All compounds with one chiral carbon atom have a pair of enantiomers.Compounds containing two different chiral carbon atoms have four optically active isomers (two enantiomers).With the further increase of the number of different chiral carbon atoms in the molecule, the number of optical isomers will also increase. The number of optically active isomers of the molecule containing n different chiral carbon atoms is theoretically 2n,EnantiomerIs 2n-1yes.
If two chiral carbon atoms contain an identical atom or group, it is also customary to place two identical atoms or groups on theFischer projectionThe same side of is called red;It is called Su type on the opposite side.Therefore, the four isomers can also be named after the red and Soviet type.[1]
Judgment method
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Chiral carbon atom
1. Chiral carbon atoms must be saturated carbon atoms;
2. If the four groups connected by the chiral carbon atoms are different.
optical rotation
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For most organic molecules, molecular chirality is mainly caused by the chiral carbon atoms in the molecule.Chiral molecules are optically active, and non chiral molecules are not optically active.For example, the real propionic acid molecule and the mirror image can overlap completely, and there is a symmetry plane in the molecule, which has no chirality, so it has no optical activity;The physical and mirror images of lactic acid (2-hydroxypropionic acid) molecule cannot overlap completely. There is no symmetry plane or symmetry center in the molecule, which is chiral, so it has optical activity.Therefore, the chirality of compounds is a sufficient and necessary condition to produce optical activity.[2]
Racemate
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Some compounds have chiral carbon atoms in their molecules, but due to symmetry factors in the molecules, the molecules have no optical activity, such as (2R, 3S) - tartaric acid. Such compounds are called racemates.Both the racemate and the racemate have no optical activity, but the racemate can be separated into left and right racemates, and the racemate cannot be separated.[2]
classification
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Classification of chiral carbon atoms and chiral molecules.
Chiral carbon atoms: The carbon atoms connected with four different atoms or groups are called chiral carbon atoms.There are molecules containing one chiral carbon atom, two chiral carbon atoms, three chiral carbon atoms and four chiral carbon atoms.Some molecules do not contain chiral carbon atoms.
Classification of chiral molecules: chiral molecules can be divided into chiral molecules with chiral carbon atoms and chiral molecules without chiral carbon atoms according to whether they contain chiral carbon atoms.However, isomers of molecules containing multiple chiral carbon atoms are not necessarily chiral molecules.[3]
