CIE AS Chemistry (9701) exams from 2022

Revision Notes

4.1.4 Molecular Ion Peak & Fragmentation

Mass Spectrometry: Deducing Molecular Formula

  • Each peak in the mass spectrum corresponds to a certain fragment with a particular m/e value
  • The peak with the highest m/e value is the molecular ion (M+) peak which gives information about the molecular mass of the compound
  • The molecular ion is the entire molecule that has lost one electron when bombarded with a beam of electrons

Interpreting Mass Spectra equation 1

  • The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13
  • The height of the [M+1] peak for a particular ion depends on how many carbon atoms are present in that molecule; The more carbon atoms, the larger the [M+1] peak is
    • For example, the height of the [M+1] peak for an hexane (containing six carbon atoms) ion will be greater than the height of the [M+1] peak of an ethane (containing two carbon atoms) ion

Worked example: Analysing mass spectra

Analytical Techniques Worked example - Analysing mass spectra, downloadable AS & A Level Chemistry revision notes

Analytical Techniques Spec 1_Mass Spectrometry, downloadable AS & A Level Chemistry revision notes

Answer

The mass spectrum corresponds to propanal as the molecular ion peak is at m/e = 58

Propanal arises from the CH3CH2CHO+ ion which has a molecular mass of 58

Butanal arises from the CH3CH2CH2CHO+ ion which has a molecular mass of 72

Identifying Molecules using Fragmentation

  • The molecular ion peak can be used to identify the molecular mass of a compound
  • However, different compounds may have the same molecular mass
  • To further determine the structure of the unknown compound, fragmentation is used
  • Fragments may appear due to the formation of characteristic fragments or the loss of small molecules
    • For example, a peak at 29 is due to the characteristic fragment C2H5+­­
    • Loss of small molecules give rise to peaks at 18 (H2O), 28 (CO), and 44 (CO2)

Alkanes

  • Simple alkanes are fragmented in mass spectroscopy by breaking the C-C bonds
  • M/e values of some of the common alkane fragments are given in the table below

m/e values of fragments table

Analytical Techniques Table 1_Identifying Molecules using Fragmentation, downloadable AS & A Level Chemistry revision notes

Analytical Techniques Alkane Spectrum, downloadable AS & A Level Chemistry revision notes

Mass spectrum showing fragmentation of alkanes

Halogenoalkanes

  • Halogenoalkanes have often multiple peaks around the molecular ion peak
  • This is caused by the fact that there are different isotopes of the halogens

 

Analytical Techniques Halogenoalkane Spectrum, downloadable AS & A Level Chemistry revision notes

Mass spectrum showing different isotopes of the halogens in the molecular ion

Alcohols

  • Alcohols often tend to lose a water molecule giving rise to a peak at 18 below the molecular ion
  • Another common peak is found at m/e value 31 which corresponds to the CH2OH+­­ fragment
  • For example, the mass spectrum of propan-1-ol shows that the compound has fragmented in four different ways:
    • Loss of H to form a C3H7O+ fragment with m/e = 59
    • Loss of a water molecule to form a C3H6+ fragment with m/e = 42
    • Loss of a C2H5 to form a CH2OH+ fragment with m/e = 31
    • And the loss of CH2OH to form a C2H5+ fragment with m/e = 29

Analytical Techniques Alcohols Spectrum_2, downloadable AS & A Level Chemistry revision notes

Mass spectrum showing the fragmentation patterns in propan-1-ol (alcohol)

Worked example: Ion fragmentation

Analytical Techniques Worked example - Ion fragmentation, downloadable AS & A Level Chemistry revision notes

Answer

The correct answer is 4 as bromomethane (CH3Br) will fragment into 3 peaks

    • CH381Br → [CH381Br]+ + e at m/e 96
    • CH379Br → [CH379Br]+ + e at m/e 94
    • CH3Br → [CH3]+ + Br at m/e 15

The last two peaks (which correspond to the molecular ion peak) therefore are equal in size and occur at m/e values of 94 and 96

Analytical Techniques Answer Worked example - Ion fragmentation, downloadable AS & A Level Chemistry revision notes

Worked example: Alcohol fragmentation

Analytical Techniques Worked example - Alcohol fragmentation, downloadable AS & A Level Chemistry revision notes

Answer

The correct answer is 4 because a line at m/e = 43 corresponds to an ion with a mass of 43 for example:

    • [CH3CH2CH2]+
    • [(CH3)2CH]+

2-butanol is not likely to have a fragment at m/e = 43 as it does not have either of these fragments in its structure.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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