1 IUPAC 2003, Ottawa, Canada August 10-15, 2003

2 Le 39e Congrès de l’IUPAC et la 86e conférence de la Société canadienne de chimie Du 10 au 15 août 2003

3 The 39th IUPAC Congress and 86th Conference of The Canadian Society for Chemistry August , 2003

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A Mass Spectral Chlorine Rule for Sophomore Organic Chemistry Ray A Gross, Jr.

6 Abstract If n is the number of chlorine atoms and m the number of bromine atoms in the formula of an organic compound, then n can be found from the equation I = 3 n, where I is the intensity of the lowest-mass molecular ion in the mass spectrum relative to the highest-mass ion attributable to m and n. The value of m is then found from the number of molecular-ion peaks (m + n + 1) attributable to m and n. The equation is derived, and its use is exemplified.

7 Introduction The intensity of the m + n + 1 molecular-ion peaks attributable to m bromine and n chlorine atoms may be modeled by the expression (a + b) m (3a + b) n. The coefficients of the expanded binomial pair give relative abundances of the molecular ions. For C 6 H 3 Br 1 Cl 2, the expression is (a + b) 1 (3a + b) 2 = 9a a 2 b + 7ab 2 + 1b 3. The model intensities of the molecular-ion peaks are 9:15:7:1 as compared to the actual values of 10:16:7:1 for 2-bromo-1,4-dichlorobenzene, a real compound. See Table 1. The model-equation results are sufficiently accurate so that the general solution of the model is applicable to real compounds, because n and m must be whole numbers.

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9 Methods The general expression (a + b) m (3a +b) n will be expanded. The coefficient of the first term in the resulting polynomial will be divided by the coefficient of the last term. The resulting ratio represents the relative numbers of molecular ions or the corresponding intensities of their mass spectral peaks. We find the ratio I to be a function of n and independent of m. The intensities of the lowest-mass and highest-mass molecular ions attributable to the presence of bromine and chlorine are determined by n only, giving rise to a chlorine rule.

10 Results (a + b) m (3a + b) n = 1 m 3 n a (m + n) + …. + 1 m 1 n b (m + n) I = 1 m 3 n /1 m 1 n I = 3 n Chlorine Rule: When I equals 1, 3, 9, 27 or 81; n is 0, 1, 2, 3, or 4, respectively, where n = number of chlorine atoms. The number of bromine atoms m equals the number of peaks attributable to m and n minus the sum of n + 1.

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16 Conclusions The mass spectra of compounds that contain C, H, N, and O atoms together with m Br, and n Cl atoms show m + n + 1 molecular-ion peaks 2 amu apart due to Br and Cl. The value of n is found from the equation I = 3 n. The magnitude of I is found from the mass spectrum of an unknown as a ratio of peak intensities (blue over red in the spectra of Figures 1-5). The value of m is found from the number of A + 2 molecular-ion peaks; m = the number of peaks minus (n + 1).

17 Acknowledgements Table 1: Junhua Yan’s Isotope Pattern Calculator (accessed May 2003). Figures 3-5: Institute of Advanced Industrial Science and Technology; Tsukuba, Ibaraki, Japan SDBSWeb: (accessed May 2003). NSF Grant: DUE Submitted to JCE