Abstract:

Proposal of experiments for the physical chemistry laboratory is a challenge in developing the physical chemistry curriculum to better reflect the range of activities found in current physical chemistry research. Here we report on an integrated physical-organic chemistry experiment, suitable for undergraduate students aimed to attract their attention to control chemical reactivity by employing simple instrumentation available in most, if not all, chemical laboratories. The topic of choice involves a physical organic chemistry experiment proposed involves a kinetic study of the spontaneous decomposition of an arenediazonium ions, ArN2+, in aqueous acid solution.

Experiments are designed to fit into the undergraduate physical and organic chemistry curriculum and can be easily adapted to a number of different laboratory settings according to the instructor or course requirements. Experiments can be carried out by the same students, synthetizing the necessary compounds that will employ later in the physical chemistry laboratory. Alternatively, experiments can be done by different students or the instructors if considered more convenient. Students carrying out the complete set of experiments will be able to learn synthetic procedures, kinetics and mechanisms of organic reactions. Complementary topics include determining product yields and selectivities of ions towards different nucleophiles.

Arenediazonium salts have been proved to be very versatile for both fundamental and applied research and their reactions are part of the material included in most organic chemistry books. ArN2+ ions are not thermally stable and undergo spontaneous decomposition in aqueous solution following a Dn + An mechanism, i.e., a rate determining formation of a very reactive aryl cation that reacts immediately with available nucleophiles. A Scheme wil show a simplified version of this mechanism, applicable when water is the solvent, which acts as nucleophile.

According to this reaction scheme, H3O+ ions are formed in the course of the spontaneous dediazoniation, and thus can be easily monitored by potentiometric measurements. This also allows estimation of product yields as well as the selectivity of ArN2+ towards different nucleophils such as halide, X- , ions. Advantages and inconveniences of the method will be presented as well as alternative experiments specifically designed for undergraduate students taking introductory courses of chemical kinetics are proposed.

Keywords:

Physical Chemistry, Potentiometry, Areediazonium ions, Laboratory experiment.