In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomericreactionproducts from different stereoisomeric reactants, or which operates on only one of the stereoisomers. In contrast, stereoselectivity is the property of a reactantmixture where a non-stereospecific mechanism allows for the formation of multiple products, but where one of the products is favored by factors, such as steric access, that are independent of the mechanism. A stereospecific mechanism specifies the stereochemical outcome of a given reactant, whereas a stereoselective reactionselects products from those made available by the same, non-specific mechanism acting on a given reactant. Given a single, stereoisomerically pure starting material, a stereospecific mechanism will give 100% of a particular stereoisomer, although loss of stereochemical integrity can easily occur through competing mechanisms with different stereochemical outcomes. A stereoselective process will normally give multiple products even if only one mechanism is operating on an isomerically pure starting material. The term stereospecific reaction is ambiguous, since the term reaction itself can mean a single-mechanism transformation, which could be stereospecific, or the outcome of a reactant mixture that may proceed through multiple competing mechanisms, specific and non-specific. In the latter sense, the term stereospecific reaction is commonly misused to mean highly stereoselective reaction. Chiral synthesis is built on a combination of stereospecific transformations and stereoselective ones, where also the optical activity of a chemical compound is preserved. The quality of stereospecificity is focused on the reactants and their stereochemistry; it is concerned with the products too, but only as they provide evidence of a difference in behavior between reactants. Of stereoisomeric reactants, each behaves in its own specific way. Stereospecificity towards enantiomers is called enantiospecificity.
Examples
at sp3 centres can proceed by the stereospecific SN2 mechanism, causing only inversion, or by the non-specific SN1 mechanism, the outcome of which can show a modest selectivity for inversion, depending on the reactants and the reaction conditions to which the mechanism does not refer. The choice of mechanism adopted by a particular reactant combination depends on other factors. For example, tertiary centres react almost exclusively by the SN1 mechanism whereas primary centres react almost exclusively by the SN2 mechanism. When a nucleophilic substitution results in incomplete inversion, it is because of a competition between the two mechanisms, as often occurs at secondary centres, or because of double inversion. The addition of singlet carbenes to alkenes is stereospecific in that the geometry of the alkene is preserved in the product. For example, dibromocarbene and cis-2-butene yieldcis-2,3-dimethyl-1,1-dibromocyclopropane, whereas the transisomer exclusively yields the trans cyclopropane. This addition remains stereospecific even if the starting alkene is not isomerically pure, as the products' stereochemistry will match the reactants'. The disrotatoryring closing reaction of conjugated trienes is stereospecific in that isomeric reactants will give isomeric products. For example, trans,cis,trans-2,4,6-octatriene gives cis-dimethylcyclohexadiene, whereas the trans,cis,cis reactant isomer gives the trans product and the trans,trans,trans reactant isomer does not react in this manner.
