Acid-catalysed hydration of olefins

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. 

Since the rate-determining stage in an electrophilic addition reaction often involves the attack of the electrophile upon the unsaturated system, factors which affect the electronegativity of the atom being attacked will influence the rate of the reaction. In the acid-catalysed hydration of olefins, which in dilute solutions follows the simple kinetic form... 

The acid-catalysed hydration of olefins is a reversible process (Scheme 1, p. 4) for which the rate-determining stage involves the protonation of the olefin. As a result, the kinetics of the reaction may be complicated if hydration is not substantially complete under the reaction conditions. In dilute solution, however, this is generally not the case, and some equilibrium constants for the reaction... 

Table 3 contains kinetic results for the acid-catalysed hydration of olefins. In the second column of the table, rate coefficients are quoted in units of 10" l.mole. sec . A , is defined by the expression... 

Within this latter category, two new possibilities occur. The initial attack upon the unsaturated system may be due (i) to a positively charged species (X+) or to the positively charged end of the X-Y dipolar molecule, when the addition is initiated by an electrophilic attack, or it may be due (ii) to a negatively charged species (Y ) or to the negatively charged end of the X-Y dipole, when a nucleophilic addition is said to take place. The first mode of reaction is exemplified by the acid-catalysed hydration of an olefin, viz. 

The generalisation is often made that acetylenes undergo electrophilic attack less readily than olefins this is usually attributed to the lower polarisa-bility of the triple bond. In some cases, as in the acid-catalysed hydration of acetylenic ethers , the generalisation does not hold. However, the lower reactivity of acetylenic bonds in many electrophilic reactions explains the use of metal-ion catalysts, such as the well-known mercuric-ion-catalysed hydration of acetylene, and the greater tendency towards nucleophilic addition reactions. 

Dehydration of cyclohex-2-en-l-ols with methyl triphenoxyphosphonium iodide in hexamethylphosphoric triamide provides a simple route to the conjugated dienes. In the acid-catalysed hydration of cycloalka-l,3-dienes an approximately linear inverse relationship between ring strain energy and log(rate of hydration) was foundfor olefins hydrated via a conjugated carbonium ion. 

The addition of weak acids (MeOH, CHgCOOH) and strong acids (HBr, H,SO ) to olefins can proceed in a similar manner to the acid-catalysed hydration reactions. A similar reaction scheme involving initial attack on the double bond by a proton can be proposed, viz. 

It is with the limitation of being unable to assess the acidity of the medium independently of the type of indicator employed, that interpretation of the dependency of the rate of acid-catalysed dehydration of alcohols and hydration of olefins must be approached. As each of the various acidity functions run parallel to each other, a plot of the logarithm of the rate coefficient of an acid-catalysed reaction against an acidity function should give a linear correlation. The slope of such a plot, however, will only be unity if the ratio of activity coefficients of the substrate and its activated complex vary in the same way with changes in the reaction medium as the ratio of activity coefficients of the indicator molecule and its conjugate acid. 

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... 

The inactivity of pure anhydrous Lewis acid haUdes in Friedel-Crafts polymerisation of olefins was first demonstrated in 1936 (203) it was found that pure, dry aluminum chloride does not react with ethylene. Subsequentiy it was shown (204) that boron ttifluoride alone does not catalyse the polymerisation of isobutylene when kept absolutely dry in a vacuum system. However, polymers form upon admission of traces of water. The active catalyst is boron ttifluoride hydrate, BF H20, ie, a conjugate protic acid H" (BF20H) . 

Acid-exchanged zeolite A can also be used to catalyse the hydration of a wide range of olefins, such as terpenes (e.g. equation 4.34) [140, 141]. 

The several attempts, published in the literature, to describe the kinetics of vapour phase olefin (mostly ethylene) hydration can be classified into two groups according to the basic model used. One model, for reactions catalysed by phosphoric acid supported on solids, treats the kinetics as if the process were homogeneous acid catalysis and takes into account the acid strength of the supported acid. Thus, a semiempirical equation for the initial reaction rate [288]... 

The observed structure effects are similar, as in the reaction catalysed by sulphuric acid. On this basis and with the notion of the strong acidity of the heterogeneous catalysts used, it is possible to assume a mechanism similar to olefin hydration (Sect. 3.1) or alkylation (Sect. 3.3). Olefin protonation by the catalyst seems to be the first step, which is followed by the interaction with the nucleophile, in this case the alcohol. 

Despite the reversibility of these reactions, many acidic oxides catalyse the dehydration of alcohols but show no significant activity for olefin hydration. The high partial pressure of water thermodynamically necessary leads to excessive surface coverage by water, with a marked fall in effective acidity. 

Other processes include the alkylation of phenol using alkenes, and the manufacture of acrylate and methacrylate esters from alcohols and the corresponding acids. Olefin hydration reactions require more extreme conditions but Deutsche Texaco have developed a resin-catalysed propene hydration process to form isopropyl alcohol [125]. The reaction is run at 130 C near the upper limit for sulphonic acid resins, but a species with sufficient lifetime is available. There is even some evidence that butene hydration is now carried out similarly. Finally, B.P. Chemicals have recently disclosed [126] a new olefin isomerisation process yielding 2,3-dimethylbut-l-ene. Here the conditions required to favour the isomerisation versus rapid oligomerisation had to be identified to establish a viable process. 

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