17.6 Deduction of Chlorine and Bromine in a Molecule by Mass Spectrometry

Mass spectrometry is a powerful analytical technique that can identify the presence of certain elements in a molecule based on their isotopic abundances. Chlorine and bromine, in particular, have distinctive isotopic patterns that produce characteristic peaks in a mass spectrum, making them easily identifiable.

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The Role of Isotopes in Mass Spectrometry

Many elements exist naturally as a mixture of isotopes, atoms with the same number of protons but different numbers of neutrons, resulting in different masses. Mass spectrometry separates ions based on their mass-to-charge ratio ($m/z$). The presence of heavier isotopes leads to additional peaks in the mass spectrum, typically at higher $m/z$ values than the main molecular ion peak ($M^{+}$). A peak appearing two mass units higher than the molecular ion peak is called the M+2 peak.

1. Deduction of Chlorine (Cl)

Chlorine has two stable isotopes, ${}^{35}\mathrm{Cl}$ and ${}^{37}\mathrm{Cl}$, with significant natural abundances.

• Isotopes and Abundances:

  • ${}^{35}\mathrm{Cl}$: Relative abundance of 75.77%
  • ${}^{37}\mathrm{Cl}$: Relative abundance of 24.23%

• Mass Spectrum Pattern:

  • A molecule containing one chlorine atom will show two peaks in the molecular ion region.
  • The molecular ion peak ($M^{+}$) corresponds to the molecule containing the ${}^{35}\mathrm{Cl}$ isotope.
  • An M+2 peak appears for the molecule containing the heavier ${}^{37}\mathrm{Cl}$ isotope.
  • The ratio of the heights of the $M^{+}$ peak to the $M+2$ peak is approximately 3:1, reflecting the natural abundance of the isotopes.

Example: Mass Spectrum of Chloromethane ($\mathrm{C}{\mathrm{H}}_3\mathrm{Cl}$)

The mass spectrum of chloromethane clearly illustrates this pattern.

• The molecular ion peak ($M^{+}$) for $\mathrm{C}{\mathrm{H}}_3^{35}\mathrm{Cl}$ appears at $m/z=50$.
• The M+2 peak for $\mathrm{C}{\mathrm{H}}_3^{37}\mathrm{Cl}$ appears at $m/z=52$.
• The height of the peak at $m/z=52$ is approximately one-third the height of the peak at $m/z=50$, confirming the presence of a single chlorine atom.

2. Deduction of Bromine (Br)

Bromine also has two stable isotopes, ${}^{79}\mathrm{Br}$ and ${}^{81}\mathrm{Br}$, with nearly equal abundances.

• Isotopes and Abundances:

  • ${}^{79}\mathrm{Br}$: Relative abundance of 50.50%
  • ${}^{81}\mathrm{Br}$: Relative abundance of 49.50%

• Mass Spectrum Pattern:

  • A molecule containing one bromine atom will show two prominent peaks in the molecular ion region.
  • The molecular ion peak ($M^{+}$) corresponds to the molecule with the ${}^{79}\mathrm{Br}$ isotope.
  • The M+2 peak corresponds to the molecule with the ${}^{81}\mathrm{Br}$ isotope.
  • Because their abundances are almost equal, the $M^{+}$ and $M+2$ peaks have approximately the same height (a 1:1 ratio).

Example: Mass Spectrum of Bromoethane (${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{Br}$)

The mass spectrum of bromoethane provides a classic example of bromine's signature.

• The molecular ion peak ($M^{+}$) for ${\mathrm{C}}_2{\mathrm{H}}_5^{79}\mathrm{Br}$ appears at $m/z=108$.
• The M+2 peak for ${\mathrm{C}}_2{\mathrm{H}}_5^{81}\mathrm{Br}$ appears at $m/z=110$.
• The heights of these two peaks are nearly identical, which is a definitive indicator of a single bromine atom in the molecule.

Possible Questions and Answers

Q: How can you distinguish between a molecule containing chlorine and one containing bromine using only a mass spectrum?

A: Look at the ratio of the $M^{+}$ and $M+2$ peaks.

• Chlorine: The $M+2$ peak is about one-third the height of the $M^{+}$ peak (3:1 ratio).
• Bromine: The $M+2$ peak is approximately the same height as the $M^{+}$ peak (1:1 ratio).

Q: What causes the M+2 peak in the mass spectrum of a halogenated compound?

A: The M+2 peak is caused by the presence of a heavier, naturally occurring isotope of the halogen (e.g., ${}^{37}\mathrm{Cl}$ or ${}^{81}\mathrm{Br}$) in the molecule.

Significance: This technique is crucial in structural elucidation in organic chemistry, helping chemists to quickly determine the presence and number of halogen atoms in an unknown compound.