What is optical isomerism and how many pairs of optical isomers are there?

When there are four different substituents on a carbon atom, optical isomerism is produced, and this carbon atom is called a chiral carbon atom.

Optical isomerism. Open Categories:

Chemistry, Science.

The phenomenon that two or more molecules exhibit different optical rotation properties due to differences in configuration. These molecules are optical isomers of each other.

There are two reasons for the phenomenon of optical isomerism

A molecule contains one or more chiral atoms (see Chirality). When it contains one chiral carbon atom, there are two optical isomers, which have the relationship of being physical objects and mirror images of each other, so they are also called enantiomers. Enantiomers have equal optical rotation capacity, but the direction of rotation is opposite, and their physical and chemical properties are very similar, such as glyceraldehyde:

A molecule containing two identical chiral carbon atoms has 3 optical isomers like tartaric acid:

where A and B are enantiomers, and the optical rotation ability of C cancels each other out due to the presence of two chiral atoms within the molecule that have the same structure but opposite configuration (see Molecular Configuration), which is called racemate. It and A or B have parts of the same configuration and parts of the molecule that are enantiomerized with each other, and this relationship is called diastereoisomerism. Diastereomers differ not only in optical rotation, but also in terms of physical and chemical properties.

When the cyclic compound contains chiral atoms, the above optical isomerism can also occur. When the molecule contains several different chiral atoms, the number of optical isomers is 2n, and n is the number of different chiral atoms. Optical isomers can be labeled with the D- and L- methods (see Monosaccharides), and the R- and S-methods can be used to label the configuration of each atom in their molecule (see Sequential Rules).

Although some molecules do not contain chiral atoms, they can also cause optical isomerism due to the hindrance of internal rotation in the molecule, such as propadiene or biphenyl optical rotation compounds, which distinguish left and right chirality by one axis in the molecule; Another example is the optical rotation basket type compound molecule, which distinguishes the chiral nature by one plane.

Optical isomers, also known as enantiomers, are metapalmar isomers, optical isomers, mirror isomers or enantiomers, and cannot completely overlap with each other's stereoisomer mirrors.

If we look at the mathematical permutation, there are two stereoisomers with one chiral carbon atom, and the number of stereoisomers of a compound with n chiral carbon atoms should be 2 to the nth power.

However, it is necessary to pay attention to the situation of racemates. Racemates mean that if there are two identical chiral centers in a molecule, and their optical rotation is opposite, the molecule has no stereoisomers.

As shown in the figure, (2R,3R) tartaric acid and (2S,3S) tartaric acid are a pair of stereoisomers, but (2R,3S) tartaric acid and (2S,3R) tartaric acid are the same compound, the so-called racemates. (I'm sorry, but I can't plug in the network at home.) Go find an organic chemistry book, it's all examples.

Therefore, for compounds with different chiral centers, the number of stereoisomers should be 2 to the nth power. For compounds with the same chiral center, the number of stereoisomers may be reduced due to the possible presence of racemates. As for whether there are really racemates, and how many racemates exist, it is necessary to analyze them on a case-by-case basis.

There is no difference between optical isomerism and enantiomerism.

The isomerism phenomenon in which the different arrangements of atoms or atomic clusters in asymmetric molecules in space have different effects on the polarization plane of plane polarized light is called optical isomerism (alias: enantioisomerism, mirror isomerism, chiral isomerism), and the isomer it produces is called optical isomer. There are two reasons for the phenomenon of optical isomerism

A molecule contains one or more chiral atoms.

First of all, it is clear that the configuration isomerism includes optical isomerism, and in addition to the cis-trans isomerism, so the configuration isomerism number should be the optical rotation isomerization number.

The compound is asymmetrical and there is one chiral carbon, so there are two optical isomers; There is no cis-trans isomerism, so the configuration isomerism number is still 2.

When 2 hydroxyl groups are facing the outside of the paper at the same time or towards the paper at the same time, there is a symmetry plane of the compound, and there is no optical rotation at this time, and when one hydroxyl group is outward and the other is inward, there is 1 optical rotation isomerism; The two hydroxyl groups are cis-isomeric at the same time inward or outward at the same time, and one outward and one inward are trans-isomeric, so the isomerism number of the configuration is 2

There is a symmetry plane in the compound, and the optical isomerism number is 0. The presence of two configuration isomers is caused by the hydroxyl cis-trans isomerism. Same as 3Same as 1

Summary. Polarimetric positive and negative is a property that describes the direction of rotation of polarized light in an optical sample, which is closely related to the chemical structure. The difference in the stereostructure of the molecule leads to the different reactivity of the chral center phatons, resulting in isomers of the same form but different chemical properties.

These isomers also have different directions of rotation for polarized light, and their optical rotation properties can be measured in terms of optical rotation index. Therefore, the positive and negative optical rotation and isomers are closely related, and the optical rotation index has also become an important means to detect and identify chiral compounds.

Can you elaborate on that a little bit more?

Polarimetric positive and negative is a property that describes the direction of rotation of polarized light in an optical sample, which is closely related to the chemical structure. Different stereotypes of molecules lead to different reactivity of chiral centers, resulting in isomers with the same shape and nanobeat formula but different chemical properties. These isomers also differ in the direction of rotation of polarized light, and their optical rotation properties can be measured in terms of optical rotation index.

Therefore, the positive and negative optical rotation and isomers are closely related, and the optical rotation index has also become an important means to detect and identify chiral compounds.

Stereoisomers that mirror each other are called enantiomers (enantiomers).Enantiomers all have optical rotation, one of which is left-handed and one is right-handed. Therefore, enantiomers are also called optical isomers.

To put it simply, the relationship between two isomers is like the three-dimensional structure of an object looking in a mirror, and the three-dimensional structure and its image in the mirror are enantiomers of each other.

When plane-polarized light passes through a solution of chiral compounds, the direction of the polarization plane is rotated by an angle. This ability to rotate the polarization plane is called optical rotation.

Optical rotation is the property of chiral molecules To judge whether a substance has optical rotation or not, it is only necessary to see whether it is a chiral molecule. Methods for determining whether a substance is a chiral molecule:

Let's see if there is chiral carbon (that is, the same saturated carbon is connected to four different groups), and if not, it is definitely not.

See if the substance has a symmetry center or plane of symmetry, and if it has, it is not a chiral molecule.

Optical activity is optical rotation.

There are many substances in nature that do not affect the vibration surface of polarized light, such as water, ethanol tamperol, acetone, glycerol and sodium chloride. There are other substances that can deflect the vibrating surface of polarized light, such as a solution of lactic acid and glucose. Substances that can deflect the vibrational surface of polarized light are called optical rotation substances.

Substances that cannot deflect polarized light on the vibrational surface are called non-optical rotating substances, and they do not have optical rotation. When polarized light passes through a solution of optical rotating substances, it can be observed that some substances can rotate the vibration of the polarized light at a certain angle to the left (counterclockwise).

This substance is called a levorotator, which is left-handed and denoted by " "; Other substances rotate the vibration of polarized light at a certain angle to the right (clockwise), called dextras, which are right-handed, and are denoted by " ". In the past, "l, d" was used to indicate left and right rotation.