Transcript Document

IUPAC 2003, Ottawa, Canada
August 10-15, 2003
1


a86

a
aa
hm
Du10au15
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