Hydrocarbons geometric isomerism

Adjacent polyenes are hydrocarbons with double bonds between each pair of atoms in the polyene system they are called cumulenes2. Two types of cumulenes exist those with an even number of adjacent double bonds and those with an odd number. The former can exhibit chirality, the latter geometric isomerism. Allenes (propadienes) are the simplest members of the even-numbered type of cumulenes. 

The alkenes make up a homologous series of hydrocarbons with the general formula C H2 . Alkenes show two types of structural isomerism, position isomerism and chain isomerism. Geometrical isomerism also exists because of the lack of free rotation about the C=C double bond. 

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. 

The L-B films offer some advantages over aqueous-hydrocarbon interfaces of micelles and the related assemblies discussed above in terms of the magnitude of their orienting ability and the ease of interpretation of selectivity in photoreactions conducted in them. Molecules in the films have very little freedom of motion (stiff reaction cavities), their interfaces are very well defined, and therefore the alignment of reactant molecules can be readily expressed in the products. Photodimerization of stilbazole derivatives 62, N-octadecyl-l-(4-pyridyl)-4-(phenyl)-l,3-butadiene, (63), surfactant styrene derivatives 64 and 65, and cinnamic acids have been carried out in L-B films [18, 196-200], In all cases, single isomeric head-head dimers are obtained. Geometric isomerization of olefins has not been observed in competition with photodimerization. Independent of the location of the chromophore (i.e.,... 

As shown in Figure 1.17, there are three possible dichloroethylene compounds, all clear, colorless liquids. Vinylidene chloride forms a copolymer with vinyl chloride used in some kinds of coating materials. The geometrically isomeric 1,2-dichloroethylenes are used as organic synthesis intermediates and as solvents. Trichloroethylene is a clear, colorless, nonflammable, volatile liquid. It is an excellent degreasing and dry-cleaning solvent and has been used as a household solvent and for food extraction (for example, in decaffeination of coffee). Colorless, nonflammable liquid tetrachloroethylene has properties and uses similar to those of trichloroethylene. Hexachloro-butadiene, a colorless liquid with an odor somewhat like that of turpentine, is used as a solvent for higher hydrocarbons and elastomers, as a hydraulic fluid, in transformers, and for heat transfer. 

The presence of a double bond in the hydrocarbon chain gives rise to geometrical isomerism, which is due to restricted rotation around carbon-carbon double bonds and is exemplified by fumaric and maleic acids. 

The other series of bridged 14 ir-electron compounds comprises those systems with anthracene perimeter (e.g. 32, a system which, if allowed to attain planarity, is also predicted to have aromatic character). These compounds can be considered as the next highest members in the series of bridged annulenes based on linearly fused hydrocarbons, extending the study of the naphthalene-perimeter bridged [10]annulenes. There is in the system 35 a possibility of geometrical isomerism, in that the... 

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes, because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cw-1,4 in structure. The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-lA in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. 

ALKENES, ALKYNES, AND AROMATIC HYDROCARBONS (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 (cts-trans) isomerism as well. In geometric isomeis, 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. 

Unsaturated fatty acids with one double bond, with the trivial name of monoenoic fatty acids, differ from each other by the number of carbon atoms, the double bond position in the hydrocarbon chain and the spatial organisation of molecules cis-trans isomerism also known as E/Z isomerism or geometric isomerism). The most important of these are listed in Table 3.2. Many monoenoic fatty acids have trivial names, which are commonly used. A tpical example is cis- or (Z)-octadec-9-enoic acid, also known as oleic acid (3-4). 

Examination of the structure of rubber hydrocarbon (IV) indicates the possibility of geometrical isomerism and Staudinger suggested that rubber contains mainly ds-l,4-bonds whilst gutta percha (section 20.2.7) has mainly fruns-1,4-bonds. This assignment was made mainly on the basis of the lower density of rubber it was well known that for simple molecules the cis-isomer generally has a lower density than the trans-isomer and the same relationship... 

One criterion of aromaticity is the ring current, which is indicated by a chemical shift difference between protons, in the plane of the conjugated system and those above or below the plane. The chemical shifts of two isomeric hydrocarbons are given below. In qualitative terms, which appears to be more aromatic (Because the chemical shift depends on the geometric relationship to the ring current, a quantitative calculation would be necessary to confirm the correctness of this qualitative impression.) Does Hiickel MO theory predict a difference in the aromaticity of these two compounds ... 

Alkynes are hydrocarbons with carbon-carbon triple bonds as their functional group. Alkyne names generally have the -yne suffix, although some of their common names (acetylene, for example) do not conform to this rule. The triple bond is linear, so there is no possibility of geometric (cis-trans) isomerism in alkynes. 

Structural-, positional-, stereo-isomerism (optical and geometric) in aliphatic hydrocarbon systems. 

Diastereomers are also encountered in unsaturated acyclic compounds. When two C atoms are joined together by a double bond, all the remaining four single bonds to the two C atoms lie in the same plane as the C=C bond. If each of these two carbon atoms is bonded to a H atom and a hydrocarbon (alkyl) chain, the alkyl chains can be either on the same side of the C=C bond as each other or on opposite sides, and the resulting diastereomers (which used to be known as geometric isomers), shown in Fig. 2.2b, are termed cis and tram, respectively. Again, these diastereomers have different physical properties (see also Box 2.3). Optical isomerism is not possible about a C=C bond (the mirror images are superimposable). 

The relative amounts of different hydrocarbons produced can be discussed in terms of the different possibilities for intramolecular transfer. Isomerization of a free valency on a primary carbon to the fifth carbon should be very favourable for geometrical reasons namely,... 

Other Syntheses Related to the Fischer-Tropsch Process Comparatively little is yet known of some synthetic reactions which obviously resemble the Fischer-Tropsch process very closely, but they are worth brief mention because they are also likely to be controlled by geometrical factors. The Oxo synthesis (15) of aldehydes by the interaction of ethylene or other olefins with carbon monoxide and hydrogen is carried out in contact with cobalt catalysts at temperatures in the range 110-150°, and under a pressure of 100-200 atmospheres. Cyclic olefins react similarly for example, cyclohexene gives hexahydrobenzaldehyde. There can be little doubt that a two-point adsorption of the hydrocarbon must take place and that the adsorbed molecule then reacts with carbon monoxide and hydrogen the difference between this process and that responsible for the normal hydrocarbon synthesis is that adsorbed carbon monoxide survives as such under the less drastic temperature conditions which are employed. Owing to the fact that a variety of isomeric aldehydes are produced, this system deserves further detailed study on geometrical lines. 

Functional group (1.4) Geometric isomers (1.9) Homologous series (1.10) Hybrid orbital (1.3) Hydrocarbon (1.5) Hydrophobic (1.10) Inorganic chemistry (1.1) Isomerism (1.3)... 

Now we can teach also isomerism such as geometric and positional isomers of C2H4, benzene, and chiral isomers, how Nature makes use of cis-trans geometries (retinal. Tamoxifen), and chirality. Mind you, the 3D rules can be applied also to longer chains of hydrocarbons. In brief, we show that molecular architecture can be reconstructed based on a few simple structural elements. 

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