Crossover experiment (chemistry)

In chemistry, a crossover experiment is a method used to study the mechanism of a chemical reaction. In a crossover experiment, two similar but distinguishable reactants simultaneously undergo a reaction as part of the same reaction mixture. The products formed will either correspond directly to one of the two reactants (non-crossover products) or will include components of both reactants (crossover products). The aim of a crossover experiment is to determine whether or not a reaction process involves a stage where the components of each reactant have an opportunity to exchange with each other.

The results of crossover experiments are often straightforward to analyze, making them one of the most useful and most frequently applied methods of mechanistic study. In organic chemistry crossover experiments are most often used to distinguish between intramolecular and intermolecular reactions.Inorganic and organometallic chemists rely heavily on crossover experiments, and in particular isotopic labeling experiments, for support or contradiction of proposed mechanisms. When the mechanism being investigated is more complicated than an intra- or intermolecular substitution or rearrangement, crossover experiment design can itself become a challenging question. A well-designed crossover experiment can lead to conclusions about a mechanism that would otherwise be impossible to make. Many mechanistic studies include both crossover experiments and measurements of rate and kinetic isotope effects.

Crossover experiments allow for experimental study of a reaction mechanism. Mechanistic studies are of interest to theoretical and experimental chemists for a variety of reasons including prediction of stereochemical outcomes, optimization of reaction conditions for rate and selectivity, and design of improved catalysts for better turnover number, robustness, etc. Since a mechanism cannot be directly observed or determined solely based on the reactants or products, mechanisms are challenging to study experimentally. Only a handful of experimental methods are capable of providing information about the mechanism of a reaction, including crossover experiments, studies of the kinetic isotope effect, and rate variations by substituent. The crossover experiment has the advantage of being conceptually straightforward and relatively easy to design, carry out, and interpret. In modern mechanistic studies, crossover experiments and KIE studies are commonly used in conjunction with computational methods.

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