Question

Halogenated compounds are particularly easy to identify by their mass spectra because both chlorine and bromine occur naturally as mixtures of two abundant isotopes. Chlorine occurs as \({ }^{35} \mathrm{Cl}(75.8 \%)\) and \({ }^{37} \mathrm{Cl}(24.2 \%)\); bromine occurs as \({ }^{79} \mathrm{Br}(50.7 \%)\) and \({ }^{81} \mathrm{Br}(49.3 \%)\). At what masses do the molecular ions occur for the following formulas? What are the relative percentages of each molecular ion? (a) Bromomethane, \(\mathrm{CH}_{3} \mathrm{Br}\) (b) 1-Chlorohexane, \(\mathrm{C}_{6} \mathrm{H}_{13} \mathrm{Cl}\)

Short answer

Bromomethane mass ions: 94 (50.7%) and 96 (49.3%). 1-Chlorohexane mass ions: 120 (75.8%) and 122 (24.2%).

Step-by-step solution

Determine Isotopic Combinations

First, identify the isotopic combinations for each compound based on the given isotopic abundances of chlorine and bromine.

Calculate Masses for Bromomethane

Bromomethane, CH₃Br, has the following isotopic combinations: - ** 79Br:** The molecular mass is computed as 12 (C) + 1×3 (3 Hs) + 79 (Br) = 94. This combination corresponds to C₁H₃⁷⁹Br. - ** 81Br:** The molecular mass is calculated as 12 (C) + 1×3 (3 Hs) + 81 (Br) = 96. This combination corresponds to C₁H₃⁸¹Br.

Calculate Relative Percentages for Bromomethane

For CH₃Br, since two isotopes of bromine are nearly equally abundant, the percentages are nearly equal: - Mass 94 (C₁H₃⁷⁹Br): 50.7% - Mass 96 (C₁H₃⁸¹Br): 49.3%

Calculate Masses for 1-Chlorohexane

1-chlorohexane, C₆H₁₃Cl, involves the chlorine isotopes: - **³⁵Cl:** The molecular mass is computed as 6×12 (C) + 1×13 (13 Hs) + 35 (Cl) = 120. This corresponds to C₆H₁₃³⁵Cl. - **³⁷Cl:** The molecular mass is calculated as 6×12 (C) + 1×13 (13 Hs) + 37 (Cl) = 122. This corresponds to C₆H₁₃³⁷Cl.

Calculate Relative Percentages for 1-Chlorohexane

For C₆H₁₃Cl, using the natural abundance probabilities of chlorine isotopes: - Mass 120 (C₆H₁₃³⁵Cl): 75.8% - Mass 122 (C₆H₁₃³⁷Cl): 24.2%

Key concepts

Isotopic Abundance

Isotopic abundance refers to the natural occurrence of isotopes of an element. Isotopes are atoms with the same number of protons but different numbers of neutrons. Because they have different masses, isotopes of the same element do not produce the same mass spectra. They appear as separate peaks based on their respective masses and abundances.

For example, chlorine has two main isotopes: \(^ {35} \text{Cl}\) and \(^ {37} \text{Cl}\), with natural abundances of 75.8% and 24.2%, respectively. Similarly, bromine also has two commonly occurring isotopes: \(^ {79} \text{Br}\) and \(^ {81} \text{Br}\), with nearly equal abundances of 50.7% and 49.3%. These percentages indicate how likely each isotope is to occur naturally in a given sample.

Understanding isotopic abundance is crucial in mass spectrometry as it directly influences the intensity of the peaks for different isotopic molecular ions. Therefore, an element with multiple isotopes will show additional peaks corresponding to each possible isotopic combination, providing valuable clues to the compound's identity.

Molecular Ions

A molecular ion is often the largest ion observed in a mass spectrum. It is formed when a compound is ionized but not fragmented, meaning the entire molecule gains or loses an electron. This ion appears in the mass spectrum as a peak at a specific mass-to-charge ratio (m/z) that corresponds to the whole molecular unit.

In the context of halogenated compounds, the molecular ion peak is particularly informative. For instance, in bromomethane (\( \text{CH}_3\text{Br} \)), the molecular ions are observed at masses 94 and 96. This results from the two isotopic forms of bromine: \( ^{79}\text{Br} \) and \( ^{81}\text{Br} \). The presence of these peaks at different m/z values signifies the presence of different isotopic compounds of bromine within the sample.

The molecular ion provides the molecular mass of a compound, helping to deduce the molecular formula when combined with other structural information. Although molecular ions are stable, in some cases, they can fragment into smaller ions that also appear on the spectrum, offering further details about the compound’s molecular structure.

Halogenated Compounds

Halogenated compounds are molecules that include halogen atoms such as fluorine, chlorine, bromine, or iodine. Due to their unique elemental properties, halogens lead to distinct patterns in mass spectrometry, aiding in the identification of such compounds.

Chlorine and bromine, for instance, occur as mixtures of isotopes with notable natural abundances. This influences the mass spectra significantly, yielding characteristic patterns due to the presence of molecular ions from different isotopic combinations. For example, in 1-chlorohexane (\( \text{C}_6\text{H}_{13}\text{Cl} \)), the peaks at masses 120 and 122 arise due to the isotopes \( ^{35}\text{Cl} \) and \( ^{37}\text{Cl} \) merging with the organic compound.

Mass spectrometry of halogenated compounds thus exploits these isotopic effects. The distinct spacing and intensity of isotopic peaks enable chemists to quickly spot halogenated species and infer which halogens are present, providing key insights into structural and reactive properties of the molecules.