Geometric isomerism in alkenes

As we have just seen, 2-butene exists in cis- and tm s-forms—the methyl groups may be on the same side (cis) or opposite sides (trans) of the double bond. These are usually described as geometric isomers and are distinct from the structural isomers discussed in the last chapter. All the same bonds are present, but the directional property of the bonds means that they are distinct compounds with different chemical and physical properties. This is a subclass of stereoisomers, which we will discuss in more detail in Chapter 7. The terms cis and trans are adequate 

we rank the substituents at each end of the double bond, and to do this, we use a method called the Cahn-Ingold-Prelog system. This is a very powerful method, designed to cope with almost any eventuality, and is used in ranking groups to describe stereochemistry in a range of compounds, not only alkenes. All the rules are listed here, with examples for the most important for now, simply skip the last two, which will seem obscure, until we return to them later  

For each of the following sets of substituents, rank them according to the Cahn-Ingold-Prelog system  

Designate the double bond geometry as Z or for each of the following molecules  

5(a) At each end of the double bond, chlorine takes precedence over carbon so the priority groups are on opposite sides of the double bond, and we would designate this as E. 

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The two areas of orbital overlap in the n bond (above and below the plane of the molecule) cause ethene to be a rigid molecule there is no rotation around the carbon—carbon bond. This is in contrast to ethane, and other non-cyclic alkanes, and leads to the existence of cis-trans (geometric) isomerism in alkenes (Chapter 20). 

In the rather short history of organic photochemistry, the geometrical E-Z photoisomerization has been exceptionally intensively studied for half a century and a number of reviews have been published [11-18], Although the geometrical isomerization of alkenes can be effected thermally, catalytically, and photochemically, one of the unique features of photoisomerization is that the photostationary EfZ ratio is independent from the ground-state thermodynamics but is instead governed by the excited-state potential surfaces, which enables the thermodynamically less-stable isomers... 

STEREO-ISOMERISM IN ALKENES - GEOMETRIC OR cis-trans ISOMERISM... 

Analysis of such results is often complicated by the observation that simple alkenes are not necessarily stable under the reaction conditions and suffer geometric isomerization in the presence of both the Lewis acid and the glyoxylate, but not with either alone68. It is likely that the reaction proceeds, at least in such cases, through reversible formation of cationic intermediates. followed by a necessarily slower, rate limiting and product determining step of proton loss. 

Geometric, or cis-trans, isomerism is common among alkenes. It occurs when both of the double-bonded carbon atoms are joined to two different atoms or groups. The other two structural isomers of C4H8 shown under (1) on page 597 do not show cis-trans isomerism. In both cases the carbon atom at the left is joined to two identical hydrogen atoms. 

When geometrical isomerism is due to the presence of one double bond where only two substituents are present it is easy to designate the isomers by the terms cis and trans. But if the alkene is a tri- or tetrasubstituted one, the terms cis and trans become ambiguous and do not apply at all as in the following examples. 

To illustrate what is meant by geometric isomerism, we will consider the alkene but-2-ene (H3C-HC=CH-CH3). It contains a carbon-to-carbon double bond and the diagrams on the left show the spatial arrangement of the bonds around each carbon atom. Furthermore, this arrangement of atoms and bonds is planar and all the bond angles around the C=C double bond are 120°. The bonds are fixed in relation to one another. This means that it is impossible to rotate one end of an alkene molecule around the C=C double bond while the other end is fixed. This is one of the reasons why some alkenes can exhibit geometric isomerism. 

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

Butenes or butylenes are hydrocarbon alkenes that exist as four different isomers. Each isomer is a flammable gas at normal room temperature and one atmosphere pressure, but their boiling points indicate that butenes can be condensed at low ambient temperatures and/or increase pressure similar to propane and butane. The 2 designation in the names indicates the position of the double bond. The cis and trans labels indicate geometric isomerism. Geometric isomers are molecules that have similar atoms and bonds but different spatial arrangement of atoms. The structures indicate that three of the butenes are normal butenes, n-butenes, but that methylpropene is branched. Methylpropene is also called isobutene or isobutylene. Isobutenes are more reactive than n-butenes, and reaction mechanisms involving isobutenes differ from those of normal butenes. 

Finally, intense ultraviolet irradiation of perfluoro(2.3-dimethylbut-2-ene) (14) yields terminal alkene 15 quantitatively.35 Unfortunately, photolysis of more complex perfluoroalkenes gives mixtures of products resulting from migration of both allylic fluorine and allylic per-fluoroalkyl groups. Perfluorobut-2-ene only undergoes geometric isomerization. Pyrolysis of 14 at temperatures up to 300 C in the dark does not lead to detectable rearrangement. 

Reductive alkene isomerizations can also be induced by photochemical excitation. Geometric isomerization and rearrangement can be observed upon electron transfer sensitization with molecules with inverse electron demand. Thus, a substituted cinnamyl alcohol in the presence of excited p-dimethoxybenzene gave geometric isomerization and rearrangement characteristic of a free allyl cation, eq. 32 (94) ... 

The topic of electron transfer induced geometric isomerization is treated in a separate section, because we wish to emphasize the existence of two fundamentally different mechanisms of isomerization. Although both mechanisms are initiated by an electron transfer step, the key intermediates involved in these isomerizations are fundamentally different. The interaction of acceptor sensitizers with donor olefins leads to the isomerization of alkene radical cations, whereas the reaction of donor sensitizers with acceptor olefins leads, eventually, to the population of alkene triplet states. 

Another extensively investigated system involves the interaction of two alkenes, each capable of geometric isomerization, viz., the system stilbene-dicyanoethylene, which also illustrates the involvement of ground-state charge-transfer complexes. Excitation of the ground-state complex results in efficient Z - E isomerization of the stilbene exclusively, because the stilbene triplet state lies below the radical ion pair, whereas the dicyanoethylene triplet state lies above it (Fig. 11) [163-166]. 

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