3-Hexyne, hydrogenation

Figure 7. Concentration-time profiles during 1-hexyne hydrogenation over 0.4wt.% Pd/ACF. Reaction conditions 0.5kmol/ 1-hexyne in n-heptane (lOOmL) substrate-to-Pd molar ratio of 23,000 303 K pressure of 1.3 MPa 1500 rpm.

Typical concentration-time profiles during the 1-hexyne hydrogenation over 0.4wt.% Pd/ACF catalyst are presented in Figure 7 showing the experimental and simulated curves (Langmuir-Hinshelwood mechanism). Pd/ ACF materials with the same particle size but different Pd loading (0.4, 0.6, 1.2wt.%) show identical initial activity of 0.140 0.004 kmolHj/kgp(j/s. This indicates the absence of diffusion limitations. Selectivity to 1-hexene is 97.1 +0.4% up to 80% conversion, and 95.9 + 0.4% at 90% conversion. 

CH3CH2CH2CH2C=CH + HBr per°X deS> CH3CH2CH2CH2CH=CHBr 1-Hexyne Hydrogen bromide 1-Bromo-l-hexene (79%)... 

The alkane formed by hydrogenation of (S) 4 methyl 1 hexyne is optically active but the one formed by hydrogenation of (S) 3 methyl 1 pentyne is not Explain Would you expect the products of hydrogenation of these two compounds in the presence of Lindlar palladium to be optically active" ... 

Hydrogenation of 2,5-diacetoxy-2,5-dimethyl-3-hexyne 10 over 0% palladium-on-carbon is exceptionally complex. Seven different products are formed together with acetic acid. All are hydrogenolysis products arising from the initially formed 2,5-diacetoxy-2,5-dimethyl-3-hexene 11. One of these, 2,5-dimethyl-2-acetoxy-4-hexene 12 forms in as much as 4S yield. 

The product i n this case is a cis-disubstituted alkene, so the fi rst question is, " What is an immediate precursor of a cis-disubstituted alkene " We know that an alkene can be prepared from an alkyne by reduction and that the right choice of experimental conditions will allow us to prepare either a trans-disubstituted alkene (using lithium in liquid ammonia) ora cis-disubstituted alkene (using catalytic hydrogenation over the Lindlar catalyst). Thus, reduction of 2-hexyne by catalytic hydrogenation using the Lindlar catalyst should yield cis-2-hexene. 

In contrast to the work of Peterson (38) with 3-hexyne in trifluoroacetic acid (vide supra), no trimeric adducts were observed in the hydrogen halide additions to propyne (48). 

The synthesis of Pd/ACF (0.42wt.% Pd) catalyst with monodispersed nanoparticles carried out at cuo = 3 is illustrated, as well as its catalytic performance in a liquid-phase hydrogenation of 1-hexyne in comparison with a traditional powdered Lindlar catalyst. 

Hydrogenations involving consecutive reactions are common in the organic process industry and even in the hydrogenation of fats. In the fine chemicals industry we have examples of acetylenic (triple) bonds to be selectively converted to olefinic (double) bonds. Lange et al. (1998) have shown, for the comversion of the model substance 2-hexyne into cis-2-hexene, how catalytically active microporous thin-film membranes can accomplish 100% selectivity. This unusual selectivity is attributed to avoidance of backmixing. 

The kind of surface site catalyzing alkyne half-hydrogenation has been the subject of some speculation. Two possibilities are shown in Fig. 2.2. Certainly such a site should fit the evidence Burwell and associates obtained from the hydrogenation of di-tert-butyl acetylene (2,2,5,5-tetramethyl-3-hexyne). 31 They proposed that the molecule dislocated a surface atom, pulling it up out of the plane of the surface (Fig. 2.3). That work concluded that hydrogenation... 

The use of stronger acid conditions provides somewhat better synthetic yields of alkanes from alkynes. A useful method consists of treatment of the substrate with a combination of triethylsilane, aluminum chloride, and excess hydrogen chloride in dichloromethane.146 Thus, treatment of phenylacetylene with 5 equivalents of triethylsilane and 0.2 equivalents of aluminum chloride in this way at room temperature yields 50% of ethylbenzene after 1.5 hours. Diphenylacetylene gives a 50% yield of bibenzyl when treated with 97 equivalents of triethylsilane and 2.7 equivalents of aluminum chloride after 2.8 hours. Even 1-hexyne gives a mixture of 44% n -hexane and 7% methylpentane of undisclosed structure when treated with 10 equivalents of triethylsilane and 0.5 equivalent of aluminum chloride for 0.5 hour.146... 

Finally, the only example of a polynuclear homogeneous catalyst is the dinuc-lear complex [Pt P sH ]4- [66], which catalyzed the hydrogenation of styrene, phenylacetylene, 1-octyne, and 1-hexyne (i-PrOH, 60°C, 20.7 atm H2 pressure, Pd substrate ratio 1 1800) to the corresponding alkanes within 10 h of reaction. 

Alkynes are hydrogenated to cis olefins with the same catalytic systems, and subsequently undergo hydrogenation to yield the corresponding alkanes [7, 42, 45, 47, 49, 59, 93]. For example, Jordan et al. reported the selective hydrogenation of 3-hexyne into cis 3-hexene with a TOF of 25 IT1 [25], and cis 3-hexene is... 

The Cp derivative [Ru3Cp2( 3-Ph2C2)(CO)5] [42] also hydrogenates diphenyl-acetylene and 3-hexyne, but fragmentation probably takes place in this case to an important extent. 

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