Solubility of olefins in water

Bromohydrins [1, 80, after line 3]. The preparation of bromohydrins from olefins by the combination of N BS with water has been hampered by the limited solubility of olefins in water. Dalton et al.12a report that the reaction can be carried out in good yield in DMSO containing a small quantity of water. In a special experiment, 10 moles of cyclohexene was allowed to dissolve in 50 ml. of dry dimethyl sulfoxide and and 25 mmoles of water was added. Under a nitrogen atmosphere, 20 mmoles of NBS was added with cooling below 20°. After 15 min. the reaction mixture was quenched in a large volume of water and the product removed by ether extraction. 

A severe drawback of the process arises from the limited solubility of olefins in water, which restricts the process to propylene and butylenes hydroformylation. Figure 6.14.5 shows the dependence of olefin solubility in water as a function of the number of carbon atoms. 

The hydroformylation of propene in a biphasic system using rhodium complex with TPPTS ligand can be thought of as a perfect implementation of the ideal phase-separation technique. All new hydrophilic phosphine ligands are usually first tried in hydroformylation, with two primary goals (i) to improve selectivity with respect to the ratio of normal to branched products and (ii) to enhance productivity of the biphasic system. The latter goal depends on an intrinsic limitation of the biphasic system, in that the reaction takes place in the aqueous layer and the rate (turnovers per unit time) is limited by the sparse solubility of olefins in water and by mass transfer of olefin across the very small interfacial boundary between the organic and aqueous layers. 

The Ruhrchemie/Rhone-Poulenc process is performed annually on a 600,000 metric ton scale (18). In this process, propylene is hydroformylated to form butyraldehyde. While the solubility of propylene in water (200 ppm) is sufficient for catalysis, the technique cannot be extended to longer-chain olefins, such as 1-octene (<3 ppm solubility) (20). Since the reaction occurs in the aqueous phase (21), the hydrophobicity of the substrate is a paramount concern. We overcame these limitations via the addition of a polar organic co-solvent coupled with subsequent phase splitting induced by dissolution of gaseous CO2. This creates the opportunity to run homogeneous reactions with extremely hydrophobic substrates in an organic/aqueous mixture with a water-soluble catalyst. After C02-induced phase separation, the catalyst-rich aqueous phase and the product-rich organic phase can be easily decanted and the aqueous catalyst recycled. 

ILs also present good and tunable solubility with olefins. As described above for organic/water biphasic hydroformylation systems, the partial solubility of olefins in the phase containing the catalyst is essential to obtain a viable biphasic process. 

The industry needs cheap and efficient methods for the hydroformylation of long chain olefins, the solubilities of which in water are negligible. (The log p values for Cj-Cio olefins are lower than 4.5, which means that the concentration of these hydrocarbons in water is lower than the detection limit of common analytical methods, and the values of partition coefficients are not measured directly, but estimated by, for example, HPLC retention times.) As may be expected, such olefins are either totally unreactive, or react with inappropriately low rates under pure biphasic conditions. 

To improve the productivity, several basic approaches are utilized. The most straightforward solution is to add a cosolvent like lower alcohols (MeOH, EtOH) miscible with water, thus improving the solubility of olefins in the aqueous phase. A certain additional effect may be obtained by varying the nature of the catalyst precursors, though, under the conditions used for hydroformylation, rhodium complexes are rapidly transformed into [HRh(CO)L3] thus masking the differences [128,129]. 

Biphasic catalysis rehes on the transfer of organic substrates into the aqueous phase containing the catalyst or at the interphase. Therefore, improving the affinities between the organic and aqueous phases will increase the reaction rate. Cosolvents are employed to improve mass transfer in a biphasic system because they can increase the hpophilicity of the aqueous phase, thereby increasing the solubility of olefins in the catalyst water phase and accelerating the reaction rate. 

Cationic phosphine ligands containing guanidiniumphenyl moieties were originally developed in order to make use of their pronounced solubility in water [72, 73]. They were shown to form active catalytic systems in Pd-mediated C-C coupling reactions between aryl iodides and alkynes (Castro-Stephens-Sonogashira reaction) [72, 74] and Rh-catalyzed hydroformylation of olefins in aqueous two-phase systems [75]. 

Table 3.2. HydrogensUton of olefins with water-soluble catalysts in homogeneous solution and in aqueous-organic two phase systems... 

Hydroformylation of Mid Range Olefins - Rhodium/tppts catalysts exhibit low catalytic activity in the hydroformylation of mid range olefins (C5-Cg) in a two phase system due to the much lower solubility of such olefins in water. In the Rh/tppts catalysed biphasic hydroformylation of 1-hexene, for example the conversion is only 11-22% with a n/i ratio of aldehydes of 98/2.353,373 The rate of 1-hexene hydroformylation catalysed by Rh/tppts increased by a factor 2.3 when subjected to ultrasound (35 kHz) and high stirring rates.360,361... 

As mentioned above, the poor solubility of higher olefins in water still hampers their hydroformylation in aqueous media. Since 2003, ILs have entered the hydroformylation field as an alternative to the aqueous phase thanks to the higher solubilities of long-chain olefins in these non-conventional solvents. 

Fig. 7 Solubility of olefins [30, 31 a] and of the aldehydes obtained therefrom by hydroformylation in water [31 b, c].

The Relationship Between Solubility of Linear a-Olefins in Water and the Reaction Rate under Ruhrchemie/Rhdne-Poulenc Conditions... 

Addition of HOC1 to olefins is technically very important. When chlorine and ethylene are passed into water (Gomberg s ethylene chlorohydrin synthesis267) the amount of chlorine added as such increases with increasing concentration of HC1. This undesirable side reaction is reinforced by the greater solubility of olefins and Cl2 in the Cl2-adduct than in water. The experiment must therefore be broken off at relatively low concentrations of chlorohydrin (for ethylene 8-10%), and the mixture must be worked up at that stage.226,268,269... 

The hydrogenation of olefins in supercritical carbon dioxide catalyzed by palladium nanoparticles synthesized in a water-in-C02 microemulsion was reported by Ohde et al (27). The Pd nanoparticles were prepared by hydrogen reduction of Pd ions (a PdCl2 solution) dissolved in the water core of the microemulsion. Effective hydrogenation of both C02-soluble olefins (4-methoxycinnamic acid and trans-stilbene) and a water-soluble olefin (maleic acid) catalyzed by the palladium nanoparticles in the microemulsion was demonstrated. 

Additional data on hydroformylation of 1-pentene, 1-octene and ethyl acrylate are provided in Table 1. In all the runs, the solutions became clear and yellow after a period of 10 minutes, which indicated the formation of the microemulsion with the catalyst formed in situ inside the water droplets. The solutions were clear and homogeneous during the entire run, which definitely excludes reaction via a biphasic pathway. Because of equipment limitations, the highest reaction temperature we investigated was 87.1 C. The stability of the W/CO2 microemulsion system at such a high temperature is remarkable. At the conditions employed, conversions ranged from 6 to 75%. The increase of temperature and the addition of NaOH were found to increase the reaction rate. The initial reaction rate for 1-pentene is about two times higher than that of 1-octene. In studies on hydroformylation of different olefins in aqueous biphasic systems, Brady et al. [2/] found that there is a marked dependence of the reaction rate on the solubility of the terminal olefins in water. The data shown in... 

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