2-Methylpropene hydration mechanism

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

FIGURE 6 9 Mechanism of acid catalyzed hydration of 2 methylpropene... 

IS reversible with respect to reactants and products so each tiny increment of progress along the reaction coordinate is reversible Once we know the mechanism for the for ward phase of a particular reaction we also know what the intermediates and transition states must be for the reverse In particular the three step mechanism for the acid catalyzed hydration of 2 methylpropene m Figure 6 9 is the reverse of that for the acid catalyzed dehydration of tert butyl alcohol m Figure 5 6... 

Reaction of 2-methylpropene with CH3OH in the presence of H2SO4 catalyst yields methyl tert-butyl ether, CP OQCHT, by a mechanism analogous to that of acid-catalyzed alkene hydration. Write the mechanism, using curved arrows for each step. 

Only in one case (the hydration of 2-methylpropene over a H2S04—Si02 catalyst [278]) was the so-called Rideal mechanism proposed as a preferable model and expressed by the rate equation for single-site adsorption with retardation by the product alcohol, viz. 

Alkenes lacking the phenyl group are somewhat less convenient to study by kinetic methods, but such data as observation of general acid catalysis and solvent isotope effects are also consistent with rate-limiting protonation in simple alkenes such as 2-methylpropene 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 would be governed by the... 

The mechanism for the hydration of an alkene is simply the reverse of the mechanism for the dehydration of an alcohol. We can illustrate this by giving the mechanism for the hydration of 2-methylpropene and by comparing it with the mechanism for the dehydration of 2-methyl-2-propanol given in Section 7.7A. 

The mechanism shown in Figure 9.34 predicts, indeed requires, that the Markovnikov rule be followed, and that sets up a mechanistic test. We will examine the hydration of 2-methylpropene to probe the regiochemistry of the reaction. Before we even start, it is worth thinking a bit about the results. What will we know after we have done the experiment There are only two general results possible Either the Markovnikov rule is followed or it is not (Fig. 9.35). If it is not, we can say with certainty that the mechanism we have suggested for this reaction is wrong. 

This addition process is the reverse of the acid-induced elimination of water from alcohols (dehydration, Section 11-7). Its mechanism is the same in reverse, as illustrated in the hydration of 2-methylpropene, a reaction of industrial importance leading to 2-methyl-2-propanol (tert-butyl alcohol). 

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