Propene hydration rate

With ion exchangers as catalysts for olefin hydration, special attention was paid to transport problems within the resin particles and to their effects on the reaction kinetics. In all cases, the rate was found to be of the first order with respect to the olefin. The role of water is more complicated but it is supposed that it is absorbed by the resin maintaining it in a swollen state the olefin must diffuse through the water or gel phase to a catalytic site where it may react. The quantitative interpretation depends on whether the reaction is carried out in a vapour system, liquid-vapour system or two-phase liquid system. In the vapour system [284, 285], the amount of water sorbed by the resin depends on the H20 partial pressure it was found at 125—170°C and 1.1—5.1 bar that 2-methyl-propene hydration rate is directly proportional to the amount of sorbed water... 

Acrolein (CHj=CHCHO, also known as 2-propenal) is a a,P-unsaturated aldehyde that can be transformed reducfively to saturated or unsaturated alcohols by reduction of the C = 0 or C = C double bonds (Claus 1998). In addition, a,P-unsaturated aldehydes may undergo hydration reactions in aqueous solutions. It was observed that, under acidic (pH12) conditions, acrolein is hydrated to 3-hydroxypropanal (Jensen and Hashtroudi 1976). In a natural subsurface environment, where pH may range from 6.5 to 8.5, the hydration rate of acrolein increases with the pH and its half-life decreases. Based on an experiment to analyze effects of iron on acrolein transformation, Oh et al. (2006) note that, under acidic conditions (e.g., pH = 4.4), acrolein disappears rapidly from solution in the presence of elemental iron (Fig. 16.1). Moreover, the formation of... 

CH3. OH. CH2HgI, might serve as that intermediate but that would be difficult to reconcile with relative reactivities. The rate-determining step for allylmercuric iodide cleavage could be written as shown in equation (58) if the carbonium ion were the intermediate. This is analogous to the rate-determining step for propene hydration (equation... 

Arrange the following compounds in order of increasing rate of acid-catalyzed hydration ethylene, 2-cyclopropylpropene, 2-methylpropene, propene, 1-cyclopro-pyl-l-methoxyefliene. Explain the basis of your prediction. 

The rates of hydration of alkenes increase dramatically with increasing alkyl substitution (see table at left). This is usually attributed to the relative stabilities of carbocations formed as intermediates in the initial (and rate-hmiting) step of the reaction, e.g., for hydration of propene. 

Figures 2a and 2b display the acid catalyzed E2 and El mechanisms for the dehydration of 1-propanol and 2-propanol. Note that the El mechanism involves four more rate constants (kinetic parameters) than the related E2 dehydration mechanism. Chemists employ the terminology (1) Adg3 to describe the hydration mechanism which forms 2-propanol from propene in Figure 2a, and Ad 2 to refer to the mechanism which forms 2-propanol from propene in Figure 2b. In this paper we do not distinguish between bare carbocations, Il-complexes, encumbered carbocations and symmetrically solvated carbocations, since these intermediates all manifest themselves similarly in the El kinetic model.

The unexpectedly low K value of crotonaldehyde can be easily explained on the basis of electronic effects, excepting for the observation that its rate of hydration is considerably greater than those of crotonic acid or of propene. A mechanism involving proton attack upon the ethylenic bond would require the sequence of reactivity... 

Alkenes lacking phenyl substituents appear to react by a similar mechanism. Both the observation of general acid catalysis and a solvent isotope effect are consistent with rate-limiting protonation with simple alkenes such as 2-methyl-propene and 2,3-dimethyl-2-butene. The observation of general acid catalysis rules out an alternative mechanism for alkene hydration, namely, water attack on an alkene-proton complex. The preequilibrium for formation of such a complex would be governed by the acidity of the solution, and so this mechanism would exhibit specific acid catalysis. 

The structure of the alkene affects the rate of the hydration reaction. The order of reactivity is 2-methylpropene > propene > ethene. This order reflects the effect of the stability of the carbocation intermediate. 2-Methylpropene reacts faster than propene or ethene because a 3° carbocation forms faster than a secondary or a primary carbocation. 

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