Chlorohydrins formation

Another example of a famous organic chemical reaction being replaced by a catalytic process is furnished by the manufacture of ethylene oxide. For many years it was made by chlorohydrin formation followed by dehydrochlorination to the epoxide. Although the chlorohydrin route is still used to convert propylene to propylene oxide, a more efficient air epoxidation of ethylene is used and the chlorohydrin process for ethylene oxide manufacture has not been used since 1972. 

There are two important methods for the manufacture of propylene oxide, each accounting for one half the total amount produced. The older method involves chlorohydrin formation from the reaction of propylene with chlorine water. Before 1969 this was the exclusive method. Unlike the analogous procedure for making ethylene oxide from ethylene, which now is obsolete, this method for propylene oxide is still economically competitive. Many old ethylene oxide plants have been converted to propylene oxide synthesis. 

A method of considerable industrial importance for the large-scale preparation of ethylene oxide is direct oxidation of ethylene at elevated temperatures over a suitably prepared metallic silver catalyst. Although the reaction may be written aa indicated in Eq. (09), in actual practice only about half the ethylene is converted into ethylene oxide, the remainder being oxidized further to carbon dioxide and water. In spite of this seeming disadvantage, catalytic oxidation appears at present to bo economically competitive with chlorohydrin formation aa a means for the commercial production of ethylene oxide.MM Unfortunately, other olefins, such as propylene and mo-butylene for example, apparently give only carbon dioxide and water under the usual oxidation conditions,1310 so that until now the patent hu balance ethylene oxide has been the only representative accessible by tins route. 

For many years the manufacture of ethylene oxide was carried out by chlorohydrin formation followed by dehydrochlorination to the epoxide. 

There are two important methods for the manufacture of propylene oxide. The older method involves chlorohydrin formation from the reaction of propylene with chlorine water (Fig. 1). 

The formation of halohydrins can be promoted by peroxidase catalysts.465 Recently 466 it has been shown that photocatalysis reactions of hydrogen peroxide decomposition in the presence of titanium tetrachloride can produce halohydrins. The workers believe that titanium(IV) peroxide complexes are formed in situ, which act as the photocatalysts for hydrogen peroxide degradation and for the synthesis of the chlorohydrins from the olefins. The kinetics of chlorohydrin formation were studied, along with oxygen formation. The quantum yield was found to be dependent upon the olefin concentration. The mechanism is believed to involve short-lived di- or poly-meric titanium(IV) complexes. 

Winterboum, C. C., J. J. van den Berg, E. Roitman, and F. A. Kuypers. 1992. Chlorohydrin formation from unsaturated fatty acids reacted with hypochlorous add. Arch Biochem 296(2) 547-55. 

Stoichiometric amounts of Pd(II) and exchange of the CuCl by CaClj led to no chlorohydrin formation. Jira proposed a reaction sequence of oxypalladation to AJ as observed in the Wacker process and oxidative-induced reductive C-Cl bond formation of the Pd alkyl intermediate AK (Scheme 16.41). 

Scheme 16.41 Oxidative chlorohydrine formation from Pd(ll) complexes.

The mystery becomes the reason for the anti stereochemistry observed at high chloride and cupric chloride concentrations, conditions for ethylene chlorohydrin formation. An examination of the early data of Stangl and lira provide a clue. Thus in Table 1 the data indicate that the chlorohydrin is formed only when both [CUCI2] and [Cr] concentrations are high. 

Thionyl chloride, as well as other inorganic acid chlorides, reacts with polyols to form mixed esters (see under Sulfate esters. Chapter III). In the presence of pyridine, partial chlorohydrin formation may occur (95), Selenium oxychloride forms a selenite ester upon reaction with mannitol (96), Phosphorus pentachloride yields unsaturated chlorohydrins of mannitol and galactitol which have the composition CeHeCU (97), Extremely interesting are the so-called complexes of alditols with various inorganic polybasic acids, their salts, or anhydrides in aqueous solutions. Complexes with boric, molybdic, tungstic, and other acids, as well as the oxides of antimony and arsenic, have been reported. It is believed that these complexes are true esters with one or more moles of alditol, a chelate type of structure being involved at some point. For the hexitols a compound with boric acid such as the following is postulated (98),... 

Quentela, A. L., and Abuin, S. E, Epoxidation reaction of trimethylol propane with epichlorohydrin Kinetic study of chlorohydrin formation, Polym. Engng. Set, 36, 568, 1996. 

Scheme 4 Geminal trichlorosilyl chlorohydrin formation from aliphatic aldehydes...

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