Transcript The Chlorine Rule:

Teaching the Chlorine Rule
Ray A. Gross, Jr.
Prince George’s Community College
The 19th Biennial Conference
on Chemical Education
July 31, 2006
Home of the boilermakers.
The Chlorine Rule
A way to analyze the mass spectra of
BrmCln compounds, where m and n
are small numbers.
J. Chem. Educ. 2004, 81, 1162-1168
100
Br
61 59
m=1
# Significant Cluster
Peaks = m + 1
1:1
Brm
xx
156 158
52
Cl
Cln
100 31
n=1
# Significant Cluster
Peaks = n + 1
3:1
x
x
112 114
Brm and Cln Clusters
Cln
Brm
1:1
Br1
Cl1
3:1
1:2:1
Br2
Cl2
9:6:1
1:3:3:1
Br3
ideal compounds
78 100 24
Br
m = 1, n = 1
# Sig. Peaks = m + n + 1
Cl
BrmCln
3:4:1
xx
x
190 194
What we (need to) know
• The end cluster peaks for Brm compounds are
always ~1:1 in intensity.
• The end cluster peaks for Cln compounds are
~3:1 for Cl1, ~9:1 for Cl2, and ~3n:1 for Cln.
• The number of significant cluster peaks is one
more then the number of halogens, m + n.
New Idea
• Bromine makes the same 1:1
contribution to the end peaks of
a BrmCln spectrum that it makes
to a Brm spectrum.
Which means
• Differences in end-peak
intensities are attributable to the
number of Cl atoms only.
78 100 24
ratio of end peaks = 74/24 = 3:1 = Cl1
number of Sig. Peaks = 3
number of halogens = 2 or Br1
3
1
xx
x
190 194
78 100 24
Cl = 35
Br = 79
114
ratio of end peaks = 74/24 = 3:1 = Cl1
number of Sig. Peaks = 3
number of halogens = 2 or Br1
3
1
190
-114
76
Br
Cl
o, m or p
xx
x
190 194
The Chlorine Rule
• For BrmCln compounds, the
intensity ratio of the leftmost to
rightmost cluster peak rises with
the nth power of three.
• I = 3n
Unknown
62 100 45 6
1
10
number of peaks = 4
three halogens
ratio of end peaks ~9:1 = Cl2 and Br1
x
xx
224 230
Unknown
62 100 45 6
10
1
number of peaks = 4
three halogens
ratio of end peaks ~9:1 = Cl2 and Br1
2 Cl = 70
Br = 79
149
Br
Cl
224
-149
75
Cl
x
xx
224 230
Unit Sample of ArBr2Cl
7 5
1
3
Br
Cl
Br
Br
Cl
Br
Cl
Br
Cl
Br
Br
Br
Br
Cl
Br
Br
Br
Br Cl
Br
Br
Br
Cl
Br
Br
Cl
Br
Br
3
Cl
Br
Cl
Br Cl
Br
Cl
Br
1
Br
7
Br
Br
5
Br
Cl
Br
Cl
Br
Br
Cl
Br
Cl
O
Cl
Cl
Cl
NHCCH 3
Cl
CHO
Cl
Br
Cl
M = 224
220
m/z
240
Normal Pattern
Even M
M = 203
200
m/z
220
Normal Pattern
Odd M, contains N
M = 174
170
m/z
180
Abnormal Pattern
Strong M-1, M +1 peaks, etc.
Contains aldehyde, etc.
Conclusions
• The chlorine rule provides an efficient
method for determining small numbers
of Br and Cl atoms in an unknown’s mass
spectrum.
• It complements the nitrogen rule.
Acknowledgement
• SDBS Website
http://www.aist.go.jp/RIODB/SDBS/cgibin/cre_index.cgi (accessed July 2006)