Resumen de Kinetics of Carbaryl Hydrolysis: An Undergraduate Environmental Chemistry Laboratory
Kinetics is an important part of undergraduate environmental chemistry curricula and relevant laboratory exercises are helpful in assisting students to grasp concepts. Such exercises are also useful in general chemistry courses because students can see relevance to real-world issues. The laboratory exercise described here involves determination of the second-order rate constant for base-catalyzed hydrolysis of the carbamate pesticide, carbaryl (1-naphthyl methylcarbamate), still widely used in the US and other countries. The exercise employs relatively simple and readily available equipment and exploits a bathochromic (red) absorbance shift of a product (1-naphthoxide) at pH ≥ 10 that minimizes absorbance overlap between reactant and product. Using a pseudo-first-order approach, carbaryl half-life at pH 10 to 11 ranges from approximately 1.5 h to less than 10 min, enabling completion within a 3 h laboratory period. The exercise can also be adapted for higher-level courses by using transition state theory to discriminate between hydrolysis mechanisms. To emphasize the practical utility of this exercise to students, at its conclusion, they can prepare waste carbaryl-containing solutions for disposal by this same hydrolysis process.
