Mechanism alkene hydration

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. 

Excess acidity correlations have been used to show that some aromatic sulfonic acid desulfonations have an A-SE2 mechanism.188,189 This mechanism (alternative terminologies are Ad-E2 and A(E) +A(N))190 has also been found to apply to the hydration of acetylene itself,191 to ynamines192 and to many other alkynes,193-195 as well as to many different alkenes196-199 and vinyl ethers.200-203 The excess acidity method has been used to evaluate aA values for several alkene hydrations.204 205... 

The mechanisms of these reactions have much in common and have been studied extensively from this point of view. They also have very considerable synthetic utility. The addition of water to alkenes (hydration) is particularly important for the preparation of a number of commercially important alcohols. Thus ethanol and 2-propanol (isopropyl alcohol) are made on a very large scale by the hydration of the corresponding alkenes (ethene and propene) using sulfuric or phosphoric acids as catalysts. The nature of this type of reaction will be described later. 

As was already mentioned, the standard procedure for acid catalyzed alkene hydration exhibits a rather low selectivity. On the other hand, the use of a hydroxymercuration-reduction sequence leads to the exclusive formation of Markovnikov s alcohols. A nearly exclusive anti-Markovnikov s hydration is achieved via a hydroboration-oxidation reaction (see Section 2.4). The result in both these cases is the net addition of H2O, but the basic differences in the reaction mechanisms unambiguously determine a reversed regioselectivity pattern. 

By applying Step 2 of the mechanism for hydration, we can understand how HCl and HBr react with alkenes ... 

General acid catalysis was observed in the hydration of both trans-cyclooctene and 2,3-dimethyl-2-butene, which helped to establish the Ase2 mechanism for hydration of alkenes. Furthermore, 1,1-dicyclopropyl-ethene was foimd to react much faster than either cis- or trans-l,2-dicyclo-propylethene, which indicates that substituent location (and not just the total electron-donating ability of the substituents) affects the rate constant for hydration reactions. It appears that the approaching proton is undergoing bond formation to one of the olefinic carbon atoms in the transition structure but not to the other, so the regiochemistry of the hydration reaction is determined by the approach that leads to the development of the more stable carbocation. ... 

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... 

Mechanism 10.2 Electrophilic Addition of H2O to an Alkene—Hydration Step [1] Addition of the electrophile (H ) to the n bond... 

When the following compound is hydrated in the presence of acid, the unreacted alkene is found to have retained the deuterium atoms. What does this tell you about the mechanism for hydration ... 

In the mechanism of alkene hydration, all the steps are reversible. The proton acts only as a catalyst and is not consumed in the overall reaction. Indeed, without the add, hydration would not occur alkenes are stable in neutral water. The presence of acid, however, establishes an equilibrium between alkene and alcohol. This equilibrium can be driven toward the alcohol by using low reaction temperatures and a large excess of water. Conversely, we have seen (Section 11-7) that treating the alcohol with concentrated acid favors dehydration, especially at elevated temperatures. 

Explain why the following mechanism for hydration of an alkene cannot occur... 

In the space below, draw out a possible mechanism for alkene hydration shown in Model 6. Hints for getting started can be found in the Check Your Work section below. 

This elimination reaction is the reverse of acid-catalyzed hydration, which was discussed in Section 6.2. Because a carbocation or closely related species is the intermediate, the elimination step would be expected to favor the more substituted alkene as discussed on p. 384. The El mechanism also explains the general trends in relative reactivity. Tertiary alcohols are the most reactive, and reactivity decreases going to secondary and primary alcohols. Also in accord with the El mechanism is the fact that rearranged products are found in cases where a carbocation intermediate would be expected to rearrange ... 

Water adds to alkenes to yield alcohols, a process called hydration. The reaction takes place on treatment of the alkene with water and a strong acid catalyst (HA) by a mechanism similar to that of HX addition. Thus, protonation of an alkene double bond yields a carbocation intermediate, which reacts with water to yield a protonated alcohol product (ROH2+). Loss of H+ from this protonated alcohol gives the neutral alcohol and regenerates the acid catalyst (Figure 7.2). 

Mechanism of the acid-catalyzed hydration of an alkene to yield an alcohol. Protonation of the alkene gives a carbocation intermediate that reacts with water. 

In each mechanism above, the first step involves protonation of the alkene to form a carbocation. Then, in both cases, a nucleophile (either X or H2O) attacks the car-bocation to give a product. The difference between these two reactions is in the nature of the product. The first reaction above (hydrohalogenation) gives a product that is neutral (no charge). However, the second reaction above (hydration) produced a charged species. Therefore, one more step is necessary at the end of the hydration reaction— we must get rid of the positive charge. To do this, we simply deprotonate ... 

Addition of a proton occurs to give the more-substituted carbocation, so addition is regioselective and in accord with Markovnikov s rule. A more detailed discussion of the reaction mechanism is given in Section 6.2 of Part A. Owing to the strongly acidic and rather vigorous conditions required to effect hydration of most alkenes, these conditions are applicable only to molecules that have no acid-sensitive functional groups. The reaction is occasionally applied to the synthesis of tertiary alcohols. 

The mechanism for the acid-catalysed hydration reaction is very similar to that for the hydrohalogenation of alkenes and also proceeds via a carbocatlon intermediate. It is outlined below using water and propene. 

The general reaction for the catalytic hydration of an alkene to produce an alcohol is shown in Figure 3-6, and the mechanism is in Figure 3 7. This process is an excimple of a Markovnikov addition (as seen in Organic Chemistry 1). 

Problem 6.26 (a) What principle is used to relate the mechanisms for dehydration of alcohols and hydration of alkenes (b) What conditions favor dehydration rather than hydration reactions ... 

Addition of concentrated H2SO4 to alkenes yields acid-soluble alkyl hydrogen sulphates. The addition follows Markovnikov s rule. The sulphate is hydrolysed to obtain the alcohol. The net result is Markovnikov addition of acid-catalysed hydration to an alkene. The reaction mechanism of H2SO4 addition is similar to that of acid-catalysed hydration. 

Water can be added indirectly, with anti-Markovnikov orientation, by treatment of the alkene with a 1 1 mixture of PhCH2NEt3+ BH4 and Me3SiCl, followed by addition of an aqueous solution of K2C03.152 For another method of anti-Markovnikov hydration, see 5-12. With substrates of the type C=C—Z (Z is as defined on p. 741) the product is almost always HO—C—CH—Z and the mechanism is usually nucleophilic,153 though electrophilic... 

---