User:134330mikrokosmos/Enantiomer

= Enantiomer = In chemistry, an enantiomer (/ɪˈnænti.əmər, ɛ-, -oʊ-/ ih-NAN-tee-ə-mər; from Ancient Greek ἐνάντιος (enántios) 'opposite', and μέρος (méros) 'part') – also called optical isomer, antipode, or optical antipode – is one of two stereoisomers that are non-superposable onto their own mirror image. Enantiomers are much like one's right and left hands, when looking at the same face, they cannot be superposed onto each other. No amount of reorientation will allow the four unique groups on the chiral carbon (see Chirality (chemistry)) to line up exactly. The number of stereoisomers a molecules has can be determined by the number of chiral carbons it has. Stereoisomers include both enantiomers and diastereomers.

Diastereomers, like enantiomers, share the same molecular formula and are non-superposable onto each other however, they are not mirror images of each other.

A molecule with chirality is considered optically active due to its ability to rotate a plane of polarized light. Enantiomers can also do this optical rotation, but a mixture with equals amounts of each enantiomer is optically inactive. This is referred to as a racemic mixture or a racemate. A racemic mixture is optically inactive because the opposite and equal rotation of the enantiomers result in a net zero rotation hence their lack of optical activity.

Naming conventions
The naming convention for enantiomers can be divided into three different categories. These naming systems are the R/S system and the (+)- and (-)- system (the dextrorotatory (d-) and levorotatory (l-) system is also synonymous with the (+)- and (-)- system).

The R/S system is based on the molecule's geometry with respect to a chiral center. The R/S system is assigned to a molecule based on the priority rules assigned by Cahn–Ingold–Prelog priority rules, in which the group or atom with the largest atomic number is assigned the highest priority and the group or atom with the smallest atomic number is assigned the lowest atomic number.

In contrast the (+)- and (-)- is used when a molecule is designated by its optical rotation properties (see Optical rotation). When a molecule is denoted dextrorotatory it is rotating the plane of polarized light clockwise and can also be denoted as (+). When it is denoted as levorotatory it is rotating the plane of polarized light counterclockwise and can also be denoted as (-). The Latin words for left are laevus and sinister, and the word for right is dexter (or rectus in the sense of correct or virtuous). The English word right is a cognate of rectus. This is the origin of the L/D and S/R notations, and the employment of prefixes levo- and dextro- in common names.

Enantioselective preparations
There are two main strategies for the preparation of enantiopure compounds. The first is known as chiral resolution. This method involves preparing the compound in racemic form, and separating it into its isomers. In his pioneering work, Louis Pasteur was able to isolate the isomers of tartaric acid because they crystallize from solution as crystals each with a different symmetry. A less common method is by enantiomer self-disproportionation.

The second strategy is asymmetric synthesis: the use of various techniques to prepare the desired compound in high enantiomeric excess. Techniques encompassed include the use of chiral starting materials (chiral pool synthesis), the use of chiral auxiliaries and chiral catalysts, and the application of asymmetric induction. The use of enzymes (biocatalysis) may also produce the desired compound. However, this procedure is highly complex in that it requires asymmetric induction to be taken into account in order to receive the desired enantiomer, making this method less desirably than existing ones.

A third strategy is Enantioconvergent synthesis, the synthesis of one enantiomer from a racemic precursor, utilizing both enantiomers. By making use of a chiral catalyist, both enantiomers of the reactant result in a single enantiomer of product.

A fourth and more effective method that is continuously being built upon is chiral chromatographic separation. One of the most commonly used being, high-performance liquid chromatography (HPLC). This method can be indirect or direct based on the type of stationary phase used; the indirect one uses an achiral stationary phase while the direct method uses a chiral one.

It is considered efficient and has been widely used in the past few years due to a few factors. The first being a direct injection into the column of the HPLC system has considerably reduced the time of sample preparation. Secondly, its good selectivity in that it can distinguish between two enantiomers with the use of an effective stationary phase like, the widely used cellulose derivative, which has a large capacity for chiral recognition. Lastly, HPLC has a super resolution performance.

However, there are problems with choosing a chiral column that is specific for a certain enantiomer. Coupled with the high costs and limited resolution of chiral stationary phases there is ongoing to research in the field to find a cost effective way to produce an efficient and low cost chiral stationary phase.

Enantiomers may not be isolable if there is an accessible pathway for racemization (interconversion between enantiomorphs to yield a racemic mixture) at a given temperature and timescale. For example, amines with three distinct substituents are chiral, but with few exceptions (e.g. substituted N-chloroaziridines), they rapidly undergo "umbrella inversion" at room temperature, leading to racemization. If the racemization is fast enough, the molecule can often be treated as an achiral, averaged structure.

Quasi-enantiomers
Quasi-enantiomers are molecular species that are not strictly enantiomers, but behave as if they are. In quasi-enantiomers majority of the molecule is reflected; however, an atom or group within the molecule is changed to a similar atom or group. Quasi-enantiomers can also be defined as molecules that have the potential to become enantiomers if an atom or group in the molecule is replaced. An example of quasi-enantiomers would (S)-bromobutane and (R)-iodobutane. Under normal conditions the enantiomers for (S)-bromobutane and (R)-iodobutane would (R)-bromobutane and (S)-iodobutane respectively. Quasi-enantiomers would also produce quasi-racemates, which are similar to normal racemates (see Racemic mixture) in that they form an equal mixture of quasi-enantiomers

Though not considered actual enantiomers, the naming convention for quasi-enantiomers also follows the same trend as enantiomers when looking at (R) and (S) configurations - which are considered from a geometrical basis (see Cahn–Ingold–Prelog priority rules).

Quasi-enantiomers also have applications in parallel kinetic resolution.