The Hydration of Olefins

While the last topic is the focal point of this chapter, the other three represent an essential complement to its understanding, particularly in the li t of much important work recently published. 

In an excellent monograph published more than ten years ago, de la Mare and Bolton discussed the various aspects of electrophilic addition to unsaturated systems. The hydration of olefins was postulated to occur via the rate determining reaction 

In conclusion, overwhelming evidence points today to the general acid catalysis of these reactions and to the formation of a short-lived intermediate with the characteristics of a symmetrical open ion. This behaviour includes an enormous range of nucleophili-city of the substrate, from ethylene to 1,1-diethoxyethylene. Thus, the postulation of a rr-complex precursor in the mechanism of acid-catalysed hydration of olefins is now unjustified, and the second order rate constants experimentally obtained are in fact a reflection of the protonation reaction of the hydronium ion onto the double bond. 

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Many catalysts for the hydration of olefins in general, and of ethylene in particular, are described in the patent Hterature. Practically all of them are acidic. There has been a patent Hterature review through 1937 of the types of catalysts used (47,48) and a general review of olefin hydration (88). 

With phosphoric acid-based catalysts, in which the active component is Hquid acid absorbed in the pores of the support, the reaction probably follows the path (119) for the hydration of olefins in aqueous solution ... 

The reversibly formed re-complexes precursor to the hydration of olefins have the proton imbedded in the re-electron cloud of the double bond somewhere between the two carbon atoms. They are therefore... 

Hydration of Olefins. The earliest and still the largest production of chemicals from petroleum hydrocarbons was based on the hydration of olefins to produce alcohols by the employment of sulfuric acid. The addition of olefins to sulfuric acid to form alkyl sulfates and dialkyl sulfates takes place on simple contact of the hydrocarbons with the acid. To keep down polymerization and isomerization of the hydrocarbons, the temperature is kept relatively low, usually below 40° C. and commonly considerably lower than that (18). The strength of the sulfuric acid used depends on the olefin to be absorbed. Absorption of ethylene requires an acid concentration higher than 90%, whereas propylene and butylenes are readily absorbed in 85% acid or less. The alkyl and dialkyl sulfate solutions, on dilution and heating, are hydrolyzed to the alcohols plus small amounts of by-product ethers. After distilling off the organic products, the dilute sulfuric acid is reconcentrated and re-used. 

A major limitation on the production of alcohols by olefin hydration is the fact that the products consist almost solely of secondary or tertiary alcohols (excepting, of course, ethyl alcohol). The normal or primary alcohols are made by other means (but also from petroleum hydrocarbons). It appears more difficult to prepare C5 and higher alcohols by the hydration of olefins since they are produced commercially by other means. One of the problems encountered (81) is excessive polymerization of the higher olefins when contacted with aqueous sulfuric acid. 

The oxo reaction (31) is carried out in the liquid phase at high pressure using a cobalt catalyst. A mixture of aldehyde isomers is always produced, each isomer being one carbon number higher than the starting olefin. As a group the oxygenated products of the hydrocarbon synthesis (Fischer-Tropsch) process and the oxo process are primary compounds and thus (except, of course, the methyl and ethyl derivatives) differ fundamentally from the products based on alcohols made by the hydration of olefins, which are always secondary or tertiary in structure. 

Originally, the hydration of olefins to alcohols was carried out with dilute aqueous sulphuric acid as the catalyst. Recently, the direct vapour phase hydration of olefins with solid catalysts has become the predominant method of operation. From the thermodynamic point of view, the formation of alcohols by the exothermic reaction (A) is favoured by low temperatures though even at room temperature the equilibrium is still in favour of dehydration. To induce a rapid reaction, the solid catalysts require an elevated temperatue, which shifts the equilibrium so far in favour of the olefin that the maximum attainable conversion may be very low. High pressures are therefore necessary to bring the conversion to an economic level (Fig. 11). To select an optimum combination of reaction conditions with respect to both rate limitation and equilibrium limitation,... 

Manassen, J., Klein, F.S. Reactions of n-Butene and Butan-2-ol in Dilute Acid. The Elucidation of the Mechanism and the Intermediate in Elimination from Secondary Alcohols and in the Hydration of Olefins. J. Chem. Soc. 1960, 4203. 

