Prochirality

Prochiralität is a term coined by Kenneth R. Hanson in 1966 stereochemical term He describes the property of a planar molecule which carries three different functional groups or substituents, or a non-chiral, tetrahedral molecule with more than two identical substituents. The compound can form a planar chiral center with a new substituent by an addition reaction, in the tetrahedral substitution of the identical substituents.

Stereochemical Statement

General

A prochiral compound has a plane of symmetry which divides the combination into two parts. The both sides are referred to as " enantiotopic ". Depending on which side will now be given of the attack, creates one or the other of the two mirror images. Is not in the reaction of the sides preferentially attacked, produces a racemic 1:1 mixture of the respective stereoisomers.

Furthermore, where a prochiral compound or a chiral center present, the compound is decomposed into two parts diastereotopic. Re- creates one or the other diastereomer, depending on the side from which the attack takes place. Even if a prochiral molecule is bound to a stereochemically differentiating surface or a receptor protein, the two sides of a planar or a substituent identical tetrahedral molecule are not stereochemically equivalent.

Addition of a planar molecule having a double bond

The trigonal planar sp2 -hybridized carbon atom of an organic compound can be converted into a chiral now sp3 -hybridised carbon atom in a single step by addition of HCl, Cl2, H2, etc.. Here, the new substituent may either above or below the molecular level occur. The sp2 -hybridized, planar substituted carbon atom is then called prochiral. The side that will appear from which the R configuration of the connection, Re is called; The other side is displayed from which the compound (S) -configuration is referred to as Si. An attack from the Re face ( Si face ) did not signify, however, that the product (R ) - configured [(S) -configuration ] is, rather it depends on the priority of the substituents and the priority of the added substance. An example is the addition of hydrogen to pyruvic acid, wherein the chiral lactic acid is formed.

Replacement of a substituent ( stereodescriptors )

It is not necessary that the prochiral atom is sp2 - hybridized; it can also be sp3 - hybridized and carry two identical substituents ( enantiofaces groups), so this molecule is also non- chiral. In the case of such compounds, the designations per - (R) and pro- (S) are used:

Whichever of the two identical substituents, which is (initially only in thought ) replaced, one assigns the higher CIP priority over his enantiofaces partner arbitrary. All other priorities of the substituents with each other otherwise remain untouched. If, in this way (in your mind ) a (S ) -configured " product", the substituent is called ( or group ) "pro- (S )"; and the other is referred to as " pro- (R) ".

By so doing, it is possible that substitution of the pro- (S)- substituent (R)- product is formed.

For example produced from propionic acid by replacing one of the α - hydrogen atoms enantiofaces ( hydrogen atoms of the methylene group -CH2-) by an OH group, the chiral lactic acid. Replacing the pro- (S)- hydrogen atom of the propionic acid leads to (S)- lactic acid. In this case, the above rule that the priorities of the other substituents may not modify, injured. However, this happens twice, because the introduced hydroxy group by CIP to both the carboxyl group and with respect to the methyl group has a higher priority, so cancel the injuries each other and the (S) - product is formed by substitution of the pro- (S ) -hydrogen atom. Replacing the same atom but by a ethyl group, so there is not a violation of the rule, and there is the corresponding ( R) - product ( here: 2-methyl butanoic acid ).