Dry media reaction
Encyclopedia
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
Drawbacks to overcome:
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.
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
system in the absence of a solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
. The drive for the development of dry media reactions in chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
is
- economics (save money on solvents)
- ease of purificationPurificationPurification is the process of rendering something pure, i.e. clean of foreign elements and/or pollution, and may refer to:* List of purification methods in chemistry* Water purification** Organisms used in water purification...
(no solvent removal post-synthesis) - high reaction rateReaction rateThe reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
(due to high concentrationConcentrationIn chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of reactants) - environmentally friendly (solvent is not required), see green chemistryGreen chemistryGreen chemistry, also called sustainable chemistry, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances...
Drawbacks to overcome:
- reactants should mix to a homogeneous system
- high viscosityViscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
in reactant system - unsuitable for solvent assisted chemical reactions
In one type of solventless reaction a liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
reactant is used neat
Neat
Neat may refer to:* Neat , a single, unmixed liquor served in a rocks glass.* An old word for an individual animal of the species of cattle Bos taurus* Neat , a Canadian television series* Neat Records, a British record label...
, for instance the reaction of 1-bromonaphthalene
Naphthalene
Naphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...
with Lawesson's reagent
Lawesson's reagent
Lawesson's reagent, or LR, is a chemical compound used in organic synthesis as a thiation agent. Lawesson's reagent was first made popular by Sven-Olov Lawesson, who did not, however, invent it. Lawesson's reagent was first made in 1956 during a systematic study of the reactions of arenes with...
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
Knoevenagel condensation
The Knoevenagel condensation reaction is an organic reaction named after Emil Knoevenagel. It is a modification of the Aldol condensation.A Knoevenagel condensation is a nucleophilic addition of an active hydrogen compound to a carbonyl group followed by a dehydration reaction in which a molecule...
of ketones with (malononitrile
Malononitrile
Malononitrile, also propanedinitrile, is a nitrile, with formula CH22. Malononitrile is relatively acidic, with an pKa of 11 in water...
) where a 1:1 mixture of the two reactants (and ammonium acetate
Ammonium acetate
Ammonium acetate is a chemical compound with the formula CH3COONH4 . It is a white solid, which can be derived from the reaction of ammonia and acetic acid...
) is irradated in a microwave oven
Microwave oven
A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...
.
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
Aldol reaction
The aldol reaction is a powerful means of forming carbon–carbon bonds in organic chemistry.Discovered independently by Charles-Adolphe Wurtz and Alexander Porfyrevich Borodin in 1872, the reaction combines two carbonyl compounds to form a new β-hydroxy carbonyl compound...
are combined together with the asymmetric catalyst S-proline
Proline
Proline is an α-amino acid, one of the twenty DNA-encoded amino acids. Its codons are CCU, CCC, CCA, and CCG. It is not an essential amino acid, which means that the human body can synthesize it. It is unique among the 20 protein-forming amino acids in that the α-amino group is secondary...
in a ball mill
Ball mill
A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.-Description:...
in a mechanosynthesis
Mechanosynthesis
Mechanosynthesis is any chemical synthesis in which reaction outcomes are determined by the use of mechanical constraints to direct reactive molecules to specific molecular sites.-Introduction:...
. The reaction product has 97% enantiomeric excess
Enantiomeric excess
The enantiomeric excess of a substance is a measure of how pure it is. In this case, the impurity is the undesired enantiomer .-Definition:...
.
A reaction rate
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
acceleration is observed in several systems when a homogeneous solvent system is rapidly evaporated in a rotavap in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, one of them a Wittig reaction
Wittig reaction
The Wittig reaction is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide to give an alkene and triphenylphosphine oxide....
. The reaction goes to completion in 5 minutes with immediate evaporation whereas the same reaction in solution after the same 5 minutes (dichloromethane
Dichloromethane
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
) has only 70% conversion and even after 24 hours some of the aldehyde remains.