Hydrogenation of Propadiene

During the reaction, part of the propadiene is converted into products of higher molar mass these are linear and branched Ce products. Their formation has been explained by isomerisation of the half-hydrogenated state 

TABLE 8. Kinetic Parameters for the Hydrogenation of Propadiene on Pumice-Supported Metals r oc Pn Pfy 

Propadiene reacted with deuterium over Pd/Al203 at 293 K to give deuterated propenes and propanes, but neither exchanged propadiene nor hydrogen deuteride was observed the concentration of adsorbed hydrogen and deuterium atoms was therefore low, and propadiene must have been strongly adsorbed. Burwell s N profile was applied to the distribution of deuteropropenes. This pro- 

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Fig. 26. Reaction scheme for the hydrogenation of propadiene to propene and propane.

BRICI/ABB Lummus Propylene C3s Selective hydrogenation of methyl acetylene and propadiene to propylene 12 1998... 

The lack of a plane of symmetry as a criterion for potential optical activity allows us to identify other molecules that are chiral. For example, if one hydrogen on each terminal carbon of propadiene is substituted for then the resulting molecule is chiral. 1,3- Difluoropropadiene, for example, has a twofold axis of symmetry but no plane of symmetry. The enantiomers are shown in Figure 6.15. Construct models of these and convince yourself that they are non-superimposable. 

The principal subject of this chapter is the hydrogenation of molecules containing two carbon-carbon double bonds. These may exist in a variety of situations thus in a linear chain of carbon atoms they may be adjacent as in propadiene... 

What happens if two double bonds are even closer together than in the conjugated case Successive double bonds with no intervening single bonds are called cumulated double bonds. Consider 1,2-pentadiene, which contains cumulated double bonds. Such 1,2-diene systems are also called allenes, after the simplest member of the class, 1,2-propadiene or allene, H2C = C=CH2- The heat of hydrogenation of 1,2-pentadiene is —292 kJ/mol (—69.8 kcal/mol). 

Hydrogenation of methylacetylene and propadiene, its isomer (MAPD) in the separated C3 stream is very similar to tail-end acetylene removal. The possible reactions are ... 

The overhead of the depropanizer is sent to the propylene fractionator. The methylacetylene (MA) and propadiene (PD) are usually hydrogenated before entering the tower. An MAPD converter is similar to an acetylene converter, but operates at a lower temperature and in the Hquid phase. Due to recent advances in catalysis, the hydrogenation is performed at low temperatures (50—90°C) in trickle bed reactors (69). Ordy rarely are methylacetylene and propadiene recovered. 

Figure 3-13. The influence of conversion severity on the theoretical product yield for the cracking of propane. Acetylene, methyl acetylene, and propadiene are hydrogenated and both ethane and propane are recycled to extinction (wt%)." ...

The a-selectivity for carbon radical addition to propadiene (la) is retained on substituting chlorine or fluorine for hydrogen in radicals of the type CX3 (X=F, Cl), no matter whether the reaction is conducted in the liquid or in the gas phase (Table 11.4) [14, 49-51]. /3-Selective addition to allenes becomes progressively more important for the CC13 radical with an increase in number of methyl substituents [14, 47]. For example, treatment of optically active (P)-(+)-2,4-dimethylpenta-2,3-diene [(P)-(lc)] with BrCCl3 affords a 59 41 mixture of a- and /3-monoadducts [47]. The a-addition product consists of a 20 80 mixture of E- and Z-stereoisomers, whereas the product of /3-addition exclusively exhibits the Z-configuration. The fraction of 2,4-dimethylpenta-2,3-diene (P)-(lc) that was recovered from this reaction mixture had completely retained its optical activity. These results indicate that the a-and the /3-CCl3 addition proceed under kinetic control. If one of the addition steps were reversible, at least partial racemization would inevitably have taken place. 

Tetrahydrofuran (12) adds to propadiene when heated in the presence of DTBP to 160°C to afford a minor fraction of diadduct in addition to 71% of a mixture of monoaddition products 13 and 14 (Scheme 11.8). The reaction proceeds via the nucleophilic 2-tetrahydrofuryl radical (not shown) that adds with a low a-selectivity to propadiene (la), thus leading after hydrogen atom trapping to a 66 34 ratio of functionalized heterocydes 13 and 14 [59]. 

C3 Hydrorefining. The aim of C3 hydrorefining is to hydrogenate methylacetylene and propadiene present in the cut. Efficient liquid-phase processes were developed by Bayer314-316 (cold hydrogenation process carried out at 10-20°C) and IFP,317 but hydrogenation in the gas phase is also practiced. 

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