Addition of Water to Alkenes Acid-Catalyzed Hydration

5 Addition of Water to Alkenes Acid-Catalyzed Hydration 

The acid-catalyzed addition of water to the double bond of an alkene (hydration of an alkene) is a method for the preparation of low-molecular-weight alcohols. This reaction has its greatest utility in large-scale industrial processes. The acids most commonly used to catalyze the hydration of alkenes are dilute aqueous solutions of snlfnric acid and phosphoric acid. These reactions, too, are usually regioselective, and the addition of water to the double bond follows Markovnikov s rule. In general, the reaction takes the form that follows  

Because the reactions follow Markovnikov s rule, add-catalyzed hydrations of alkenes do not yield primary alcohols except in the special case of the hydration of ethene  

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 alkene donates an electron pair to a proton to form the more stable 3° carbocation. 

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ADDITION OF WATER TO ALKENES ACID-CATALYZED HYDRATION... 

An alternative method of Markovnikov addition of water to alkenes involves an intermediate rather similar to the bromonium ion. This is a mercurinium ion, formed by the reaction of an alkene with mercury (II) acetate (check out the sizes of mercury and bromine, and you will see why these are so similar). This is often preferred to acid-catalyzed hydration, as conditions... 

Addition of Water Alkenes don t react with pure water, but in the presence of a strong acid catalyst such as sulfuric acid, a hydration reaction takes place to yield an alcohol. An -H from water adds to one carbon, and an -OH adds to the other. For example, nearly 300 million gallons of ethyl alcohol (ethanol) are produced each year in the United States by the acid-catalyzed addition of water to ethylene ... 

Acid-catalyzed hydration (Section 6.9) Addition of water to the double bond of an alkene takes place in aqueous acid. Addition occurs according to Markovnikov s rule. 

Hydration of alkenes is the addition of the elements of water (H and OH) across the carbon-carbon double bond. There is substantial evidence that acid-catalyzed addition of water to an alkene involves a cationic intermediate. Rate constants for hydration increase with the electron-donating ability of the substituents on the double bond, and rate constants for hydration of im-symmetrical alkenes with the general formula RiR2C=CH2 give a good correlation with cr values. 

STRATEGY AND ANSWER We recognize that synthesis by path (a) would require a Markovnikov addition of water to the alkene. So, we could use either acid-catalyzed hydration or oxymercuration—demercuration. 

In the second step of the reaction, water acts as a nucleophile and adds to the Lewis acid (the protonated carbonyl) to generate a protonated diol (Fig. 16.25). In the final step of this acid-catalyzed reaction, the protonated diol is deprotonated by a water molecule to generate the neutral diol and regenerate the add catalyst, H30. A diol with both OH groups on the same carbon is called a em-diol (gem stands for geminal) or a hydrate. Note how closely related are the acid-catalyzed additions of water to an alkene and to a carbonyl group. Each reaction is a sequence of three steps protonation, addition of water, and deprotonation. 

Treatment of compound A with ethoxide gives alkene B, shown below, which undergoes acid-catalyzed hydration (Markovnikov addition of water) to give alcohol C ... 

Mercuric Ion-Catalyzed Hydration Alkynes undergo acid-catalyzed addition of water across the triple bond in the presence of mercuric ion as a catalyst. A mixture of mercuric sulfate in aqueous sulfuric acid is commonly used as the reagent. The hydration of alkynes is similar to the hydration of alkenes, and it also goes with Markovnikov orientation. The products are not the alcohols we might expect, however. 

The mercuric ion-catalyzed hydration of alkynes probably proceeds in a similar manner to the oxymercuration of alkenes (see Section 5.1). Electrophilic addition of Hg to the triple bond leads to a vinylic cation, which is trapped by water to give an vinylic organomercury intermediate. Unlike the alkene oxymercuration, which requires reductive removal of the mercury by NaBH4, the vinylic mercury intermediate is cleaved under the acidic reaction conditions to give the enol, which tautomerizes to the ketone. Hydration of terminal alkynes follows the Mai kovnikov rule to furnish methyl ketones. ° ... 

