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2000
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March
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In the Laboratory
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Dediazoniations in Water: An Integrated Physical Organic Chemistry Experiment
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Ugo Costas-Costas and Román Pazo-LLorente
Facultad de Ciencias, Departamento de Química Física y Orgánica,
Universidad de Vigo, As Lagoas-Macrosende, 36200 Vigo-Pontevedra, Spain
Elisa González-Romero
Facultad de Ciencias, Departamento de Química Analítica y Alimentaria,
Universidad de Vigo, As Lagaas-Macrosende, 36200 Viga-Pontevedra, Spaing
Carlos Bravo-Díaz
Facultad de Ciencias, Departamento de Química Física y Orgáica,
Universidad de Vigo, As Lagaas-Macrosende, 36200 Vigo-Pontevedra, Spain
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March 2000
Vol. 77 No. 3
p. 384
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| Abstract |
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Most organic chemistry textbooks include sections where arenediazonium salt chemistry is discussed
but this material is not normally included in undergraduate laboratory experiments. We propose
an integrated physical organic experiment that includes synthesis and isolation of an arenediazonium
salt in the solid state and a kinetic study of its stability in water by employing both
spectrophotometric and potentiometric measurements. Spectrophotometric kinetic data can be
obtained by following the decrease in absorbance due to diazonium ion disappearance.
Hydroxy-dediazoniations yield H3O+, so dediazoniation can be monitored
by measuring pH changes
with time, allowing determination of the rate constants for product formation. Comparison of the
two rate constants allows simple mechanistic considerations: heterolytic dediazoniations proceed
via a Dn + An mechanism, that is, rate-determining
irreversible dissociation of the arenediazonium
ion into N2 and an aryl cation that reacts immediately, at diffusion-controlled rates,
with water or other available nucleophiles present in solution. Therefore, the observed
rate constants for
product formation and for loss of arenediazonium ion should be the same and essentially independent
of added nucleophiles.
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| Supplement |
Synthetic procedures, Safety Note, Instructor Notes, and Data Analysis are
included.
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| More Information |
Citation |
Costas-Costas, Ugo; Pazo-LLorente, Román; González-Romero, Elisa; Bravo-Díaz, Carlos. J. Chem. Educ. 2000 77 384.
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Keywords |
Physical Chemistry; Organic Chemistry; Kinetics; Synthesis; Mechanisms; Organic Synthesis
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History |
Created:
Last Updated: |
February 14, 2000
August 31, 2005
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