Alkenes continued hydrogenation

Acid-Gatalyzed Synthesis. The acid-catalysed reaction of alkenes with hydrogen sulfide to prepare thiols can be accompHshed using a strong acid (sulfuric or phosphoric acid) catalyst. Thiols can also be prepared continuously over a variety of soHd acid catalysts, such as seoHtes, sulfonic acid-containing resin catalysts, or aluminas (22). The continuous process is utilised commercially to manufacture the more important thiols (23,24). The acid-catalysed reaction is commonly classed as a Markownikoff addition. Examples of two important industrial processes are 2-methyl-2-propanethiol and 2-propanethiol, given in equations 1 and 2, respectively. 

In the case of the reaction class we are considering, that is, the dehydrogenation of alkanes to alkenes and hydrogen, continuous removal of either the alkene or hydrogen from the system will shift the conversion of the reactant alkane further to the products. This is the phenomenon we wish to examine. 

In total, over the past six years, the chelating P,N-ligands have shown considerable promise in a variety of enantioselective processes, including transfer-hydrogenation and hydrosilylation of ketones, hydroboration of alkenes, conjugate addition to enones and Lewis-acid catalysed Diels-Alder reactions, in addition to those described above.128,341 It is anticipated that this list will continue to grow, and... 

The bromine atom then adds to the alkene, generating a new carbon radical. In the case of propene, as shown, the bromine atom bonds to the terminal carbon atom. In this way, the more stable secondary radical is generated. This is preferred to the primary radical generated if the central carbon were attacked. The new secondary radical then abstracts hydrogen from a further molecule of HBr, giving another bromine atom that can continue the chain reaction. 

Figure 3.11 Propane metathesis catalyzed by (=SiO)2TaH (3) in a continuous flow reactor. Contact time effect (520mg of 5.3% Ta/SiOj 1 atm, 150 C, 1 to lOOmLmin or VHSV = 38 to 3800 h ) (a) alkane selectivity (b) alkene selectivity (c) hydrogen selectivity (d) alkane alkene ratio.

Supercritical hydrogenation is just one example of continuous reactions which can be carried out in SCCO2 solution. Other reactions which have been carried out successfully include Friedel-Crafts alkylation of aromatics by alcohols [64], the dehydration of alcohols to form ethers [65] (using acid catalysts), and the hydroformylation of alkenes [52] (using rhodium catalysts immobilized on Si02). In each of these reactions, it is possible to obtain a selectivity which is at least as good, and often better, than with conventional solvents. However, the precise role of the scCC>2 in these reactions is not as obvious as in supercritical hydrogenation. 

The rales of the IUPAC system for naming the alkenes are as follows 1. Select as the parent structure the longest continuous chain that contains the carbon-carbon double bond then consider the compound to have been derived from this structure by replacement of hydrogen atoms by various alkyl groups. 

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