What is a sterioselective reaction

Stereospecificity

Stereospecificity, a stereochemical feature of such reactions in which stereochemically differentiated end products are formed from stereochemically differentiated starting materials according to a defined mechanism (see selectivity). So is z. B. the electrophilic bromine addition to but-2-ene a stereospecific reaction. Electrophilic addition is one trans-Addition, off trans-But-2-en is formed as a result meso-2,3-dibromobutane, from which cis-Isomers (D, L) -2,3-dibromobutane. The reverse reaction, i.e. H. the splitting off of bromine, catalyzed by iodide, is stereospecific from which meso-Product is created trans-, from the D, L connection that cisIsomers. In the case of stereospecific reactions, one can clearly infer the stereochemistry of the end product from the stereochemistry of the starting product.

As stereoselective one denotes the reactions in which one of two or more possible stereoisomeric products is preferentially formed, taking into account the statistical relationships, e.g. B. the elimination of hydrogen chloride from 1-chloro-1,2-diphenylethane, with in excess trans-Stilben arises. The stereoselectivity can be given in percent, e.g. B. 70% stereoselective or 100% stereoselective.

From the definitions given (H. E. Zimmermann, 1959, and E. L. Eliel, 1966) it follows that every stereospecific reaction is also stereoselective, but not every stereoselective process is also stereospecific. A reaction can be stereospecific in a certain temperature range, above it only stereoselective, e.g. B. the radical addition of hydrogen bromide to 2-bromobut-2-en at low temperatures.