Cycloalkanes from alkenes

The methods discussed for separation of alkanes and cycloalkanes from alkenes, aromatics, resins and other more polar constituents of petroleum can be employed also for synthetic mixtures, asphalts, bitumens, etc. However, for the quantification and identification of such homologous families as n-alkanes, branched alkanes, etc., further analysis is needed. Whereas the GTA relates to functionality and differences in polarity, the separation within the alkane family cannot be based on these characteristics. 

Cis-trans isomerism in cycloalkanes. Like alkenes, cycloalkane rings are restricted from free rotation. Two substituents on a cycloalkane must be either on the same side (cis) or on opposite sides (trans) of the ring. 

Ans. Alkenes and cycloalkanes have the same general formula and are constitutional isomers of each other. Knowing that the molecular formula of some unknown such as C5H10 fits the general formula C H2 tells us that it is not an alkane and that it is either an alkene or a cycloalkane. The molecular formula alone does not allow us to distinguish an alkene from a cycloalkane. However, alkenes and cycloalkanes have very different chemical properties. Cycloalkanes have similar chemical behavior as alkanes since both have the same kinds of bonds (C — C and C — H single bonds). The chemistry of alkenes is very different due to the presence of the carbon-carbon double bond (Sec. 12.6). 

Saturated hydrocarbons are stable. Only cycloalkanes with a tight ring are unstable. Alkenes and alkynes have a strong endothermic character, especially the first homologues and polyunsaturated conjugated hydrocarbons. This is also true for aromatic compounds, but this thermodynamic approach does not show up their real stability very well. Apart from a few special cases, the decomposition of unsaturated hydrocarbons requires extreme conditions, which are only encountered in the chemical industry. 

The volumes of reaction calculated for the hypothetical cyclizations of n-alkenes to the corresponding cycloalkanes by the use of experimentally observed partial molar volumes190 confirm the trend derived from the ring enlargements shown in Scheme 25. 

The first mechanistic concepts of aromatization 16) originate from pregas-chromatography times. A direct alkane- cycloalkane reaction was proposed by Kazansky and co-workers 47). Several authors have interpreted the formation of six-membered rings over metal catalysts in terms of alkene-alkyl insertion (i.e., analogous to the Twigg mechanism) (7, 8, 14). 

Yttrium-catalyzed enyne cyclization/hydrosilylation was proposed to occur via cr-bond metathesis of the Y-G bond of pre-catalyst Cp 2YMe(THF) with the Si-H bond of the silane to form the yttrium hydride complex Ig (Scheme 8). Hydrometallation of the C=G bond of the enyne coupled with complexation of the pendant G=G bond could form the alkenylyttrium alkyl complex Ilg. Subsequent / -migratory insertion of the alkene moiety into the Y-C bond of Ilg could form cyclopentylmethyl complex Illg. Silylation of the resulting Y-C bond via cr-bond metathesis could release the silylated cycloalkane and regenerate the active yttrium hydride catalyst. Predominant formation of the /ra //j--cyclopentane presumably results from preferential orientation of the allylic substituent in a pseudo-equatorial position in a chairlike transition state for intramolecular carbometallation (Ilg —IHg). 

Similar stoichiometric reactions can be conducted with other organic substrates. Beside mechanistic importance, such reactions are a convenient way for estimating the potential of a-oxygen oxidation. For that, various organic substrates were tested for their room temperature interaction with a-oxygen to identify the primary oxidation products extracted from the surface. Substrates included alkanes, cycloalkanes, alkenes and aromatics [121,122]. Analysis of products showed that in all cases selective formation of hydroxylated compounds took place. 

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