What Are Geometric Isomers?

Geometric isomers include: cis-trans isomers, isotactic polymers and syndiotactic polymers, rotational isomers caused by steric obstacles, and the like. Regarding the above-mentioned inventions of several stereoisomers, in principle, they are all treated as different inventions. The specification must provide identification data that can distinguish the isomers and confirm the isomers. This identification data serves to define new compounds.

The geometric isomers cannot rotate due to the carbon-carbon double bond. Therefore, if two carbon atoms constituting the double bond are connected with different atoms or

Generally, when two double bond carbons have the same atom or simple group attached, the trans isomers have central symmetry, the dipole distance is zero, and the cis isomers are plane symmetrical, and the dipole distance is not Is zero. When the atoms or groups attached to the two double bond carbons are different, the dipole distance of the trans isomer is not zero. For example, although the dipole distance of trans 2-pentene is still smaller than cis 2-pentene, it is not zero. However, this does not mean that the dipole distance of all cis isomers is large. The melting points,

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Geometric isomers often have some differences in chemical properties due to their different spatial arrangements. The more typical manifestations are as follows.

(1) Acid. A more typical example is the comparison of the acidity of maleic acid and fumaric acid. In the cis isomer, the two carboxyl groups are relatively close in space, and the repulsive force between them is relatively large, so the hydrogen atom ionization tendency is increased; and compared to the trans isomer, the cis isomer The steric repulsion attenuates the conjugation of the double bond to the carbonyl electron donor, and makes it easier to ionize hydrogen ions, so the first ionization constant of the cis isomer is larger. However, due to the field effect in the cis isomer, the second-stage ionization is more difficult than the trans isomer.

(2) Ring formation reaction. Considering the stability of the ring, the ring-forming reaction is generally based on the formation of five-membered ring and six-membered ring, so the cis-isomer ring-forming reactivity is usually higher than the trans-isomer. More typical is the reaction of dehydration to anhydride or lactone. For example, maleic acid is easily dehydrated to form maleic anhydride, while trans is more difficult to react. First, the configuration conversion must be completed under high temperature or ultraviolet light. A further ring formation reaction occurred. The cis-trans configuration of the isomers can usually be determined by dehydration to anhydride or ester formation. In addition, some other ring-forming reactions can also be used to determine the configuration. For example, cis-1,2-cyclopentanediol can react with aldehydes and ketones to form ketals, but trans cannot. ^[1]