In 1,2-dichloroethane a , free rotation about the C—C bond allows the two structures to be interconverted by a twist of one end relative to the other. In 1,2-dichloroethene b , restricted rotation about the double bond means that the relative positions of substituent groups above or below the double bond are significant.
In 1,2-dichloroethane part a of Table 1 , there is free rotation about the C—C bond. The two models shown represent exactly the same molecule; they are not isomers. You can draw structural formulas that look different, but if you bear in mind the possibility of this free rotation about single bonds, you should recognize that these two structures represent the same molecule:. In 1,2-dichloroethene T able 1 , however, restricted rotation about the double bond means that the relative positions of substituent groups above or below the double bond become significant.
This leads to a special kind of isomerism. These two compounds are c is -trans isomers , compounds that have different configurations groups permanently in different places in space because of the presence of a rigid structure in their molecule.
We could name it 2-butene, but there are actually two such compounds; the double bond results in cis-trans isomerism Figure 2. Cis-trans isomers have different physical, chemical, and physiological properties. Cis butene has both methyl groups on the same side of the molecule. Trans butene has the methyl groups on opposite sides of the molecule.
Stereoisomers may be described using other notation besides cis- and trans-. The E-Z system is used instead of cis-trans for compounds that have more than two substituents. When used in a name, E and Z are written in italic type. Actively scan device characteristics for identification.
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From the Editors at Merriam-Webster. What does 'cisgender' mean? Statistics for cis Look-up Popularity. Style: MLA. More Definitions for cis. Medical Definition of cis Entry 1 of 2.
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