Stereoisomers:
They can rotate polarized light either to the:
1- Right = Clockwise rotation => (+) or (D) or (R) D: Dextrorotatory
OR to the
2- Left = Counterclockwise rotation => (-) or (L) or (S) L: Levorotatory
Note:
1- An Optically inactive substance is the one that is not capable of rotating the plane of polarized light.
2- An optically active substance is the one that is capable of rotating the plane of polarized light.
Enantiomers:
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They are type of stereoisomers.
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They are molecules that are not superimposable
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One molecule is mirror image to the second molecule.
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In enantiomers , the carbon atom is chiral ( bonded to 4 different groups of atoms)
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Note that if the carbon atom is bonded to 2 similar groups or more, this carbon is said Achiral.
Note: Rotate molecule II, either clockwise or anti-clockwise, it becomes superimposable with molecule I => molecule I and II are not enantiomers. => Optically inactive.
Characteristics of Enantiomers:
Enantiomers have identical physical properties, except for the rotation of the plane of polarized light.
- Two enantiomers rotate plane-polarized light to an equal extent but in opposite directions. Thus, if enantiomer A rotates polarized light +5°, the same concentration of enantiomer B rotates it –5°.
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Enantiomers have identical chemical properties except toward optically active reagents ( Same rate of reaction, same products, same catalyst, …)
Examples about optically active reagents: enzymes, some catalysts…
Example:
(+) glucose is the main sugar in animals; it plays a unique role in animal metabolism.
(-) glucose can’t be metabolized by animals or fermented by yeasts.
How to determine whether the configuration of the carbon atom is R (+) or S (-)?
Rules:
1- The carbon atom should be Chiral (attached to 4 different groups).
2- Apply the following sequence rule for the Chiral carbon:
Sequence rule:
1- Number the groups attached to the carbon atom according to their priority. (The priority depends on the atomic number.)
2- If two atoms are identical, compare the atoms to which these atoms are joined.
3- If the direction is clockwise => R, and if the direction is anticlockwise => S.
Note:
If the group with lowest priority is pointed forward (toward you),
Then if it’s:
1- clockwise
(To the right)=> S
2- anticlockwise
(To the left) => R
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Summary For Isomers
For more information and if you like to practice exercise about enantiomers, you can visit the following website:
http://www.chemistry.msu.edu/faculty/reusch/VirtTxtJml/sterism3.htm
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