Mr Cole Chemistry

Mass Spec Questions

Mass Spectrometry

Organic Analysis Worksheet

Spectrum Analysis

1. An alcohol has the following mass spectrum.

Mass spectrum of an alcohol
a) What is the relative molecular mass of the compound? Explain how you chose that value.
Answer

60

b) What causes the peak at 31?
Answer Fragment causing peak at 31
c) Draw the structure of the compound and justify your answer. Remember to consider isomers in your justification.
Answer

It has the correct Mr and it has a peak at 31. Propan-2-ol would have the same Mr but wouldn’t have a CH2OH fragment, so wouldn’t have a peak at 31.

Structure of the compound

2. A nitrogen containing compound has the following mass spectrum.

Mass spectrum of nitrogen compound
a) What is the relative molecular mass of the compound. You may assume that there are no carbon-13 atoms in the sample.
Answer

69

b) What is the peak at 15 likely to be?
Answer

CH3

c) What is the peak at 54 likely to be?
Answer Fragment at 54
d) Draw the structure of the molecule.
Answer Structure of the molecule

3. A molecule is formed from oxidising an alcohol. It has the following spectrum.

Mass spectrum of oxidised alcohol
a) What is the Mr of the compound?
Answer

72

b) What are the possible functional groups that this molecule could have.
Answer

aldehyde, ketone, carboxylic acid

c) What is the peak at 29 likely to be?
Answer

CH2CH3

d) What is the peak at 43 likely to be?
Answer
Fragment 1 OR Fragment 2
e) What are the two possible structures of the compound based on this?
Answer
Structure 1 OR Structure 2
f) What further test(s) could you do to determine which of these structures it is.
Answer

Tollens’ reagent or Fehlings solution. Propanal would provide a positive result (silver mirror for Tollens’; dark red precipitate for Fehlings) whereas propanone would not.

4. The following four compounds all have a relative molecular mass of 78. However, when you use high-resolution mass spectrometry, you can tell them apart.

Compound Relative Molecular Mass
1-chloropropane 78.44
2-chloropropane 78.44
Benzene 78.11
2-chlorooxirane 78.40
a) Explain why it is impossible to tell 1-chloropropane and 2-chloropropane apart using mass spectrometry, no matter the resolution.
Answer

They are made of exactly the same atoms so their mass spectra will be identical.

b) Both 1-chloropropane and 2-chloropropane show two molecular ion peaks when analysed with a mass spectrometer. Explain why this is the case, state the M/Z of the two peaks, and state the relative intensity of the peaks.
Answer

They contain chlorine, which has two common isotopes, Cl-35 and Cl-37. There will be two peaks: 78 and 80. The peak at 78 will be three times the abundance of the peak at 80.

c) What is the relative molecular mass of the compound. You may assume that there are no carbon-13 atoms in the sample.
Answer

78.5

5. A molecule of trichloroethane has multiple peaks. Sketch a MS spectrum for this molecule.
Answer

The molecule contains 3 Chlorine atoms. We can use a probability tree to determine the ratio of peaks based on the relative abundance of 35Cl (3) and 37Cl (1).

Cl Atom 1 Cl Atom 2 Cl Atom 3 Probability m/z Start 35 3 37 1 35 3 37 1 35 3 37 1 3×3×3 = 27 132 3 3×3×1 = 9 134 1 3×1×3 = 9 134 3 3×1×1 = 3 136 1 1×3×3 = 9 134 3 1×3×1 = 3 136 1 1×1×3 = 3 136 3 1×1×1 = 1 138 1

Summary of Ratios:

  • m/z 132: 27
  • m/z 134: 9 + 9 + 9 = 27
  • m/z 136: 3 + 3 + 3 = 9
  • m/z 138: 1
132
134
136
138
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