Alumina alkyne hydrogenation

Dehydration of propargyl alcohols occurs commonly on mononuclear ruthenium complexes.[ l Water is formed from the terminal alkynic hydrogen and by the alcoholic OH this is the more common dehydration process (it is denoted Route A). These reactions afford organic intermediates leading to cumulene complexes useful for the synthesis of doped polyacetylenes or of non-linear optic materials. Dehydration of rhodium-coordinated propargyl alcohols " leads to free or coordinated cumulenes and can be catalyzed by alumina and chloride ions, Fig. 16. 

The structure of Os3(/r-H)2(CO)10 has been established by X-ray8 and neutron diffraction.9 The 46-electron complex displays a relatively high reactivity under mild conditions, associated with a stable triosmium framework and has been extensively studied as a model for the chemisorption of alkenes and alkynes on surfaces and in the catalytic isomerization and hydrogenation of alkenes.10 When supported onto alumina it is a catalyst for the methanation of CO and C02 slightly less efficient than NiOs3(/r-H)3(CO)9(,5-CsH5)>... 

Hydrogen halides may add to acetylenes in a similar way to afford alkenyl halides.565 The use of silica and alumina, in this case, provides a simple means for facilitating addition of hydrogen halides to alkynes that does not occur readily in solution. Arylalkylacetylenes yield the corresponding syn-addition products (45) which undergo isomerization on extended treatment ... 

A Ni-As(B) catalyst prepared by the borohydride reduction of alumina-supported nickel arsenate gave, on hydrogenation of 1-bromo-l 1-hexadecyne (13) in the presence of a small amount of acetone, a 97% yield of the alkene (14) having a 92 5 cis/trans ratio. No hydrogenolysis of the carbon-bromine bond occurred. "> 2 Borohydride reduction of cobalt acetate gave a Co(B) catalyst that was somewhat less active than Ni(B) but that was quite selective in alkyne semihydrogenations (Eqn. 16.20). ... 

In the liquid phase at room temperature, using alcohol as a solvent and palladium supported on barium sulfate as catalyst, the only products observed from 1-butyne hydrogenation were 1-butene (98%) and n-butane (2%) (57). The gas phase reaction using 0.03% palladium on alumina catalyst gave 1-butene (99.1%), cis- and product distributions were maintained until at least 76% removal of the parent hydrocarbon but isomerization and hydrogenation of the 1-butene occurred after complete removal of the alkyne. Thus, l-butjme must displace 1-butene from the surface before its isomerization can occur, and it must prohibit the re-entry of 1-butene into the reacting surface layer. This represents the operation of a powerful thermodynamic factor. 

We shall consider first the simplest reaction so far reported (56, 94), which is the hydrogenation of 2-butyne in a flow system at room temperature and a little above, catalyzed by alumina-supported palladium (0.03%). This reaction proceeds very selectively, only a trace of butane being formed in the presence of 2-hutyne, as long as the catalyst is not completely fresh. Moreover, the reaction is highly stereoselective for the formation of cis-2-butene and only traces of mw -2-butene and 1-butene were observed. After the removal of 2-hutyne the cts-olefin both isomerized and hydrogenated, showing that a powerful thermodynamic factor is again operative (as was observed for 1-butyne, propyne and acetylene) when alkyne is present. 

Hydrogen chloride does not add easily to alkenes at a preparatively useful rate and reacts even slower with alkynes. It appeared that the presence of silicagel or alumina appreciably facilitates hydrohalogenation of alkenes and alkynes when the reaction is performed in methylene chloride. Initially, a product of suprafacial addition is formed, which equilibrates to the thermodynamic E/Z-equilibrium. For instance. 

The reaction is one you have not yet met, but it is quite a simple one, and it follows an unsurprising mechanism. It is the reaction of an alkyne with hydrogen chloride in the presence of alumina (AI2O3). The reaction produces two geometrical isomers of a chloroalkene. 

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