Stereochemistry - Organic Chemistry summary - Chapter 5

Stereochemistry is the field of chemistry that deals with the structures of molecules in three dimensions. Compounds that have the same molecular formula but are not identical are called isomers; they fall into two classes: constitutional isomers and stereoisomers. Constitutional isomersdiffer in the way their atoms are connected. Stereoisomers differ in the way their atoms are arranged in space. There are two kinds of stereoisomers: cis–trans isomers and isomers that contain chirality centers.

A chiral molecule has a nonsuperimposable mirror image. An achiral molecule has a superimposable mirror image. The feature that is most often the cause of chirality is an asymmetric carbon. An asymmetric carbon is a carbon bonded to four different atoms or groups. An asymmetric carbon is also known as a chirality center. Nitrogen and phosphorus atoms can also be chirality centers. Nonsuperimposable mirror-image molecules are called enantiomers. Diastereomers are stereoisomers that are not enantiomers. Enantiomers have identical physical and chemical properties; diastereomers have different physical and chemical properties. An achiral reagent reacts identically with both enantiomers; a chiral reagent reacts differently with each enantiomer. A mixture of equal amounts of two enantiomers is called a racemic mixture.

A stereocenter is an atom at which the interchange of two groups produces a stereoisomer:asymmetric carbons— where the interchange of two groups produces an enantiomer—and the carbons where the interchange of two groups converts a cis isomer to a trans isomer (or an E isomer to a Z isomer)—are stereocenters. The letters R and S indicate the configuration about an asymmetric carbon. If one molecule has the R and the other has the S configuration, they are enantiomers; if they both have the R or both have the S configuration, they are identical.

Chiral compounds are optically active—they rotate the plane of polarized light; achiral compounds are optically inactive. If one enantiomer rotates the plane of polarization clockwise its mirror image will rotate the plane of polarization the same amount counterclockwise Each optically active compound has a characteristic specific rotation. A racemic mixture is optically inactive. A meso compound has two or more asymmetric carbons and a plane of symmetry; it is an achiral molecule. A compound with the same four groups bonded to two different asymmetric carbons will have three stereoisomers, a meso compound and a pair of enantiomers. If a reaction does not break any bonds to the asymmetric carbon, the reactant and product will have the same relative configuration—their substituents will have the same relative positions. The absolute configuration is the actual configuration. If a reaction does break a bond to the asymmetric carbon,the configuration of the product will depend on the mechanism of the reaction.

A regioselective reaction selects for a particular constitutional isomer; a stereoselective reaction selects for a particular stereoisomer. A reaction is stereospecific if the reactant can exist as stereoisomers and each stereoisomeric reactant leads to a different stereoisomeric product or a different set of stereoisomeric products. When a reactant that does not have an asymmetric carbon forms a product with one asymmetric carbon,the product will be a racemic mixture.

In syn addition the two substituents add to the same side of the double bond; in anti addition they add to opposite sides of the double bond. Both syn and anti addition occur in electrophilic addition reactions that take place by way of a carbocation or a radical intermediate. Addition of to an alkene is a syn addition reaction. Hydroboration–oxidation is overall a syn addition of water. Addition of is an anti addition reaction. An enzyme-catalyzed reaction forms only one stereoisomer; an enzyme typically catalyzes the reaction of only one stereoisomer.

from - Organic chemistry - Bruice - Chapter 5 summary