2- Butene geometric isomerism

When two different substituents are attached to each carbon atom of the double bond, cis-trans isomers can exist. In the case of c T-2-butene (Fig. 1.11a), both methyl groups are on the same side of the double bond. The other isomer has the methyl groups on opposite sides and is designated as rran5--2-butene (Fig. l.llb). Their physical properties are quite different. Geometric isomerism can also exist in ring systems examples were cited in the previous discussion on conformational isomers. 

Because 1,2-diphenylcyclo-l-butene (DPCB) has a rigid planar structure with the c-St-chromophore structurally constrained by the cyclobutene ring, and because it is stable for geometrical isomerizations, TPCB" is expected to have the same rigid planar structure. Pulse radiolysis of DPCB in DCE produces DPCB, which shows bands at... 

The main aim of the results concerns the comparison between E and Z isomers, both towards hydrogenation and/or isomerization processes. In particular, the less stable Z isomers favor the geometric isomerization products, whereas the most stable (E)-2-buten-l-ol mainly affords a hydrogenated compound. This agrees with earlier findings, where Z isomers generally favor isomerized (both geometric and double bond) derivatives (3). 

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. 

Consider the two configurations of 2-butene in Figure 1-9 and remember that the carbon-carbon double bond does not permit the rotation of the carbon atoms. Notice that in one case the two methyl groups are located on the same side of the basic structure while in the other case the methyl groups are located on opposite sides of the structure. These two molecules exhibit geometric isomerization because of the way the atoms are oriented in space, even though they are alike... 

In addition to the locations of the double bonds, another difference of alkenes is the molecule s inability to rotate at the double bond. With alkanes, when substituent groups attach to a carbon, the molecule can rotate around the C-C bonds in response to electron-electron repulsions. Because the double bond in the alkene is composed of both sigma and pi bonds, the molecule can t rotate around the double bond (see Chapter 6). What this means for alkenes is that the molecule can have different structural orientations around the double bond. These different orientations allow a new kind of isomerism, known as geometrical isomerism. When the non-hydrogen parts of the molecule are on the same side of the molecule, the term cis- is placed in front of the name. When the non-hydrogen parts are placed on opposite sides of the molecule, the term trans- is placed in front of the name. In the previous section, you saw that the alkane butane has only two isomers. Because of geometrical isomerism, butene has four isomers, shown in Figure 19.12. 

Over the range of conditions, 1-butene decomposes more rapidly than either of the 2-butene isomers. Double-bond shift and geometrical isomerization accompany the decomposition of the n-butenes however, skeletal isomerization does not occur, as isobutene is not found among the products of the pyrolysis. Isomerization reactions apparently are kinetically controlled, as equilibrium distributions are not generally observed. Trans cis ratios in the products do not correspond to equilibrium at either the maximum or the average reactor temperatures, and in some cases the ratio falls below equilibrium values based on American Petroleum Institute (API) data (14). However, none of these data exceed the equilibrium values based on more recent thermodynamic data (15). 

Since the l-but-2-enyl radical (III) can be an intermediate to geometrical isomerization, this reaction path will play a major role in the low temperature region. It has been reported that the rate of isomerization of cis- or trans-2-butene exceeds the rate of cracking (5). An alternate path for decomposition of the radical III is by loss of a hydrogen atom to give butadiene as a product. 

Figure 6.4.2 Butenes exhibit structurai and geometrical isomerism...

There is hindered rotation about any carbon-carbon double bond, but it gives rise to geometric isomerism only if there is a certain relationship among the groups attached to the doubly-bonded carbons. We can look for this isomerism by drawing the possible structures (or better yet, by constructing them from molecular models), and then seeing if these are indeed isomeric, or actually identical. On this basis we find that propylene, 1-butene, and isobutylene should not show... 

Cyclopropanation of C=C bonds by carbenoids derived from diazoesters usually occurs stereospecifically with respect to the configuration of the olefin. This has been confirmed for cyclopropanation with copper s.si.eo.ss) palladium and rhodium catalysts However, cyclopropanation of cw-Dj-styrene with ethyl diazoacetate in the presence of a (l,2-dioximato)cobalt(II) complex occurs with considerable geometrical isomerization Furthermore, CuCl-catalyzed cyclopropanation of cis-2-butene with co-diazoacetophenone gives a mixture of the cis- and rrans-1,2-dimethylcyclopropanes... 

Make a model of ethyne. Now replace the hydrogens with methyl groups to get 2-butyne. Compare to 2-butene which exhibits cis-trans isomerism. Why does 2-butene show geometric isomerism but not 2-butyne ... 

Briston and Dainton 16), who investigated the interpolymerization of sulfur dioxide with cfs-butene-2 and benzoyl peroxide, observed a considerable geometrical isomerization above 250°. Prolonged heating with sulfur dioxide resulted eventually in the attainment of cis-trans equilibrium, while in the absence of sulfur dioxide no isomerization occurred. At 100° and high catalyst concentrations, a slow double-bond migration leading to the formation of butene-1 was also detected. 

Geometric isomerism indicated in l-chloro-2-methyl-2-butene. 

Certain alkenes possess an additional type of isomerism, referred to as cis-trans or geometrical isomerism. For example, there are two compounds, not one, with structure Kb) (2-butene) of Problem 12.5 ... 

In addition to geometry, alkenes also differ from open-chain alkanes in that the double bonds prevent the relatively free rotation that is characteristic of carbon atoms bonded by single bonds. As a result, alkenes can exhibit geometric isomerism, the same type of stereoisomerism seen earlier for the cycloalkanes (Section 1.9). There are two geometric isomers of 2-butene ... 

The biradical pathway for the cyclobutane cleavage and geometric isomerization was first pursued by Walters with cis- and ran -l,2-dimethylcyclobutane which interconvert roughly one-fifth as fast as they undergo cleavage to propylene at 400°C (Scheme 5.23). There is also 5-10% ethylene and 2-butene formed in the reaction, and, further, there is some preservation of stereochemistry at Cl and C2. The ratio of cis- to trans-2-buionQ formed from the trans compound is 0.125 while that from the cis compound is 2 at low conversions. ... 

Comparisons of the activation enthalpy for this reaction to that for geometric isomerization of ethylene and 2-butene led to an estimate of 13 2 kcal/mol for allyl radical resonance energy once correction for hexatriene resonance energy was made. 

Indicate whether each of the following molecules is capable of geometrical isomerism. For those that are, draw the structures (a) 1,1-dichloro-1-butene, (b) 2,4-dichloro-2-butene, (c) 1,4-dichlorobenzene, (d) 4,4-dimethyl-2-pentyne. 

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