Double bonds, alkene hydrogenation

An incorrect statement to the effect that there is no evidence for the complexing of double bonds with hydrogen halides must be corrected. This writer himself has quoted several times the demonstration by O. Maas and his collaborators that the HX (X = Cl, Br) do form complexes with alkenes [A, B],... 

Solvents exert control on the chemose-lective hydrogenation of alkenes bearing a benzyloxy protecting group [160]. In the unpolar solvent benzene, only the double bond is hydrogenated, while in methanol, acetone, and ethyl acetate, the benzyloxy group is also removed. Selective... 

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

Alkenes get converted to alcohols by treatment with aqueous acid (sulphuric or phosphoric acid (Following fig.)). This electrophilic addition reaction involves the addition of water across the double bond. The hydrogen adds to one carbon while a hydroxyl group adds to the other carbon. 

While the FMO treatment depicted in Fig. 4.12 fully describes the surface interactions taking place when a double bond is hydrogenated, this approach is more complex than is needed for most purposes. The reaction sequences shown in Schemes 3.2, 3.4 and 3.5, which are based on an octahedral species however, are incorrect. Scheme 4.1 shows the reaction sequence for alkene hydrogenation over a MH site that is analogous to Scheme 3.2 but uses a more descriptive surface site model. Instead of the depiction of the MH site as a comer atom as... 

Isomerization. With Pd catalysts, and, to a lesser extent, with Pt catalysts, a mixture of isomeric products may be obtained due to positional isomerization of double bonds during hydrogenation. As illustrated below, 5yn-addition of H2 to either face of the double bond in alkene A furnishes cw-decalin C. However, 5yn-addition of Hj to the isornerized alkene B can produce the cw-decalin C and/or trans-decalin D, depending on which face of the double bond undergoes addition by H2. In fact, hydrogenation of A in the presence of Pt furnishes 80% of the thermodynamically more stable trans-decalin and only 20% of cw-decalin. 

In this reaction, the catalyst surface interacts with the hydrogen molecule and in doing so weakens the H-H bond. The bond is then broken and the hydrogen atoms are bound to the catalyst. The catalyst can then interact with the alkene molecule, so weakening the pi bond of the double bond. The hydrogen atoms and the alkene molecule are positioned on the catalyst conveniently close to each other to allow easy transfer of the hydrogens from catalyst to alkene. The alkane product then departs, leaving the catalyst as it was before the reaction. 

Alkenes contain one or more double bonds. Alkenes, alkynes, and aromatics are unsaturated because they have fewer hydrogen atoms.Alkenes are also called olefins. The suffix used in the naming of alkenes is -ene, and the number roots are those used for alkanes of the same length. 

In aqueous/organic biphasic medium the reaction rate for the hydrogenation of linear and cyclic alkenes with several Ru(II) complexes including [HRuCl-(TPPMS)2]2, [HRuC1(TPPMS)2(L)2] and [HRuCl(TPPTS)2(L)2] (L = aniline or tet-rahydroquinoline) followed the order linear C2 f-6 linear C7-C10 > cyclic alkenes > branched alkenes [54]. This reactivity pattern is similar to the case of al-kene hydrogenations with [HRuCl(PPh3)3], i.e., the least-substituted double bonds are hydrogenated the fastest. 

Catalytic hydrogenation of these three compounds all yield cis-1,2-dimethylcyclopropane. Note that the tetrasubstituted alkene yields only the cis product upon catalytic hydrogenation. The other two yield cis and trans products, depending upon which side of the double bond was hydrogenated. 

The physical properties of the alkenes are very similar to those of the alkanes. Chemically, however, they are much more reactive than the alkanes because they contain double bonds. Alkenes are said to be unsaturated hydrocarbons, because of the double bond, their molecules can add on more atoms of hydrogen. The unsaturation of alkenes gives rise to reactions with substances other than hydrogen. The chemical properties of alkenes are as follows ... 

The net reaction from hydroboration and subsequent oxidation of an alkene is hydration of a carbon-carbon double bond. Because hydrogen is added to the more substituted carbon of the double bond and —OH to the less substituted carbon, we refer to... 

The hydrogenation of an alkene to an alkane can be accomplished with heterogeneous Pd catalyst under a variety of conditions. Less substituted double bonds are hydrogenated more rapidly (Scheme Alcoholic solvents are used often, but many nonpolar aprotic solvents, such as EtOAc and THE, are also commonf w (Scheme 12). 

Rank the following alkenes in order of stability of the double bond to hydrogenation (orda- of AH° of hydrogenation) 2,3-dimethyl-2-butene, cis-3-hexene, fro s-4-octene, and 1-hexene. 

Hydrogenation Pi Alkene (double bond) -t- hydrogen > alkane (single bonds) Ni Unsaturated fat (double bonds) -1- hydrogen > saturated fat (single bonds)... 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

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