Phase transfer chemistry

The development of a sustainable chemistry known as "eco-compatible chemistry" or "green chemistry" is a major challenge for synthetic chemists. One of the aims of green chemistry is to design chemical products and processes that reduce or eliminate the use and synthesis of hazardous and toxic substances. To achieve this goal, the synthetic chemist must integrate a certain number of basic principles (12 principles of green chemistry), including limiting the use of organic solvents (toxic and flammable) and costly and dangerous reagents, reducing the quantity of by-products and reaction waste by including separation and purification steps, saving atoms and steps by favoring catalytic rather than stoichiometric reactions. This approach must include the entire process, i.e. the reaction itself as well as its treatment, while limiting separation and purification problems.

The classic approach of the synthetic organic chemist, which consists of carrying out reactions in a homogeneous medium using an organic solvent common to all the reaction partners, not only leads to costly separation and purification steps that generate additional waste, but also to dilution and solvation effects that can limit efficiency.

The use of multiphase systems (e.g. liquid-liquid or solid-liquid), with the transfer of one or more partners from one phase to the other, is a more environmentally-friendly alternative that can be applied to most organic chemistry reactions. Phase transfer chemistry can be applied to both stoichiometric and catalytic processes, with phase transfer taking place either during or after the reaction. Phase transfer processes offer a number of advantages in terms of sustainable chemistry: they limit, or even eliminate, the use of organic solvents (with the possibility of carrying out reactions only in the presence of water) and facilitate separation (ideally by simple decantation), thus minimizing the quantity of toxic effluents and energy costs. They are generally highly efficient, selective and operate under mild conditions.

In this article, we focus on phase transfer applications in synthetic chemistry, using three main approaches: phase transfer catalysis, which involves transfer to the organic phase; reverse phase transfer, which involves transfer to the aqueous phase; and temperature-controlled systems, which involve phase transfer or temperature-induced phase separation.