Dry media reaction

A dry media reaction or solid-state reaction or solventless reaction is a chemical reaction system in the absence of a solvent^[1] . The drive for the development of dry media reactions in chemistry is

• economics (save money on solvents)
• ease of purification (not required to remove a solvent post-synthesis)
• high reaction rate (due to high concentration of reactants)
• environmentally friendly (solvent is not required) see green chemistry

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Drawbacks to overcome:

• reactants should mix to a homogeneous system
• high viscosity in reactant system
• unsuitable for solvent assisted chemical reactions

One type of solventless reaction is one where a liquid reactant is used neat, for instance the reaction of 1-bromonaphthalene with Lawesson's reagent is done with no added liquid solvent, but the 1-bromonaphthalene acts as a solvent.

A reaction which is closer to a true solventless reaction is a Knoevenagel condensation of ketones with (malononitrile) where a 1:1 mixture of the two reactants (and ammonium acetate) is irradated in a microwave oven^[2]

Collin Raston's research group have been responsible for a number of new solvent free reactions^[3][4] In some of these reactions all the starting materials are solids, they are ground together with some sodium hydroxide to form a liquid, which turns into a paste which then hardens to a solid.

In another development the two components of an Aldol reaction are combined together with the asymmetric catalyst S-proline in a ball mill in a mechanosynthesis. The reaction product has 97% enantiomeric excess.^[5]

A reaction rate acceleration is observed in several systems when a homogeneous solvent system is rapidly evaporated in a rotavap in a vacuum, one of them a Wittig reaction^[6] :

The reaction goes to completion in 5 minutes with immediate evaporation whereas the same reaction in solution after the same 5 minutes (dichloromethane) has only 70% conversion and even after 24 hours some of the aldehyde remains.

References

[1]