Enantiomer

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In chemistry, two stereoisomers are said to be enantiomers if they are mirror images of each other. Much as a left and right hand are different but one is the mirror image of the other, enantiomers are stereoisomers whose molecules are nonsuperimposable mirror images of each other.

Enantiomers have - when present in a symmetric environment - identical chemical and physical properties except for their ability to rotate plane-polarized light by equal amounts but in opposite directions. A solution of equal parts of an optically-active isomer and its enantiomer is known as a racemic solution and has a net rotation of plane-polarized light of zero.

Contents

1 Naming conventions
2 More definitions
3 Enantioselective preparations
4 Enantiopure medications
5 External links
6 See also

Naming conventions

main article: optical isomerism

There are several conventions used for naming chiral compounds, all displayed as a prefix before the chemical name of the substance:

(+)- vs. (−)-; also written d- vs. l-

- based on the substance's ability to rotate polarized light.

D- vs. L-

- based on the actual geometry of each enantiomer, with the version synthesized from naturally occurring (+)-glyceraldehyde being considered the D- form.

(R)- vs. (S)-

- based on the actual geometry of each enantiomer, using the Cahn-Ingold-Prelog priority rules to classify the form. Molecules with multiple chiral centers will have a corresponding number of letters; e.g. natural d-alpha-tocoperol is R,R,R-alpha-tocoperol.

The (+)- vs. (−)- convention is the only one based on optical properties. The other two conventions are based on the actual geometry of each enantiomer. There is no correspondance between any convention. In nature, many chiral substances are only produced in one optical form, while (most) man-made chiral substances are racemic mixtures. the purity of enantiomers can be determined by optical rotation.

More definitions

Any non-racemic chiral substance is called scalemic
A chiral substance is enantiopure or homochiral when only one of two possible enantiomers is present.
A chiral substance is enantioenriched or heterochiral when an excess of one enantiomer is present but not to the exclusion of the other.
Enantiomeric excess or ee is a measure for how much of one enantiomer is present compared to the other. For example, in a sample with 40% ee in R, the remaining 60% is racemic with 30% of R and 30% of S, so that the total amount of R is 70%.

Enantioselective preparations

Several strategies exist for the preparation of enantiopure compounds. This first method is the separation of a racemic mixture into its isomers. Louis Pasteur in his pioneering work 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

In a non-symmetric environment such as a biological environment, enantiomers may react at different speeds with other substances. This is the basis for chiral synthesis.

Enantiopure medications

Advances in industrial chemical processes have made it economical for pharmaceutical manufacturers to take drugs that were originally marketed in racemic form and market the individual enantiomers, each of which may have unique properties. For some drugs, such as zopiclone, only one enantiomer (eszopiclone) is active; the FDA has allowed such once-generic drugs to be patented and marketed under another name. In other cases, such as ibuprofen, both enantiomers produce the same effects.

Examples of racemic mixtures and the corresponding single-enantiomer products that have been marketed include:

Ofloxacin (Floxin) and Levofloxacin (Levaquin)
Methylphenidate (Ritalin) and Dexmethylphenidate (Focalin)
Bupivacaine (Marcain) and Levobupivacaine (Chirocaine)
Cetirizine (Zyrtec) and Levocetirizine (Xyzal)
Albuterol (Ventolin) and Levalbuterol (Xopenex)
Omeprazole (Prilosec) and Esomeprazole (Nexium)
Citalopram (Celexa / Cipramil) and Escitalopram (Lexapro / Cipralex)
Zopiclone (Imovane) and Eszopiclone (Lunesta)
Modafinil (Provigil) and Armodafinil (Nuvigil)

Thalidomide is an example of a racemic drug, in which one enantiomer produces a desirable antiemetic effect, whereas the other is toxic and produces a teratogenic side-effect. However, the enantiomers are converted into each other in vivo, so chemical processes may not be used to mitigate its toxicity.

External links