Fig. 1.11. Net reaction (a) for the hydration of olefins (R = CH3, R = H) or (b) for the addition of alcohol to olefins (R = CF3, R = alkyl) via the reaction sequence (1) solvomercuration of the olefin (for mechanism, see Figure 3.37 regioselectivity Figure 3.38) (2) reduction of the alkylmercury compound obtained (for mechanism, see Figure 1.12).

The evidence for the operation of the A—SE 2 mechanism in the hydration of olefins and other related reactions has been reviewed [42]. 

The application of organometallic compounds in medicine, pharmacy, agriculture and industry requires the accurate determination of these metals as part of their application. Most % complexes characterised by direct carbon-to-carbon metal bonding may be classified as organometallic and the nature and characteristics of the n ligands are similar to those in the coordination metal-ligand complexes. The -complex metals are the least satisfactorily described by crystal field theory (CFT) or valence bond theory (VBT). They are better treated by molecular orbital theory (MOT) and ligand field theory (LFT). There are several uses of metal 7i-complexes and metal catalysed reactions that proceed via substrate metal rc-complex intermediate. Examples of these are the polymerisation of ethylene and the hydration of olefins to form aldehydes as in the Wacker process of air oxidation of ethylene to produce acetaldehyde. 

This indicates the two-fold function of petroleum chemicals the hydration of olefins led to alcohols and to the family of derivatives of alcohols already made from other sources and, on the other hand, the olefin oxides and their derivatives were new industrial chemicals not previously made. 

The hydration of olefins to alcohols has been carried out on a large scale by hydrolyzing the sulfuric acid esters formed by the absorption of the olefins in sulfuric acid. In the case of the higher olefins these reactions occur with comparative ease. Thus, isobutylene may be hydrated to tertiary butanol in cold, moderately concentrated sulfuric acid.81 Some of the pentenes and heptenes may be hydrated in dilute (5 to 10 per cent) solutions of formic, acetic, or oxalic acids as well as in weak solutions of the mineral acids.83 With 60 per cent concentrations of hydroiodic acid, isobutylene yields the iodide almost exclusively, but at lower concentrations increasing amounts of the alcohol are fonned84 Similar phenomena attend the absorption of the higher olefins in hydrobromic acid. Hydrochloric acid, on the other hand, does not show such marked activity toward the higher olefins ind is practically devoid of activity toward ethylene. 

A number of other processes have been proposed and patented for the hydration of olefins in the presence of aqueous solutions of various salts and acids under such conditions that liquid is present at temperatures of about 200° C. and above. However, commercial yields of alcohols have not been reported from these processes. Usually, fairly dilute solutions only of the stronger mineral acids have been used or proposed in order to avoid the polymerizing effects otherwise encountered. 

Several patents have been granted for the hydration of olefins under... 

The difficulties attending the catalytic vapor phase hydration of olefins, while not apparent from the claims made in the patents which have been obtained for such processes, are serious and numerous. Aside from those already mentioned, the difficulties of separating the alcohol from the dilute liquid condensate by distillation and of purifying the alcohols from hydrocarbon polymers by a process of chlorination or selective absorption must be overcome. In view of the success that has attended the hydration of olefins, particularly those higher than ethylene, by means of absorption in sulfuric acid followed by dilution and distillation, it is probable that direct hydration processes at the present stage of the art will be unable to compete as long as cheap sulfuric acid is available. 

Reviews of the hydration of olefins are given by Asinger59 and Seka,60 and of the use of boron trifluoride for that purpose by Topchiev et al.61... 

Hydration of Ethylene. Taft and associates have proposed the following mechanism for the hydration of olefins in an aqueous aci[Pg.767]

Predict the products of each precursor by the hydration of olefins method. 

Scheme 1 Simplified mechanism of the hydration of olefins in acid.

Much of the reported work on the hydration of olefins has tried to fit the acid-dependence of the reaction either to the stoichiometric hydrogen ion concentration or to the origin Hammett function. In the latter case, even when the reaction shows linear dependence upon //o, the slope of the plot is often greater than unity. It has been suggested that this discrepancy is due to changes in the activity coefficient terms similar to the variations found between the different acidity functions , viz-... 

There is a considerable amount of evidence opposing the intermediacy of TT-complexes in the acid-catalysed dehydration of alcohols and in the hydration of olefins. In the 1,2-diphenylethanol system , if before or during the course of the rate-determining step the two central carbon atoms become equivalent then substituents in both phenyl groups should exert a similar influence on the... 

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