Teaching the chlorine rule
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Teaching the Chlorine Rule. Ray A. Gross, Jr. Prince George’s Community College. The 19 th Biennial Conference on Chemical Education. July 31, 2006. Home of the boilermakers. The Chlorine Rule. A way to analyze the mass spectra of Br m Cl n compounds, where m and n are small numbers.

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Teaching the Chlorine Rule

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Teaching the chlorine rule

Teaching the Chlorine Rule

Ray A. Gross, Jr.

Prince George’s Community College

The 19 th biennial conference on chemical education

The 19th Biennial Conference on Chemical Education

July 31, 2006

Teaching the chlorine rule

Home of the boilermakers.

The chlorine rule

The Chlorine Rule

A way to analyze the mass spectra of BrmClncompounds, where m and n are small numbers.

J. Chem. Educ.2004, 81, 1162-1168

Br m and cl n clusters

Brm and ClnClusters

What we need to know

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

New Idea

  • Bromine makes the same 1:1 contribution to the end peaks of a BrmClnspectrum that it makes to a Brm spectrum.

Which means

Which means

  • Differences in end-peak intensities are attributable to the number of Cl atoms only.

The chlorine rule1

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

Teaching the chlorine rule

Structure begets properties!



  • 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.



  • SDBS Website http://www.aist.go.jp/RIODB/SDBS/cgi-bin/cre_index.cgi (accessed July 2006)

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