Depto. Química Orgánica

CHIRALITY AND OPTICAL ACTIVITY

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15/07/2017
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Your hands are 'isomeric' because they do have the same finger pattern but are object and non-superimposable mirror image. They look pretty much the same but are definitely different.
This property is called CHIRALITY.
Threfore, a molecule is CHIRAL if and only if its mirror image is non-superimposable, regardless whether we turn it around any of the 3D axis or even twist every of its single bonds within.
Object and non-superimposable mirror image are ENATIOMERS or ENANTIOISOMERS. They are CHIRAL, different molecules that display a different physical property: OPTICAL ROTATION of polarized light.

VIDEO Quiralidad
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In this reaction, is only one product being produced? Or is it two different compounds?
Can you superimpose the two bromoderivatives?
The answer to the last question is NO.
Consequently, the reaction does give a mixture of two different compounds, object and non-superimposable mirror image, i.e. two ENANTIOMERS. Why two ENANTIOMERS are produced?
Because the two hydrogens that can suffer the reaction are equivalent to each other and have the same chance of being attacked by bromine.
An important class of CHIRAL MOLECULES are those bearing one or more carbons with four different substituents.
Those special carbons are denoted as STEREOGENIC CENTERS or STEREOCENTERS.
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However, to have an even more complex picture of STEROISOMERISM, there exist molecules that are CHIRAL without having any STEREOCENTER.
The most relevant examples are those of ALLENES, BIPHENYLS and BINAPHTHYLS, among many others.
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ALLENES
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BIPHENYLS
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BINAPHFTHYLS
ENANTIOMERS bear the very same physical and chemical properties, importnat exception made of their response to polarized light (OPTICAL ACTIVITY). That's why they are also called OPTICAL ISOMERS.
The specific optical rotation, that is assessed by means of a POLARIMETER, is a physical property characteristic to the structure of each ENANTIOMER, its concentration and the solvent used in the measurement.
The measurement of the specific rotation, when that of each ENATIOMER is known, indicates the ENANTIOMERIC PURITY of the product.
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VIDEO Separación enantiómeros ibuprofeno
The NON-CHIRAL compounds or the 1:1 mixture of enantiomers (RACEMIC MIXTURES or RACEMATES) are optically INACTIVE and thus give a NULL optical rotation.
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