Isomerism cis-trans in alkenes

We saw in Chapter 1 that the carbon-carbon double bond can be described in two ways. In valence bond language (Section 1.8), the carbons are 5p2-hybridized and have three equivalent hybrid orbitals that lie in a plane at angles of 120° to one another. The carbons form a a bond by head-on overlap of sp orbitals and a tt bond by sideways overlap of unhybridizedp orbitals oriented 

Broken tt bond after rotation (p orbitals are perpendicular) 

Cis-trans isomerism is not limited to (//substituted alkenes. It can occur whenever both double-bond carbons are attached to two different roups. If one of the double-bond carbons is attached to two identical groups, however, then cis-trans isomerism is not possible (1-igure 6.4). 

These two compounds are identical they are not cis-trans isomers. 

The lack of rotation around carbon-carbon double bonds is of more than just theoretical interest it also has chemical consequences. Imagine the situation for a disubstifitted alkene such as 2-butene. Disubstitilted means that two substituents other than hydrogen are bonded to the double-bond carbons.) The two methyl groups in 2-bulene can be either on the same side of the double bond or on opposite sides, a situation similar to that in disubstitutecl cycloalkanes (Section 4.2). 

Test your knowledge of Key Ideas by using resources in ThomsonNOW or by answering end-of-chapter problems marked with A. 

Fig ure 6.3 Cis and trans isomers of 2-butene. The cis isomer has the two methyl groups on the same side of the double bond, and the trans isomer has the methyl groups on opposite sides. 

Since bond rotation can t occur, the two 2-butenes can t spontaneously interconvert they are different, isolable compounds. As with disubstituted cycloalkanes (Section 3,8), we call such compounds cis-trans stereoisomers. The compound with substituents on the same side of the double bond is called cis-2-butene, and the isomer with substituents on opposite sides is rofts-2-butene. 

Cis-trans isomerism is not limited to disubstituted alkenes. It can occur in any alkene that has both of its double-bond carbons attached to two dif- 

The requirement for cl -trans isomerism in alkenes. Compounds that have one of their carbons bonded to two identical groups can t exist as cis-trans isomers. Oniy when both carbons are bonded to two different groups are cis-trans isomers possible. 

Problem 6.7 Which of the following compounds can exist as pairs of cis-trans isomers Draw each cis-trans pair, and indicate the geometry of each isomer. 

Problem 6.8 Cyclodecene can exist in both cis and trans forms, but cyclohexene cannot. Explain. (Making molecular models will be helpful.) 

Change the following old names to new, post-1993 names, and draw the structure of each compound  

FIGURE 7.1 The -it bond must break for rotation to take place around a carbon-carbon double bond. 

Since bond rotation can t occur, the two but-2-enes can t spontaneously interconvert they are different, isolable compounds. As with disubstituted cycloalkanes, we call such compounds cis-trans stereoisomers. 

Because rotation at carbon-carbon double bonds is restricted, cis-trans isomerism (geometric isomerism) is possible in appropriately substituted alkenes. For example, 1,2-dichloroethene exists in two different forms  

This representation is simplified because there are actually several additional intermediates between rhodopsin and the fully dissociated frans-retinal and opsin. 

These stereoisomers are not readily interconverted by rotation around the double bond at room temperature. Like cis-trans isomers of cycloalkanes, they are configurational stereoisomers and can be separated from one another by distillation, taking advantage of the difference in their boiling points. 

Are cis-trans isomers possible for 1-butene and 2-butene Solution 2-Butene has cis-trans isomers, but 1-butene does not 

For 1-butene, carbon-1 has two identical hydrogen atoms attached to it therefore, only one structure is possible. 

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Cis-trans isomerism in alkenes arises because the electronic structure of the carbon-carbon double bond makes bond rotation energetically unfavorable at normal temperatures. Were it to occur, rotation would break the pi part of the double bond by disrupting the sideways overlap of two parallel p orbitals (Figure 23.2). In fact, an energy input of 240 kj/mol is needed to cause bond rotation. 

Another approach for a stereochemical index Q encoding information on cis-trans isomerism in alkenes was described by Estrada on including a corrected electron charge density calculated with the MOP AC version 6.0, the following... 

Cis—trans isomerism in alkenes is possible only when both of the double-bond carbon atoms have two different groups. 

Thus far, we have considered cis-trans isomerism in alkenes containing only one carbon-carbon double bond. For an alkene with one carbon-carbon double bond that can show cis-trans isomerism, two cis-trans isomers are possible. For an alkene with n carbon-carbon double bonds, each of which can show m-fraw3 isomerism, 2" ris-frawi isomers are possible. 

Some Facts about Double Bonds The Orbital Model of a Double Bond the Pi Bond Cis Trans Isomerism in Alkenes A WORD ABOUT... 

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