Analysis of alkenes

It may be mentioned in passing that the volume, and quantitative precision, of data available in this field owes much to the use of gas/ liquid chromatography for the rapid, and accurate, quantitative analysis of alkene mixtures. 

Most methods used for analysis of alkenes, such as bromination and hydrogenation, can be employed to determine the number of double bonds in polyenes. These methods were also employed to classify various petroleums ( bromine number ). However, these classical methods are employed less in analysis of conjugated dienes and polyenes mostly because the products produce a less informative mixture than in the alkene case. 

Fordham, P.J. Chamot-Rooke, J. Guidice, E. Tortajada, J. Morizur, J.-P. Analysis of Alkenes by Copper Ion Chemical Ionization Gas Chromatogra-phy/Mass Spectrometry and Gas Chro-... 

Figure 13.8. Analysis of alkene binding to different tricoordinated metal complexes.

Spectroscopic analysis of alkenes will be discussed in Secs. 13.15-13.16.)... 

The beta silicon effect , referring to the strong stabilization of carbocations -substituted with silicon, is another well-known manifestation of hyperconjugation. The C-Si empty orbital on a cationic center, very significant delocalization is observed. Finally, an important feature in the conformational analysis of alkenes is the tendency for allylic sp centers to prefer to eclipse the C=C double bond (Scheme I) (see Conformational Analysis I Conformational Analysis 2 and Conformational Analysis 3). This preference, typically on the order of 2 kcal moI , is ascribed to improved hypercon-jugative interactions of the non-eclipsing C-H bonds with the jr orbital of the alkene for this conformation. 

The proton chemical shifts of the protons directly attached to the basic three carbon skeleton are found between 5.0 and 6.8 ppm. The J(H,H) between these protons is about -5 Hz. The shift region is similar to the region for similarly substituted alkenes, although the spread in shifts is smaller and the allene proton resonances are slightly upfield from the alkene resonances. We could not establish a reliable additivity rule for the allene proton shifts as we could for the shifts (vide infra) and therefore we found the proton shifts much less valuable for the structural analysis of the allene moiety than the NMR data on the basic three-carbon system. 

Structure elucidation does not necessarily require the complete analysis of all multiplets in complicated spectra. If the coupling constants are known, the characteristic fine structure of the single multiplet almost always leads to identification of a molecular fragment and, in the case of alkenes and aromatic or heteroaromatic compounds, it may even lead to the elucidation of the complete substitution pattern. 

In the case of alkenes and aromatic and heteroaromatic compounds, analysis of a single multiplet will often clarify the complete substitution pattern. A few examples will illustrate the procedure. 

The main product, benzene, is represented by solute (B), and the high boiling aromatics are represented by solute (C) (toluene and xylenes). The analysis of the products they obtained are shown in Figure 12. The material stripped form the top section (section (1)) is seen to contain the alkanes, alkenes and naphthenes and very little benzene. The product stripped from the center section appears to be virtually pure benzene. The product from section (3) contained toluene, the xylenes and thiophen which elutes close to benzene. The thiophen, however, was only eliminated at the expense of some loss of benzene to the lower stripping section. Although the system works well it proved experimentally difficult to set up and maintain under constant operating conditions. The problems arose largely from the need to adjust the pressures that must prevent cross-flow. The system as described would be virtually impossible to operate with a liquid mobile phase. 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

In the Diels-Alder reaction a double bond adds 1,4 to a conjugated diene (a 2 + 4 cycloaddition), so the product is always a six-membered ring. The double-bond compound is called a dienophile. The reaction is easy and rapid and of very broad scope and reactivity of dienes and dienophiles can be predicted based on analysis of the HOMOs and LUMOs of these species. Ethylene and simple alkenes make poor dienophiles, although the reaction has been carried out with these compounds. 

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

Since the review by Lattes et al. on the amination of alkenes in 1983 [18] and our first review on the catalytic amination of monoolefins in 1989 [19], several review articles have appeared in the literature [13, 14, 17, 20-23]. The present review corresponds to an analysis of literature data up to the end of 1999. 

An HSAB analysis of singlet carbene reactivity based on B3LYP/6-31G computations has calculated the extent of charge transfer for substituted alkenes,122 and the results are summarized in Figure 10.3 The trends are as anticipated for changes in structure of both the carbene and alkene. The charge transfer interactions are consistent with HOMO-LUMO interactions between the carbene and alkene. Similarly, a correlation was found for the global electrophilicity parameter, co, and the ANmax parameters (see Topic 1.5, Part A for definition of these DFT-based parameters).123... 

From a synthetic point of view, the regioselectivity and stereoselectivity of the cyclization are of paramount importance. As discussed in Section 11.2.3.3 of Part A, the order of preference for cyclization of alkyl radicals is 5-exo > 6-endo 6-exo > 7-endo S-endo > 1-exo because of stereoelectronic preferences. For relatively rigid cyclic structures, proximity and alignment factors determined by the specific geometry of the ring system are of major importance. Theoretical analysis of radical addition indicates that the major interaction of the attacking radical is with the alkene LUMO.321 The preferred direction of attack is not perpendicular to the it system, but rather at an angle of about 110°. 

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