Geometrical isomers double bonds

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). 

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). 

SECTION 24.3 The names of alkenes and alkynes are based on the longest continuous chain of carbon atoms that contains the multiple bond, and the location of the multiple bond is specified by a numerical prefix. Alkenes exhibit not only structural isomerism but geometric (cis-tmiis) isomerism as well. In geometric isomers the bonds are the same, but the molecules have different geometries. Geometric isomerism is possible in alkenes because rotation about the C=C double bond is restricted. 

Since hydrogenation is not carried to completion there is considerable scope for isomerization to occur. The mechanism by which this takes place has been described by Allen and Kiess (1955). They indicate that selective conditions tend to favor isomer formation. Both positional and geometric isomers are formed. In positional isomers, double bonds have wandered from their original position along the fatty acid carbon chain in geometric isomers the position of groups attached to carbon atoms have changed relative to each other in space from the natural cis to trans. Recently the nutritional properties of isomeric fatty acids have been questioned. Applewhite (1981) in a literature review has concluded that this concern is not substantiated by the available data. 

The carbon atoms of the double bond have a trigonal planar configuration and free rotation about the C—C bond is prevented by the n bond. The inability to rotate means that geometrical isomers can be produced, with substituents a and b, thus ... 

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. 

As in all double-bond situations, the adjacent chain sections can be either cis or trans-structures [XV] and [XVI], respectively-with respect to the double bond, producing the following geometrical isomers ... 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

There are a myriad of possible isomers, including positional and geometrical isomers of the double bond(s) as well as stmctural isomers resulting from head-to-head or head-to-tad alignment of the reacting fatty acids. 

The first three members of the olefin series are ethylene, propylene, and butylene (or butene). Structural isomers exist when n > 4, as a consequence of the positioning of the double bond in normal alkenes as a result of branching in branched alkenes. In addition, geometric isomers may be possible owing to restricted rotation of atoms about the C=C bond. For instance, C H (butene) has four possible isomers instead of the expected three ... 

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. 

Because a double bond between two carbons prevents the carbons from rotating, isomers involving the atoms bonded to the carbons are possible, as shown above with dichloroethylene. Such isomers are called geometrical isomers, in contrast to the structural isomers discussed previously. When the substiuent groups are on the same side of the molecule, the compound is designated the cis- isomer. When the substituent groups are on the opposite side, the compound is the trans- isomer. Like all isomers, cis- and trans-isomers have the same molecular formula, but differ in certain physical and chemical properties. For example, cw-l,2-dichloroethylene boils at 60°C whereas 1,2-dichloroethylene boils at 48°C. 

The carbon chains of samrated fatty acids form a zigzag pattern when extended, as at low temperamres. At higher temperatures, some bonds rotate, causing chain shortening, which explains why biomembranes become thinner with increases in temperamre. A type of geometric isomerism occurs in unsaturated fatty acids, depending on the orientation of atoms or groups around the axes of double bonds, which do not allow rotation. If the acyl chains are on the same side of the bond, it is cis-, as in oleic acid if on opposite sides, it is tram-, as in elaidic acid, the tram isomer of oleic acid (Fig-... 

Besides the capacity of CRTI to introduce all four double bonds in the conversion of phytoene to lycopene, the enzyme produces different geometric isomers than does PDS/ZDS (see graphic, side-by-side comparison in Fraser and Bramley ). CRTI produces all-trans isomers. Studies that have examined the function of the paired plant desaturases acting together, from Arabidopsis, and from maize and from... 

With the availability of stable geometric isomers of doubly bonded germanium compounds, experimental determinations of the 7r-bond strength can be made. The enthalpy of activation for double bond isomerization in Mes(Tip)Ge=Ge(Tip)Mes (Tip = 2,4,6-triisopropylphenyl) has been determined for the Z-E conversion, 22.2 . 3 kcal/mol and for the E-Z conversion, 20.0 0.3 kcal/mol.15 These values agree well with recent theoretical estimations.7 The isomerization barrier in germaphos-... 

The unstable organic molecules have fixed structures because of the nature of their bonds. The structure of methane is a tetrahedron of covalent single —C — H bonds. Where the structure of even a simple compound is based on double bonds, such as in an ethylene, there are two forms labelled trans and cis geometric isomers ... 

Evidence can be presented to show that rotation around a C-C single bond happens readily, but rotation around a C=C double bond does not. Consider the compound, CH2C1CH2C1. No matter how this compound is synthesized, there is only one compound that is made with that formula. However, when CHC1CHC1 is prepared, there are two different compounds made with that formula. We call these two compounds geometric isomers. One is labeled cis and the other is trans. They have different physical and chemical properties. If there were free rotation around a double bond, this could not happen. 

Geometrical isomers differ only in the spatial orientation of groups about a plane or direction, i.e., they differ in orientation either (i) around a double bond (see 2-butene) or (ii) across the ring in a cyclic compound (see 1,2-dichlorocyclobutane). Both cis and trans isomers exist. 

The isomer distribution of the nickel catalyst system in general is similar qualitatively to that of the Rh catalyst system described earlier. However, quantitatively it is quite different. In the Rh system the 1,2-adduct, i.e., 3-methyl-1,4-hexadiene is about 1-3% of the total C6 products formed, while in the Ni system it varies from 6 to 17% depending on the phosphine used. There is a distinct trend that the amount of this isomer increases with increasing donor property of the phosphine ligands (see Table X). The quantity of 3-methyl-1,4-pentadiene produced is not affected by butadiene conversion. On the other hand the formation of 2,4-hexadienes which consists of three geometric isomers—trans-trans, trans-cis, and cis-cis—is controlled by butadiene conversion. However, the double-bond isomerization reaction of 1,4-hexadiene to 2,4-hexadiene by the nickel catalyst is significantly slower than that by the Rh catalyst. Thus at the same level of butadiene conversion, the nickel catalyst produces significantly less 2,4-hexadiene (see Fig. 2). 

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