A stereospecific mechanismspecifiesthe stereochemical outcome of a given reactant, whereas a stereoselective reactionselectsproducts 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 (or no reaction), 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 termstereospecific reactionis ambiguous, since the termreactionitself can mean a single-mechanism transformation (such as theDiels-Alder reaction), 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 termstereospecific reactionis commonly misused to mean 'highly stereoselective reaction'.

Chiral synthesisis built on a combination of stereospecific transformations (for the interconversion of existing stereocenters) and stereoselective ones (for the creation of new stereocenters), where also theoptical activityof a chemical compound is preserved.

The quality ofstereospecificityis 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 ownspecificway.
stereoselectivity-Anenantioselectivereaction is one in which oneenantiomeris formed in preference to the other, in a reaction that creates an optically active product from an achiral starting material, using either a chiral catalyst, anenzymeor a chiral reagent. The degree of selectivity is measured by theenantiomeric excess. An important variant iskinetic resolution, in which a pre-existing chiral center undergoes reaction with a chiral catalyst, an enzyme or a chiral reagent such that one enantiomer reacts faster than the other and leaves behind the less reactive enantiomer, or in which a pre-existing chiral center influences the reactivity of a reaction center elsewhere in the same molecule.

Adiastereoselectivereaction is one in which onediastereomeris formed in preference to another (or in which a subset of all possible diastereomers dominates the product mixture), establishing a preferred relative stereochemistry. In this case, either two or more chiral centers are formed at once such that one relative stereochemistry is favored,[3]or a pre-existing chiral center (which needs not be optically pure) biases the stereochemical outcome during the creation of another. The degree of relative selectivity is measured by thediastereomeric excess.

Stereoconvergencecan be considered an opposite of stereoselectivity, when the reaction of two different stereoisomers yield a single product stereoisomer.

The quality of stereoselectivity is concerned solely with the products, and their stereochemistry. Of a number of possible stereoisomeric products, the reaction selects one or two to be formed.

StereospecificReaction:A reaction in which the stereochemistry of the reactant completely determines the stereochemistry of the product without any other option.

StereoselectiveReaction:A reaction in which there is a choice of pathway, but the product stereoisomer is formed due to its reaction pathway being more favourable than the others available.