Chirality and Stereogenic Centers
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What is Chirality?
?The word "chirality" refers to the property of "handness". You right and left hands are very similar, yet they are not identical. They are related to each other as mirror images, and as such they can not be superimposed on top of each other. Molecules can also be chiral if they contain one or more chiral centers.
What is Chiral Centre?
For the purposes of introductory organic chemistry, a chiral center can be defined as an sp3 hybridized carbon which is bonded to four different groups. A more contemporary term for "chiral center" (or chiral carbon) is stereogenic center (or stereocenter), and the terms will be used interchangeably. i.e.If all the four bonds of carbon are satisfied by four different atoms / groups, it is chiral. Here it should be noted that isotopes are regarded as different atoms?groups chiral carbon is designated by an asterisk (*).
Chirality, resulting from the presence of four different groups around a carbon results from an asymmetry in the molecule. This can be seen by examining the molecules shown below; when two identical groups are on one carbon, there is an internal mirror plane passing through the molecule; when four groups are present, there is no internal mirror plane (no symmetry) and hence the carbon is chiral.
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Optical Isomerism in 2-bromobutane
The carbon which is bound to the bromine (carbon #2) is a stereogenic center; it is sp3 hybridized and it is bonded to four different groups i.e. a hydrogen atom, a bromine atom, a methyl group and an ethyl group.
All of the other carbons in this molecule are not stereogenic centers, since they are all bonded to at least two hydrogens.
2-Chloro-3-bromobutane has two stereogenic centers:
Carbon #2 in this molecule is sp3 hybridized and is attached to four different groups i.e. a hydrogen atom, a chlorine atom. a methyl group and a – CHBrCH3 group.
Likewise, carbon #3 is also sp3 hybridized and is attached to four different groups:
a hydrogen atom , a bromine atom, a methyl group, and a -CHClCH3 group.
The remaining carbons are both sp3 hybridized, but each is attached to three hydrogens, and neither is a stereogenic center.
Optical Isomerism in bromochloroiodomethane
The structural formula of bromochloroiodomethane is
The molecule has one chiral carbon as designated by star. So molecule is chiral. It is non - superimposable on its mirror image.
According to Van't Hoff rule,
Total number of optical isomers should be = 2n; when n is number of chiral centre
The Fischer projections of the two isomers are
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Non-super imposable
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Can rotate PPL
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Optically active
Stereoisomers which are mirror - image of each other are called enantiomers or enantiomorphs. (i) and (ii) are enantiomers. All the physical and chemical properties of enantiomers are same except two:
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They rotate PPL to the same extent but in opposite direction. One which rotates PPL in clockwise direction is called dextro-rotatory (dextro is Latin word meaning thereby right) and is designated by d or (+). One which rotates PPL in anti-clockwise direction is called laevo rotatory (means towards left) and designated by l or (–).
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They react with optically active compounds with different rates.
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