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The Chlorine Rule: An Analysis of Isotope Patterns of Compounds Containing Multiple Bromine and Chlorine Atoms. Ray A. Gross, Jr. With an Introduction to the Isotope-Pattern Analyzer. My Reasons for this Presentation. Present results obtained at PGCC

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Ray a gross jr

The Chlorine Rule: An Analysis of Isotope Patterns of Compounds Containing Multiple Bromine and Chlorine Atoms

Ray A. Gross, Jr.

With an Introduction to the Isotope-Pattern Analyzer

My reasons for this presentation

My Reasons for this Presentation

  • Present results obtained at PGCC

  • Show that content found in textbooks can be improved

  • Motivate students

Ray a gross jr

Isotopes of Br and Cl

Ray a gross jr

Mass Spectrometer

Ray a gross jr

Schematic diagram of a mass spectrometer

Ray a gross jr

Why Br and Cl?

Molecular-ion peaks of C10H20Br1Cl1, C10H19Br2Cl1 and C10H18Br3Cl1.



In lieu of pattern matching,it should be possible to determine the number of Br and Cl atoms in a molecular formula of a compound by analyzing the molecular-ion cluster (i.e., by cluster analysis).

Herbert c brown nobel laureate

Herbert C. BrownNobel Laureate


with BH3 (CHM 201)

Reduction with NaBH4

(CHM 202/204)

Lillian berg nvcc annandale

Lillian BergNVCC-Annandale

Chlorine constant

Chlorine Constant

Bromine constant

Bromine Constant

IM = 3n

Theoretical considerations

Theoretical Considerations

Ideal Compounds

Br (a:b) = 1:1

Cl (a:b) = 3:1

13C and 2H negligible

Bromine binomial

Bromine Binomial

  • Ratio (a:b) = 1:1

  • (1a + 1b)m for Brm

  • (1a + 1b)1 = 1a + 1b = 1:1

  • (1a + 1b)2 = 1a2 + 2ab+ 1b2 = 1:2:1

Chlorine binomial

Chlorine Binomial

  • Ratio (a:b) = 3:1

  • (3a + 1b)n for Cln

  • (3a + 1b)1 = 3a + 1b = 3:1

  • (3a + b)2 = 9a2 + 6ab + 1b2 = 9:6:1

Ray a gross jr

Ideal Model = Binomial Pair

(1a + 1b)m(3a + 1b)n


3a2 + 4ab + 1b2 = 3:4:1

Ray a gross jr


(1a + 1b)m(3a + 1b)n = 1m3na(m+ n) + …. + 1m1nb(m+ n)

I(L/R) = 1m3n/1m1n

IM = 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 A – n.

J.Chem.Educ. 2004, 81, 1161-1168 (article available at front desk)

Roald hoffmann nobel laureate

Roald Hoffmann-Nobel Laureate

Conservation of orbital symmetry

“Oxygen” Priestley vs Sheele




Ray a gross jr

Gross giving lecture with Hoffmann, Djerassi and

Woodward looking on.

Structure begets properties

Structure Begets Properties

  • Let’s examine structures.

  • Assume 3:1 and 1:1 isotopic abundances of chlorine and bromine.

  • Consider Brm, Cln and BrmClncompounds.



N = 2m4n

N = 2m2n2n

N = 2A2n

Chem. Educ. 2003, 8, 182-186

Ray a gross jr

Summary Part I for BrmClnCompounds

  • Derived a chlorine-rule equation, IM = 3n

  • Applied it to find gross structures of unknowns

  • Derived a unit-sample equation, N = 2A2n

Follow on to the chlorine rule

Follow-on to the Chlorine Rule

  • An automated A + 2 isotope-pattern analyzer (IPA)

  • IPA is on my website

J. Chem. Educ., in press

Example of a print out of a mass spectrum in the molecular ion region

Example of a Print Out of a Mass Spectrum in the Molecular-Ion Region

Molecular ion data is entered into the ipa

Molecular-Ion Data is Entered into the IPA

The Excel program returns the A + 2 (Cl, Br, S) composition of the molecular formula

Homework assignment for selected students

Homework AssignmentforSelected Students

  • Pick up slip from front desk

  • Enter data from your slip into IPA

  • Obtain the Cl, Br, S composition (e.g., Br1Cl2) and record it on your slip

  • Write your name on the slip and turn it in next Tuesday.

Acknowledgement mass spectra from the spectral data base system sdbs

Acknowledgement: Mass Spectra from the Spectral Data Base System (SDBS)

Ray a gross jr


Lecture attended by hordes of students eager to learn

Lecture attended by hordes of students eager to learn.

Gross and friends

Gross and Friends

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