A Mass Spectral Chlorine Rule for Use in Structure Determinations in Sophomore Organic Chemistry

The number of chlorine atoms, n, is stoichiometrically related to the ratio of peak intensities of the lowest-mass-to-highest-mass molecular ions in ideal molecular-ion clusters displayed in the mass spectra of Br- and Cl-containing compounds. Intensity ratios for these peaks are independent of the...

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Bibliographic Details
Published in:Journal of chemical education 2004-08, Vol.81 (8), p.1161
Main Author: Gross, Ray A
Format: Article
Language:English
Subjects:
Atomic properties
Bromine
Bromine compounds
Chlorine
Chlorine compounds
College Students
Higher Education
Mass spectrometry
Molecular structure
Organic Chemistry
Organic compounds
Rules
Science Instruction
Spectroscopy
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Summary:The number of chlorine atoms, n, is stoichiometrically related to the ratio of peak intensities of the lowest-mass-to-highest-mass molecular ions in ideal molecular-ion clusters displayed in the mass spectra of Br- and Cl-containing compounds. Intensity ratios for these peaks are independent of the number of bromine atoms and equal the nth power of 3. For ratios of 1, 3, 9, 27, and 81, the values of n are 0, 1, 2, 3, and 4, respectively. When real clusters are peak-normalized, n can be found by cluster analysis of compounds containing C, H, O, N, Br, and Cl. Given a cluster, students can find the number of chlorine atoms in these compounds directly from the normalized intensity of M, the lowest-mass molecular ion, and can find the number of bromine atoms from the number of peaks in the cluster. The chlorine rule provides a new tool for use in organic structure determination. A discovery exercise for the chlorine rule and the names of 69 compounds, the spectra of 68 of which have been successfully analyzed by students using the chlorine rule, are provided in the Supplemental Material.
ISSN:0021-9584
1938-1328
DOI:10.1021/ed081p1161