Shapes of alkenes

To take into account the absolute contribution that a single double-bond makes to the whole size and shape of alkene molecules, second-grade structural parameters were derived from a - molecular graph [Zhang et ai, 1997]. The topological descriptors representing the size w and the shape related to the presence of a double bond are, respectively ... 

Alkenes are hydrocarbons that contain a carbon-carbon double bond A carbon-carbon double bond is both an important structural unit and an important func tional group m organic chemistry The shape of an organic molecule is influenced by the presence of this bond and the double bond is the site of most of the chemical reactions that alkenes undergo Some representative alkenes include isobutylene (an industrial chemical) a pmene (a fragrant liquid obtained from pine trees) md fame sene (a naturally occurring alkene with three double bonds)... 

One example has used a manganese porphyrin and iodobenzene encapsulated within a dendrimer to bring about shape-selective epox-idation of alkenes. The important aspect of catalysts is that the reactants can move rapidly to the active site, and that the products can be removed rapidly from the active site and expelled from the dendrimer. 

Mn impregnated into MCM-4i, a silicalite containing uniform mesopores of approximately 22 A, catalyzes TBHP epoxidation of alkenes.88 Over Mn-MCM-41, both cis- and trans-stilbene yield trans-stilbene oxide, which the authors conclude signals a radical mechanism.88 In contrast, over Ti—MCM-41, trans-stilbene cannot be oxidized, only cis-stilbene is epoxidized to the cis-stilbene oxide, which suggest a radical-free mechanism.89 Finally, emphasizing the shape selectivity possibilities, only trans-stilbene (not cis-stilbene) can be epoxidized over Mn-ZSM-5, a zeolite with relatively small pores of 5.1 x 5.4 A (Fig. 6.14).88... 

Ratios of turnover frequencies (TOFs) corresponding to the degree of enhancement of the reaction rates by the imprinting revealed that the imprinted Rh-dimer catalyst (10) showed size and shape selectivities for the alkenes as shown in Figure 8.10. Selectivity for the alkene hydrogenation on the Rh2imp catalyst (10) depends on the size and shape of the template cavity as reaction site in... 

Substitution has no significant effect on the geometry of the n complex and RuHCl(PH3)2(CH2=CH2) and RuHCl(PH3)2(H2C=CH(OMe)) have very similar shape. Of interest the OMe group cannot reach the second empty coordination site (trans to H) of Ru. Remarkably, the Ru-alkene bond dissociation energy is also not affected by the presence of OCH3 (less than 3 kcal.mol 1 difference in binding dissociation energy). The methyl vinyl ether... 

An alkane is a hydrocarbon that has only single bonds. Alkanes that do not contain rings have the formula C H2n + 2 An alkane in the shape of a ring is called a cycloalkane. Cycloalkanes have the formula CnH2n- An alkene is a compound that has at least one double bond. Straight-chain alkenes with one double bond have the same formula as cycloalkanes, Cj,H22,. 

Hsia Chen, C.S. and Bridget, R. (1995) Shape-selective oligomerization of alkenes to near-linear hydrocarbons by zeolite catalysis. J. Catal., 161, 587-593. 

Treatment of neomenthyl chloride with base rapidly produces two different alkenes, i.e. 2-menthene and 3-menthene. If one considers the three-dimensional shape of neomenthyl chloride, it can be seen that, in the preferred conformer with the two alkyl groups equatorial (see Section 3.3.2), the chlorine is an axial substituent. This means there are two different hydrogen atoms adjacent that are also axial and anfi-periplanar to the chlorine. As a conseqnence, two different E2 eliminations can occnr hence the two observed prodncts. That the two prodncts are not formed in eqnal amonnts will be considered in the next section. 

Shape- and Size-Selective Hydrogenation of Alkenes on the Imprinted Rh Dimer Catalyst... 

Casey was able to prepare related zirconocene alkenyl complexes according to Scheme 8.18. Alkene coordination was established by a number of NMR techniques. While zwitterionic compounds 38 allowed the determination of the alkene dissociation energy, AG = 10.5 kcal mol , very similar to that of 35. Thermally more stable complexes were obtained by protonation of 37 with [HNMePh2][B(C5F5)4[. Dynamic NMR spectroscopy and line shape analysis allowed the measurement of the barriers of alkene dissociation (AG = 10.7 and 11.1 kcal mol ), as well as for the site epimerisation ( chain skipping ) at the zirconium center (AG = 14.4 kcal mol" ) (Scheme 8.19) [77]. 

As with alkanes, the boiling points and melting points of alkenes decrease with increasing molecular weight, but show some variations that depend on the shape of the molecule. Alkenes with the same molecular formula are isomers of one another if the position and the stereochemistry of the double bond differ. For example, there are four different acyclic structures that can be drawn for butene (C4H8). They have different b.p. and m.p. as follows. 

Reaction of alkenes with BrCl proceeds via the intermediate [alkene-Br]+ rather than the intermediate [alkene-Cl]+. Explain using a two-orbital interaction diagram. (The structure of the complex between ethylene and BrCl in the gas phase has been determined by microwave spectroscopy. It is T-shaped, with the BrCl molecule lying perpendicular to the ethylene plane and pointing bromine-end first toward the midpoint of the C=C bond Legon, A. C. Bloemink, H. I. Hinds, K. Thorn, J. C., Chem. Eng. News, 1994 Nov. 7, 26-29.)... 

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