88
An alkyl ion series is a pattern formed by alkyl groups. Identify the m/z values in Exercise \(\PageIndex{3}\) for peaks corresponding to an alkyl ion.
- Hint
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Alkyl ion series will follow a pattern formed by CH<sub>3</sub>(CH<sub>2</sub>)<sub>n</sub> where n = 0, 1, 2, 3, ....
- Answer
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15 (not shown and quite small), 29, 43, 57
The loss of an alkyl group also forms a distinct pattern. Identify the m/z values in Exercise \(\PageIndex{3}\) for peaks corresponding to loss of an alkyl ion.
- Hint
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Alkyl ion series will follow a pattern formed by CH<sub>3</sub>(CH<sub>2</sub>)<sub>n</sub> where n = 0, 1, 2, 3, .... so there loss whould follow a pattern of mass loss from the molecular ion.
- Answer
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73, 59, 45, 31
Which of the following compounds is most likely the compound in Exercise \(\PageIndex{3}\)?
2-methyl-2-propanol
1-butanol
2-butanol
1-pentanol
2-methyl-1-propanol
- Answer
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1-pentanol
Although this could be deduced solely on the molar mass, it is the only compound that would likely see any loss of a butyl group.
The compound below has a degree of unsaturation of 4 and contains two halogen atoms. Identify the molecular ion in the mass spectrum shown below.
- Hint
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You may want to think about the mass of halogen atoms and the relative ratios of the isotopes to help narrow down the formula. Remember, there are two halogen atoms which will impact the expected distribution of the peaks.
- Answer
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146
Which compound is most likely present in Exercise \(\PageIndex{7}\)?
difluorobenzene
dichlorbenzene
dibromobenzene
diiodobenzene
- Answer
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dichlorobenzene
Does the group of peaks around m/z = 100 in in Exercise \(\PageIndex{7}\) still contain a halogen atom? Why or why not?
- Hint
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Think about the halogen that is present and the distribution of the isotopes.
- Answer
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Yes, the 3:1 ratio common for chlorine is still present. One could also consider the mass of the fragment.