Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
IUPAC 2003, Ottawa, Canada August 10-15, 2003 1 a86 a aa hm Du10au15 2 The 39th IUPAC Congress and 86th Conference of The Canadian Society for Chemistry August 10 - 15, 2003 3 4 A Mass Spectral Chlorine Rule for Sophomore Organic Chemistry Ray A Gross, Jr. 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 = 3n, 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. 6 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 C6H3Br1Cl2, the expression is (a + b)1(3a + b)2 = 9a3 + 15a2b + 7ab2 + 1b3. 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. 7 C6H3Br1Cl2 % Mass 62 4 100 7 44 3 6 224 225 226 227 228 229 230 Normalized Mass % 224 225 226 227 228 229 230 231 61.5 4.0 100.0 6.5 45.5 2.8 6.4 0.4 Real Model 10 9 16 15 7 7 1 1 Table 1. Molecular ions and normalized peak intensities for 2-bromo-1,4-dichlorobenzene The actual m + n + 1 peak intensities are normalized in blue; the corresponding model intensities are shown in red. 8 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. 9 Results (a + b)m(3a + b)n = 1m3na(m + n) + …. + 1m1nb(m + n) I = 1m3n/1m1n I = 3n 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. 10 Chlorine Held Constant 3 4 1 Br1Cl1 M +2 +4 3 7 5 1 3 11 13 6 1 Br2Cl1 M +2 +4 +6 Br3Cl1 M +2 +4 +6 +8 I = 3/1 = 3n n=1 m=3-2=1 m=4-2=2 m=5-2=3 Figure 1. A + 2 Molecular-ion peaks of C10H20Br1Cl1, C10H19Br2Cl1 and C10H18Br3Cl1. I equals the intensity ratio of the blue peak to the red peak. This ratio is independent of m. The ratio equals 3n; thus n = 1 for all of these spectra. The A + 2 peaks are caused by Br and Cl atoms. The # peaks = m + n + 1, so m = # peaks - (n + 1). 11 Bromine Held Constant 3 4 1 Br1Cl1 M +2 +4 10 16 7 1 30 59 38 10 1 Br1Cl2 M +2 +4 +6 Br1Cl3 M +2 +4 +6 +8 I = 3/1 = 3n n= 1 I = 10/1 = 3n n= 2 I = 30/1 = 3n n= 3 m=3-2=1 m=4-3=1 m=5-4=1 Figure 2. A + 2 Molecular-ion peaks of C10H20Br1Cl1, C10H19Br1Cl2 and C10H18Br1Cl3. I equals the intensity ratio of the blue peak to the red peak. This ratio is independent of m. The ratio equals 3n; thus n = 1, 2 and 3 for these spectra. The A + 2 peaks are caused by Br and Cl atoms. The # peaks = m + n + 1, so m = # peaks - (n + 1). 12 NH2 Br Br 283 207 = Br2Cl1N1 76 = benzene residue Cl 3.3 1.0 M = 283 = N1 Figure 3. Mass spectrum of 2,6-dibromo-4-chloroaniline. The number of A + 2 peaks = 4. I = 3.3/1.0 = 3n n = 1 = Cl1 m = 4 - 2 = 2 = Br2 Compound contains Br2Cl1N1 13 Br Cl 224 149 = Br1Cl2 75 = benzene residue 9.9 Cl 1.0 M = 224 Figure 4. Mass spectrum of 1-bromo-2,4-dichlorobenzene. The number of A+ 2 peaks = 4. n I = 9.9/1.0 = 3 n = 2 = Cl2 m = 4 - 3 = 1 = Br1 Compound contains Br1Cl2 14 Cl CHO 174 99 = Cl2 + CHO 75 = benzene residue Cl 9.6 M - 29 = CHO 1.0 145 M = 174 Figure 5. Mass spectrum of 2,6-dichlorobenzaldehyde. The number of A + 2 peaks = 3. I = 9.6/1.0 = 3n n=2 m=3-3=0 Compound contains Cl2 plus CHO 15 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 = 3n. • 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). 16 Acknowledgements • Table 1: Junhua Yan’s Isotope Pattern Calculator http://www.geocities.com/junhuayan/pattern.htm (accessed May 2003). • Figures 3-5: Institute of Advanced Industrial Science and Technology; Tsukuba, Ibaraki, Japan SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/ (accessed May 2003). • NSF Grant: DUE-0202431 • Submitted to JCE 17