Unsaturated hydrocarbons isomerism

In unsaturated hydrocarbons, isomerism can occur because of the position of the double or triple bond. 

Many other ion-molecule reactions involving highly unsaturated hydrocarbon ions and neutral olefins or the equivalent strained cycloalkanes have been studied by mass spectrometry98. For example, we may mention here the addition of ionized cyclopropane and cyclobutane to benzene radical cations giving the respective n-alkylbenzene ions but also isomeric cyclodiene ions such as ionized 8,9-dihydroindane and 9,10-dihydrotetralin, respectively. Extensive studies have been performed on the dimerization product of charged and neutral styrene4. 

As a last example of a molecular system exhibiting nonadiabatic dynamics caused by a conical intersection, we consider a model that recently has been proposed by Seidner and Domcke to describe ultrafast cis-trans isomerization processes in unsaturated hydrocarbons [172]. Photochemical reactions of this type are known to involve large-amplitode motion on coupled potential-energy surfaces [169], thus representing another stringent test for a mixed quantum-classical description that is complementary to Models 1 and II. A number of theoretical investigations, including quantum wave-packet studies [163, 164, 172], time-resolved pump-probe spectra [164, 181], and various mixed... 

Metal-catalyzed reactions of CO with organic molecules have been under investigation since the late 1930s and early 1940s, when Roelen (/) discovered the hydroformylation reaction and Reppe (2) the acrylic acid synthesis and other related carbonylation reactions. These early studies of the carbonyla-tions of unsaturated hydrocarbons led to extremely useful syntheses of a variety of oxygenated products. Some of the reactions, however, suffered from the serious problem that they produced isomeric mixtures of products. For example, the cobalt-catalyzed hydroformylation of propylene gave mixtures of n-butyraldehyde and isobutyraldehyde. 

This conclusion is supported by observations of the isomerization of unsaturated hydrocarbons catalysed by potassium amide not only in ammonia solution (Shatenshtein and Vasil eva, 1954 Shatenshtein et al., 1954) but also by solid amides of alkali and alkaline-earth metals (Shatenshtein et al., 1958a). For example, diallyl rearranges to dipropenyl, pentene-1 to pentene-2, and 2-methylbutene-l to 2-methyl-butene-2. Subsequently, a further number of examples of isomerization... 

The following elementary processes are included unimolecular initiation, radical decomposition, radical addition to unsaturated hydrocarbons, radical isomerization, hydrogen abstraction, radical combination,... 

JBd have the general formula C H2)I. These unsaturated hydrocarbons are isomeric with the saturated... 

Because the unsaturated hydrocarbon has to bind to rhodium in the presence of bulky PPh3 groups, the catalyst favours unsubstituted double bonds (RCH=CH2 rather than RR C=CR r ). Since the alkyl intermediate is shortlived, there is little tendency to / -elimination with concomitant alkene isomerization. Although both alkene and alkyne functions are reduced, in general carbonyl or carboxylic groups and benzene rings are not, though aldehydes are frequently decarbonylated. Peroxides tend to oxidize and thus destroy the catalyst, so that substrates need to be purified carefully before use. 

Symmetrical Di-chlor Ethane.— The isomeric di-chlor ethane is obtained when the unsaturated hydrocarbon ethylene, or ethene takes up two chlorine atoms, forming an addition product. 

Ethylene and Ethylidene Compounds.—The fact that the symmetrical di-chlor ethane is readily prepared from ethylene, has given to it the name of ethylene chloride. To distinguish the two isomers by name the other, the unsymmetrical di-chlor ethane, has been called ethylidene chloride. In connection with our discussion of the constitution of the ethene series of unsaturated hydrocarbons (p. 154), we have used the constitution of ethylene chloride as proving the constitution of ethylene or ethene, as H2C = CH2. Isomerism of the character shown in these two di-chlor ethanes, as above explained, is found in all classes of di-substitution products of ethane, so that we may express the compounds by general formulas as follows ... 

The ethylene halides may be prepared by direct halogenation of ethane, but this is not a practical method as it yields a mixture of the two isomeric compounds as in the further halogenation of the monohalogen ethanes. The best method of preparation is from the unsaturated hydrocarbon, ethylene. This reaction has been fully considered already (p. 154) and need not be discussed again. 

Cyclo-hexane.—=The resulting compound CeHi2 corresponds to the olefine unsaturated hydrocarbons, CnH2n, and is isomeric with hexene. Hexene, however, readily adds two atoms of hydrogen and yields hexane, whereas hexa-hydro benzene is with difliculty converted into hexane. The compound, therefore, is not unsaturated. More important still is the fact that it proves to be identical with hexa-methy-lene or cyclo-hexane which, as we have recently shown, is a carbo-cyclic compound represented as follows ... 

Many of the significant reactions of unsaturated hydrocarbons (hydrogenation, isomerization, carbonylation, oxidation, polymerization) are catalyzed heterogeneously by metals in or near Group VIII or homogeneously by salts and complexes of these elements. Those reactions effected in both systems are discussed in terms of probable common intermediates anomalies, where they occur, are ascribed either to the ability of surfaces to form intermediate species which cannot be stabilized by single metal atoms or to the ability of the latter to coordinate simultaneously more than one hydrocarbon molecule. 

Unsaturated hydrocarbons have double or triple bonds between carbon atoms. The alkenes have C —C double bonds, described by sp hybridization of the carbon atoms. The alkynes have C —C triple bonds, described by sp hybridization of the carbon atoms. Because bond rotation does not occur readily about a carbon-carbon double bond, many alkenes exist in contrasting isomeric forms, depending on whether bonding groups are on the same (cis) or opposite sides (trans) of the double bond. When two or more double or triple bonds are separated by one single bond, the p orbitals form a conjugated system, in which the de-localized tr orbitals are best described by MO theory. 

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