• Stereoselective reactions are those reactions which give one predominant product of the two or more products possible because the reaction pathway has a choice. Either the pathway of lower activation energy is preferred (kinetic control) or the more stable product (thermodynamic control). Thus, E1 dehydration of 1-phenylpropan-1-ol provides (E)-prop-1-enylbenzene as the major product (Scheme 1).

Scheme 1

• On the other hand, stereospecific reactions lead to the production of a single isomer as a direct result of the mechanism of the reaction and the stereochemistry of the starting material. In case of E2 elimination of ((1S,2R)-1-bromopropane-1,2-diyl)dibenzene, only the E-alkene is formed as the product (Scheme 2).

Scheme 2

These two classes of reactions may further be subdivided into enantioselective or diastereoselective reactions and enantiospecific or enantiospecific reactions based on the product formed.


In some reactions, an achiral centre may be converted to a chiral centre. Similarly, a double bond may be converted into chiral centres. To understand the relationship between an achirl substrate and chiral product, two new terms need to be introduced.


Prochirality is the geometric property of an achiral object (or spatial arrangement of points or atoms) which is capable of becoming chiral in a single desymmetrization step. An achiral molecular entity, or a part of it considered on its own, is thus called prochiral if it can be made chiral by the replacement of an existing atom (or achiral group) by a different one. An achiral object which is capable of becoming chiral in two desymmetrization steps is sometimes described as proprochiral (Scheme 3).