Alkanes formation, hydrogenation

Alkanes are formed when the radical intermediate abstracts hydrogen from solvent faster than it is oxidized to the carbocation. This reductive step is promoted by good hydrogen donor solvents. It is also more prevalent for primary alkyl radicals because of the higher activation energy associated with formation of primary carbocations. The most favorable conditions for alkane formation involve photochemical decomposition of the carboxylic acid in chloroform, which is a relatively good hydrogen donor. 

The substitution of trialkylphosphine for carbon monoxide also makes the metal-hydrogen bond more hydridic in character and results in increased reduction of the initially formed aldehyde to alcohol. Slaugh and Mullineaux (52) compared Co2(CO)g and [Co2(CO)8 + 2PBu3], each at reaction conditions of 150°C, 500 psi, H2/CO I.0, for the hydroformylation of 1-pentene. The products consisted of hexyl aldehydes and hexyl alcohols in the ratios of 95 5 and 30 70, respectively. In a negative aspect of the reaction, they observed 23% hydrogenation of alkene to alkane at a reaction temperature of 195°C with the phosphine-modified catalyst. Tucci (54) reported less alkane formation (4-5%) under more favorable reaction conditions (I60°C, H2/CO 1.2, 1 hour reaction time). 

Alkene and alkane formation was suggested to take place through p cleavage and subsequent hydrogenation [Eq. (3.17)] chain branching involves the reaction of 1 with a half-hydrogenated intermediate [Eq. (3.18)] ... 

A further possibility to interpret cis addition is the so-called cA-concerted mechanism74,145. It assumes that the addition of the two hydrogen atoms takes place in a single step in a concerted fashion on a single 3M site possessing three coordinative unsaturations. The transfer of the two hydrogens to the double bond through a concerted process, where the interaction with the catalyst removes the symmetry restrictions imposed by the Woodward-Hoffman rales, leads directly to alkane formation. 

Another widely used decarboxylation procedure involves the use of lead tetraacetate. Depending on the nature of the substrate and the reaction conditions, this reagent may transform a carboxylic acid into an alkane or alkene, or into the respective acetoxy derivative (Scheme 2.144). The most favorable conditions for alkane formation utilize a good hydrogen donor as the solvent. Usually this transformation is carried out as a photochemically induced oxidative decarboxylation in chloroform solution, as is exemplified in the conversion of cyclobutanecarboxylic acid in cyclobutane.In contrast, the predominant formation of alkenes occurs in the presence of co-oxidants such as copper acetate. ... 

Racemization. Optically active 1-bromo-l-methyl-2,2-diphenylcy-clopropane, l-iodo-l-methyl-2,2-diphenylcyclopropane, and l-bromo-2,2-diphenylcyclopropane carboxylic acid were prepared to study the mechanism of alkane formation by hydrido complex. While the first two substrates could not be reduced, the a-bromo acid absorbed 87 mole % of hydrogen, being converted into optically inactive acid (Reaction 18). A sample of the optically active acid retained its configuration under reaction conditions, indicating that a symmetrical intermediate was formed at some stage of the reduction. 

Other reactions are alkane formation by hydrogenation, ketone formation (especially with ethylene ), ester formation through hydrogen transfer and formate ester synthesis. An improved catalyst system in which one CO ligand of CoH(CO)4 is substituted with a trialkylphosphine ligand , was disclosed by Shell workers in the early 1960s. With this catalyst, which is more thermally stable than the unsubstituted cobalt carbonyl, reaction proceeds at 140-190 C with 3-7 MPa of CO and Hj. Additionally, mostly linear aldehydes are obtained from linear terminal and internal olefins. This remarkable result arises from the high preference for the terminal addition to an a-olefin, and the isomerization of the olefinic position which occurs simultaneously with hydroformyiation. 

Because of the greater proportion of s character in the sp o orbitals of the carbon atoms than that in sp hybrids (alkenes) or sp hybrids (alkanes), any bonding pair of electrons can come closer to carbon nuclei in acetylene than in alkenes or alkanes. Hence hydrogen is released as a proton more easily in the acetylenic carbon atoms. The increased acidity of hydrogen attached to a carbon atom with a triple bond results in the formation of a class of carbides with certain metals, called acetylides. These acetylides are unstable and highly sensitive to shock and heat, exploding violently (see Chapter 30). 

Extracts from tea leaves in the presence of oxygen and ascorbate converted fatty acids (n-Cig to n-C32) into n-alkanes containing 2 carbons less thus a-oxidation must have preceded final loss of carbon. However, the aldehydes prepared from the C g and C24 acids were straightforwardly decarbonylated to the acids with one carbon atom less. Tea leaves in vivo produced the odd-number series of n-alkanes and it was presumed the latter route here predominated Thus the implication was that the final step in n-alkane formation was a decarbonylation rather than a decarboxylation and studies using particulate preparations from peas have confirmed this. The mechanism is obscure, but tracer studies have shown that the conversion, RCHO RH 4- CO, involves retention of the aldehydic hydrogens, and it has also been demonstrated that a metal ion is implicated (effect of chelating agents) This type of mechanism is consistent with... 

It is noteworthy that the double-bond isomerization step is faster than the overall elementary steps of alkane metathesis. Formation of lower alkanes is due to the tungsten hydride intermediate, favoring chain walking with double-bond migratirHi followed by fast cross metathesis with coordinated ethylene leading to lower alkenes in turn giving lower alkanes on hydrogenation. This intramolecular reaction pathway, without formation of the free olefin, probably is the difference between alkane and olefin metathesis. 

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