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

Additional recommended knowledge

What is the Correct Way to Check Repeatability in Balances?

Daily Visual Balance Check

Recognize and detect the effects of electrostatic charges on your balance

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]