Hydrocarbons optical isomers

While a demonstration of optical activity proves conclusively that a given aromatic hydrocarbon is not planar, the converse is not true. Failure to resolve a given compound into optical isomers may arise from a too rapid interchange of d- and Z-forms, or from use of an inappropriate resolution technique, instead of from the molecule being planar and hence inactive optically. 

Complexes with alkenes and arenes are formed when the hydrocarbons are shaken with aqueous solutions of silver(I) salts. Di- or polyalkenes often give crystalline compounds with Ag+ bound to one to three double bonds. The formation of alkene complexes of varying stability may be used for the purification of alkenes, or for the separation of isomeric mixtures (e.g., 1,3-, 1,4-, and 1,5-cyclooctadienes), or of the optical isomers of a- and /3-pinene. There is very little back-bonding contribution in the formation of Ag1 rr-complexes. For example, the planar complex (hfa)Ag(Ph-C= C-Ph) contains an almost linear acetylene ligand with a C=C... 

That the surface tensions of solutions of d- and /-optical isomers aredififerent seems doubtful. Surface tensions of normal alkanes (paraffin hydrocarbons) containing n atoms of carbon are given by a —l4-6 log ( —3)+ll 52 within experimental error. 

Dipentene is described as a product of fractionation of pine oils containing related monocyclic terpene hydrocarbons, predominantly dipentene. (Structurally, dipentene is the Same as lim-onene. D-limonene is one of the 2 optical isomers of dipentene.) Other components of commercial dipentene include carene and alpha- and beta- pinene. 

Examine the pair of substituted hydrocarbons illustrated at right, and decide whether it represents a pair of optical isomers. Explain your answer. 

In reality there are no ideal solutions, and actual mixtures only approach ideality as a limit. Ideality would require that the molecules of the constituents be similar in size, structure, and chemical nature, and the nearest approach to such a condition is perhaps exemplified by solutions of optical isomers of organic compounds. Practically, however, many solutions are so nearly ideal that for engineering purposes they can be so considered. Adjacent or nearly adjacent members of a homologous series of organic compounds particularly fall in this category. So, for example, solutions of benzene in toluene, of ethyl and propyl alcohols, or of the paraffin hydrocarbon gases in paraffin oils can ordinarily be considered as ideal solutions. 

The zig-zag in the carbon chain shown in a skeletal formula can be seen in the 3D representations of hydrocarbons in Figure 14.2. You will see more detailed 3D displayed formulae later in this chapter, when we look at optical isomers (see page 195). Figure 14.6 shows the 3D displayed formula of butan-2-ol. 

It has been observed that the formation of the olefin and carbon monoxide, 45, is ten times more important than the formation of the bicyclic hydrocarbon and carbon monoxide, 46, at 80° and 80 mm. pressure even at 3130 A. The formation of the strained bicyclic hydrocarbon is evidently not a favorable reaction although this may not be the only consideration. In the case of camphor it should be interesting to find out if an optically active isomer of the ketone on photolysis will give rise to an optically active trimethyl bicyclo [2.1.1] hexane (XXVI). A concerted reaction, analogous to the formation of cyclobutane from cyclopentanone, may lead to only an optically active product. 

Among the paraffin hydrocarbons (C H2W+2, where n is a whole number), what is the empirical formula of the compound of lowest molar mass which could demonstrate optical activity in at least one of its structural isomers ... 

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). 

Next, we examine aliphatic hydrocarbons. First we study the nomenclature and reactions of alkanes. We examine the optical isomerism of substituted alkanes and also the properties of cycloalkanes. We then study unsaturated hydrocarbons, molecules that contain carbon-to-carbon double bonds and triple bonds. We focus on their nomenclature, properties, and geometric isomers. (24.2)... 

Based on the results shown in the table, one can conclude that the largest optical nonlinearity occurs in the longest isomer, system A. This fact confirms the conclusion made for the linear model polyene (Fig. 3.8) and based on the comparison involving its different conformers. The conclusion was generalized for a larger class of condensed aromatic hydrocarbons. 

LA is a three carbon organic acid one terminal carbon atom is part of an acid or carboxyl group die other terminal carbon atom is part of a methyl or hydrocarbon group and a central carbon atom having an alcohol carbon group attached. Lactic acid exists in two optically active isomeric forms. One is known as L-lactic acid or (5)-lactic acid and the other, its mirror image, is Z)-lactic acid or (r)-lactic acid. L-Lactic acid is the biologically important isomer. 

From the methanol extract of the same species, lepidozenal (231) has been isolated and its structure elucidated by comparing its H-NMR spectrum with that of the c/.s-isomer (230) and by degrading it as shown in Scheme 27 214). The geometries of the two double bonds were based on the chemicd shift of the C-10 methyl (8 15.5 ppm), in the C-NMR spectrum of (233) and those of the C-10 (8 15.5) and C-4 methyls (8 24.1) of the hydrocarbon (234). The cis and trans-orientations of the double bonds were established by NOE experiments on the original aldehyde. The absolute configuration of (231) was based on the direct comparison of (—)-ketoacid (69) and its methyl ester (70) with compounds prepared from (-f)-methyl trans-chrysanthemate (236) in three steps. The signs of the optical rotations of (69) and (70) from (231) were in agreement with those of the two compounds (69, 70) prepared from (236). 

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