Alkenes shapes

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. 

In practice simple alkenes give poor cycloaddition so diene (49) was the chosen starting material and the aldol was protected and hydrogenated to give (48). Note that (50) is emphatically bowl shaped (Chapter T 34) so that substituents (OR) or reagents (Hg) prefer to be on the outside of the bowl. The regioselectivity of the addition of Cl2C=C=0 to the diene (49) is correct. Synthesis ... 

A single-event microkinetic description of complex feedstock conversion allows a fundamental understanding of the occurring phenomena. The limited munber of reaction families results in a tractable number of feedstock independent kinetic parameters. The catalyst dependence of these parameters can be filtered out from these parameters using catalyst descriptors such as the total number of acid sites and the alkene standard protonation enthalpy or by accounting for the shape-selective effects. Relumped single-event microkinetics account for the full reaction network on molecular level and allow to adequately describe typical industrial hydrocracking data. 

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... 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

As shown in Fig. 4.69, the HfFLi- alkene complex exhibits expected parallels with the HfFLi- H2 complex (Fig. 4.59), both in terms of molecular shape and in terms of valence interactions. The characteristic features of such weak dative bonding include long Hf—C distances (2.82 A), normal C=C bond length (1.34 A), planar alkene bond angles, and small binding energy (15.1 kcalmol-1)-... 

Diastereoisomers are stereoisomers which do NOT have a mirror image of one another. Figure 11.20 shows the diastereoisomers of 2-butene (alkenes such as this are sometimes called geometric isomers and are a consequence of the prohibition of rotation about double bonds). If a vertical mirror was placed between the two structures in Fig. 11.20 they would not reflect onto one another. If the functionality is on the same side then the isomer is the cis-form, if on the opposite side then it is the trans- form. The chemical properties are very similar because the functional groups are identical. However, as they have different shapes their physical properties are different. Interconversion requires breaking and remaking bonds so these isomers are also stable under normal conditions. 

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... 

Figure 8.10 Shape and size selectivity of the imprinted Rh-dimer catalyst in alkene hydrogenation.

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,. 

Example The observed KER values and also peak shapes may change dramatically as Eor decreases. Here, the chain length of a leaving alkene increases with an increase of the reacting alkyl substituent. Nevertheless, the mechanism of the reaction - McLafferty rearrangement of immonium ions (Chap. 6.7) - remains unaffected. [58]... 

Methanol to light alkenes in favor of methanol to aromahcs is another example of a reaction that is governed by product shape selechvity effects [13]. While aromatics are formed within the cavities of CHA and ERI framework types, as the reaction mechanism would require, aromatics would be trapped within the 8-MR charmel systems. With medium-pore size MFl aromatics are indeed observed as products, but the carbon number is limited to 10 (durene). Bulkier aromatics are observed with MOR and BEA. 

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. 

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