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Solid State Organic Synthesis and Crystal Engineering

Solid State Organic Synthesis and Crystal Engineering
Most synthetic reactions in organic chemistry occur in liquid or gaseous phase, or involve these phases. The product then has to be separated from solvents, side-products and reactants. However, many of these reactions may be conducted in the solid state. The solid state reactions usually result in only one product with 100% yield and do not involve any solvents. Moreover, the reactions are typically stereospecific and may yield a product different from the main product of the corresponding reaction in solution. Solid state synthesis becomes especially attractive nowadays, when fast economic growth causes environmental issues and so-called Green Chemistry principles become more and more important in the design of new industrial processes.

diacetylenes
Solid state polymerization of diacetylenes

 

cinnamic
Dimerization of cinnamic acid starting from different polymorphs

One well-known solid state reaction is the photopolymerization of conjugates dienes, such as diacetylenes. The molecules must be properly aligned in the reactant solid in order for the polymerization to take place. Another example is the photodimerization of unsaturated carbonic acids, such as different polymorphs of cinnamic acids. GMJ Schmidt, to whom the concept of Crystal Engineering is usually attributed, found that UV irradiation of different polymorphic forms yields different products. The molecular structure of the product depends on the crystal structure of the reactant that is how the reactant molecules are arranged in the initial crystal. If the arrangement is unfavorable, the reaction will not occur at all.
Crystal Engineering is the design of solid materials with a crystal structure that produces a desired property in the crystalline solid, such as a desired reactivity. The chemist takes molecules or other molecular species as "building blocks" and uses chemical or weaker interactions as "cement" to "glue" these blocks in a desired architecture. Coordination and hydrogen bonds are most frequently used for this purpose, although the presence of other interactions should be taken into account (for example, van der Waals forces are present in any solid and often cannot be ignored).