A dry media reaction or solid-state reaction or solventless reaction is a chemical reaction system in the absence of a solvent. The drive for the development of dry media reactions in chemistry is
- economics (save money on solvents)
- ease of purification (no solvent removal post-synthesis)
- high reaction rate (due to high concentration of reactants)
- environmentally friendly (solvent is not required), see green chemistry
Drawbacks to overcome:
- reactants should mix to a homogeneous system
- high viscosity in reactant system
- unsuitable for solvent assisted chemical reactions
In one type of solventless reaction 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.
Colin Raston's research group have been responsible for a number of new solvent free reactions. 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.
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. 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.
Famous quotes containing the words dry, media and/or reaction:
“‘Tis not such lines as almost crack the stage
When Bajazet begins to rage;
Nor a tall met’phor in the bombast way,
Nor the dry chips of short-lunged Seneca.
Nor upon all things to obtrude
And force some odd similitude.
What is it then, which like the power divine
We only can by negatives define?”
—Abraham Cowley (1618–1667)
“One can describe a landscape in many different words and sentences, but one would not normally cut up a picture of a landscape and rearrange it in different patterns in order to describe it in different ways. Because a photograph is not composed of discrete units strung out in a linear row of meaningful pieces, we do not understand it by looking at one element after another in a set sequence. The photograph is understood in one act of seeing; it is perceived in a gestalt.”
—Joshua Meyrowitz, U.S. educator, media critic. “The Blurring of Public and Private Behaviors,” No Sense of Place: The Impact of Electronic Media on Social Behavior, Oxford University Press (1985)
“The excessive increase of anything often causes a reaction in the opposite direction.”
—Plato (c. 427–347 B.C.)