Geometric isomers include: cis trans isomers, identicalStereoisomerismPolymers and isomeric polymers, rotational isomers caused by steric obstacles, etc.In principle, the above-mentioned stereoisomer inventions are treated as different inventions. The specification must provide identification data that can distinguish isomers and confirm isomers.This identification data serves to define new compounds.
Chinese name
Geometric isomer
Foreign name
Geometric isomer
Type
Cis trans isomers, etc
Role
Definition of new compounds
dipole moment
The trans dipole distance is zero, and the cis dipole distance is not zero
Boiling point
The boiling point of cis isomer is higher than that of trans isomer
Since the carbon carbon double bond of geometric isomer cannot rotate, if two carbon atoms forming the double bond are connected with different atoms orGroupDifferent isomers will be produced, for example,2-buteneThere are two different isomers.Two methyl groups are on the same side of the double bond, called cis 2-butene;The two methyl groups are on the opposite side of the double bond, which is called trans 2-butene.The two olefins have the same structure but belong to differentchemical compound。At room temperature, chemical bond rotation cannot transform each other. The only difference is that the space positions of atoms or groups in molecules are different, so they belong to stereoisomers.However, they do not have a mapping relationship, so they are notEnantiomer。Similar to disubstituted cycloalkanes, we call this isomer geometric isomer or cis trans isomer.
Cis trans isomerNot only their physical properties are different, but also their physiological activities are sometimes very different, such as twoDiethylstilbestrolOnly the trans isomers can be used to treat some gynecological diseases, which have different physical properties. The two can be separated by simple physical methods.Not all olefins have cis trans isomerization.An olefin with cis trans isomerism must have two different atoms or atomic groups attached to the two carbon atoms forming the double bond.[1]
Dipole distance, melting point and boiling point
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Generally, when two double bonded carbons are connected with the same atom or simple group, the trans isomer has a central symmetry, and the dipole distance is zero, while the cis isomer has a plane symmetry, and the dipole distance is not zero.When the atoms or groups connected by two double bonded carbons are different, the dipole distance of the trans isomer is not zero.For example, trans2-penteneAlthough the dipole distance of is still less than cis 2-pentene, it is not zero.However, this does not mean that the dipole distance of cis isomers is large, and the melting point of most geometric isomersboiling pointThe difference of is characteristic.Generally, the dipole distance of cis isomer is greater than that of trans isomer, so the boiling point of cis isomer is higher than that of trans isomer;However, the melting point of trans isomers is higher than that of cis isomers due to their large symmetry and tight arrangement between molecules.Therefore, the melting point and boiling point can be used to determine the configuration of geometric isomers.[2]
Spectral property
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isolatedCarbon carbon double bondThe ultraviolet absorption of is in vacuum ultraviolet, and when the double bond participates in conjugation, the ultraviolet absorption will move to the long wave.Therefore, if the double bond in the conjugated system has geometric isomerism, the conjugation of cis isomer will be affected due to the large space barrier, and the maximum ultraviolet absorption will move to the short wave.A typical example isStilbeneType compounds, especially when substituents are reintroduced at the ortho position of the benzene ring.Geometrically isomericInfrared absorption spectrumThere are also obvious differences in characteristics.In the symmetric substituted trans isomer, the dipole distance of the symmetric stretching vibration of the double bond changes to zero, so it is infrared inactive.For example, cis 1,2-Dichloroethylene at 1590 cm-1There is absorption at this position, but the trans isomer has no absorption at this position.For the stretching vibration of carbon carbon double bond in asymmetric substituted alkenes, the cis isomer is generally stronger than the trans isomer, because generallyCis isomerThe dipole moment of is relatively large.stayNuclear magnetic resonance spectrumIn, the symmetrically substituted R-CH-CH-R, whether cis or trans, is chemically equivalent and has the same chemical shift.For another example, the two hydrogen atoms on the anti decalin bridgehead carbon are chemically equivalent because they are both in an upright bond, while the two hydrogen atoms on the cis decalin bridgehead carbon are chemically equivalent, although one is in an upright bond and the other is in a flat bond, the cis decalin has conformational isomerism, and the two hydrogen atoms are also chemically equivalent.[2]
chemical property
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Geometric isomers often have some differences in chemical properties due to their different spatial arrangements.It is typical in the following aspects.
(1) Acidity.A typical example isMaleic acidandFumaric acidAcidity comparison.In cis isomers, the two carboxyl groups are relatively close in space, and the repulsive force between them is relatively large, so the ionization tendency of hydrogen atoms is increased;Moreover, compared with the trans isomer, the space exclusion in the cis isomer weakens the electron donor conjugation of the double bond to the carbonyl group, making it easier to ionize outHydrogen ionTherefore, the first ionization constant of cis isomers is large.However, because of the field effect in cis isomers, the second ionization is more difficult than that in trans isomers.
(2) Cyclization reaction.Considering the stability of the ring, the cyclization reaction is generally dominated by the formation of five membered ring and six membered ring, so the cyclization reactivity of cis isomers is generally higher than that of trans isomers.The typical reaction is dehydration to anhydrides or lactones, for example, maleic acid is easy to be dehydrated to formMaleic anhydrideThe trans is more difficult to react. First, the configuration transformation should be completed under high temperature or ultraviolet light, and then the cyclization reaction should be further carried out.It is usually possible to determine the cis trans configuration of isomers by dehydration to anhydride or ester.In addition, other cyclization reactions can also be used to determine the configuration. For example, cis-1,2-cyclopentanediol can react with aldehydes and ketones to form ketals, while trans can't.[1]
