-By Rohan Purohit
INTRODUCTION
The word enantiomer is derived from two Greek words that are enantios and meros which means opposite and part respectively. As it is sometimes related to optical activities it is also called an optical isomer or antipode. They usually are mirrored images of each other but are non-identical (superimposable). 
ENTAMORISM: ITS CHARACTERISTICS AND CRITERION
- The carbon atoms that are asymmetrical have four bonds with four different atoms or groups.
- These can be arranged in two different ways that they are not superimposable. Most of the compounds that have one or two asymmetrical carbon atoms show entamorism, but this is not always true there are some exceptions.
Some compounds have one or two asymmetrical carbon atom but have a plane of symmetry against the whole molecule, they are known as meso compounds.- A meso compound does not have a mirror image stereoisomer (is a form of isomerism in which molecules have the same molecular formula) because it is its mirror image.
- For example, meso tartaric acid (as you can see in the image on the right) has two asymmetrical carbon atoms but still, it does not exhibit enantiomerism because each of the halves of the molecule is equal and opposite.
OPTICAL ACTIVITIES
The ability of a chiral molecule to rotate a plane-polarized light is called optical activity. It is an important character of isomers and is measured using a polarimeter. A polarimeter consists of the following things-
- A regular light source
- polarizing lens
- sample tube
- Analyzing lens
Working of a polarimeter:-
Here first of all the light is passed from the light source to the sample tube that can rotate the plane-polarized light.
The light appears to be dim as the light no longer passes straight through the polarizing filters.
Then the rotation is quantified and compared to the degrees that the analyzing lens must be rotated by so that no dimming occurs.
OPTICAL ACTIVITY MEASUREMENT
When the rotation is calculated using a polarimeter it is known as an observed rotation, because rotation is affected by path length (l, at the time the light travels through a sample) and concentration (c indicates that what proportion of the sample is present which will rotate the light). When these effects are expelled a standard for comparison of all the molecules is received, the specific rotation, [a].
[a] = 100a / cl when concentration is expressed as g sample /100ml solution=
Specific rotation is a physical property like the boiling point or melting point of a sample.
Enantiomers will rotate the plane of polarisation is justequal amounts (same magnitude) but in opposite directions.
- Dextrorotary symbolized as d or (+), clockwise rotation (into the right)
- Levorotary symbolized as l or (-), anti-clockwise rotation (into the left)
If only one enantiomer is present in a sample, the sample is considered to be optically pure. When a sample consists of a mix of enantiomers, the effect of every enantiomer cancels out, molecule for molecule.
For example, a 50:50 mixture of two enantiomers (racemic mixture) will not rotate plane polarised light and is considered optically inactive. A mixture that contains one enantiomer in excess, however, will display a net plane of polarisation within the characteristics of the enantiomer that’s excess.
Determining Optical Purity
The optical purity or the enantiomeric excess (ee %) of a sample are often determined as follows:
Optical purity = % enantiomeric excess = % enantiomer1 – % enantiomer2
= 100 [a]mixture / [a]pure sample
ee% = 100 ([major enantiomer] – [minor enantiomer]) / ([major enantiomer] + [minor enantiomer])
Where [major enantiomer] = concentration of the main enantiomer
[minor enantiomer] = concentration of the minor enantiomer
Diastereomeric substances can have different rotations both in sign and in magnitude.
Note: A racemic mixture contains an equal mixture of two enantiomers with a specific rotation to 0o which makes them behave as optically inactive compounds/mixtures.
REFERENCES
1. “enantiomer – definition of enantiomer in English from the Oxford dictionary”. OxfordDictionaries.com. Retrieved 2016-01-20.
2. ^ IUPAC, 2nd ed. (the “Gold Book”) (1997). Online corrected version: (2006–) “optical isomers”. doi:10.1351/gold book.O043083.
3.IUPAC, , 2nd ed. (the “Gold Book”) (1997). Online corrected version: (2006–) “enantiomer”. doi:10.1351/goldbook.E02069
4.http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/chirality.html
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6. ^ 1921-2008., Eliel, Ernest L. (Ernest Ludwig) (1994). Stereochemistry of organic compounds. Wilen, Samuel H., Mander, Lewis N. New York: Wiley. ISBN 0471016705. OCLC 27642721.
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8. ^ In the sense used by particle physicists (see ref. 6), the “true” enantiomer of a molecule, which has precisely thesame mass-energy content because theoriginal molecule, maybe a mirror-image, but also built from antimatter (antiprotons, antineutrons, and positrons). Nevertheless, during thisarticle, “enantiomer” is going to be utilized inthe chemical sense and still aska nonidentical, mirror-image isomeric compound, consisting of ordinary matter elementary particles, especially.
9. ^ “P-101.8.1 Inversion of configuration” (PDF), IUPAC Provisional Recommendations—Preferred IUPAC Names, p. 48
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