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Dediazoniations in Water: An Integrated Physical Organic Chemistry Experiment

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Carlos Bravo-Díaz

Facultad de Ciencias, Departamento de Química Física y Orgáica, Universidad de Vigo, As Lagaas-Macrosende, 36200 Vigo-Pontevedra, Spain

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

J. Chem. Educ., 2000, 77 (3), p 384

DOI: 10.1021/ed077p384

Publication Date (Web): March 1, 2000

Abstract

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 D_n + A_n 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.