At the time when the Ase2 mechanism of the acid-catalyzed hydration of alkenes was finnly established , the reaction of conjugated dienes was also investigated. It was shown that the same mechanism also applietl to dienes (equation 2). The first step is generally reversible but, under well-chosen reaction conditions, the formation of an allylic carbocation by proton addition to one of the two double bonds is rate-limiting. The fast trapping of the carbocation by water in the second step affords the two allylic alcohols conesponding either to a 1,2-addition or to a 1,4-addition. Several pieces of evidence supported this mechanism. 

The reactivity of carbon-carbon double bonds toward acid-catalyzed addition of water is greatly increased by ERG substituents. The reaction of vinyl ethers with water in acidic solution is an example that has been carefully studied. With these reactants, the initial addition products are unstable hemiacetals that decompose to a ketone and alcohol. Nevertheless, the protonation step is rate determining, and the kinetic results pertain to this step. The mechanistic features are similar to those for hydration of simple alkenes. Proton transfer is rate determining, as demonstrated by general acid catalysis and solvent isotope effect data. ... 

Problem 13.13. We have described acid-catalyzed dehydration (loss of water) of an alcohol to yield an alkene. However, Sec. 12.6.1 described the opposite reaction—acid-catalyzed hydration (addition of water) of an alkene to yield an alcohol. Which is correct ... 

In the oxymercuration step, water and mercuric acetate add to the double bond in the demercuration step, sodium borohydride reduces the acetoxymercury group and replaces it with hydrogen. The net addition of H — and —OH takes place with MaJ-kovnikov t oselectivity and generally takes place without the complication of rearrangements, as sometimes occurs with acid-catalyzed hydration of alkenes. The overall alkene hydration is not stereoselective because even though the oxymercuration step occurs with anti addition, the demercuration step is not stereoselective (radicals are thought to be involved), and hence a mixture of syn and anti products results. 

The commercial production of f-butyl methyl ether has become important in recent years. In 2002, worldwide consumption of MTBE was about 7 billion gallons. With an octane value of 110, it is used as an octane number enhancer in unleaded gasolines. It is prepared by the acid-catalyzed addition of methanol to 2-methyl-propene. The reaction is related to the hydration of alkenes (Sec. 3.7.b). The only difference is that an alcohol, methanol, is used as the nucleophile instead of water. 

The acid-catalyzed addition of the elements of water across a carbon-carbon TT-bond to give an alcohol is referred to as hydration of an alkene (Eq. 10.17). Mechanistically, this process is simply the reverse of the acid-catalyzed dehydration of alcohols (Sec. 10.3). The position of the equilibrium for these two competing processes depends upon the reaction conditions. Hydration of a double bond requires the presence of excess water to drive the reaction to completion, whereas the dehydration of an alcohol requires removing water to complete the reaction. In the experiment that follows, you will examine the acid-catalyzed hydration of nor-bornene (52) to give cxo-norbomeol (53) (Eq. 10.24) as the exclusive product. 

The acid-catalyzed hydration of alkynes (Table 6.7, example 2) is commonly carried out using mercury (11) salts, such as mercuric sulfate (HgS04), as catalysts. The addition (Scheme 6.67) appears to involve a bridged mercurinium ion, which, for unsymmetrical cases such as 1-alkynes other than ethyne (acetylene [HC CH]), is subsequently attacked by water (FI2O) at the carbon that best supports a positive charge. The regiochemistry of Markownikoff addition, seen with alkenes, is followed. 

The previous section explored how acid-catalyzed hydration can be used to achieve a Markovnikov addition of water across an alkene. The utility of that process is somewhat diminished by the fact that carbocation rearrangements can produce a mixture of products ... 

In cases where protonation of the alkene ultimately leads to carbocation rearrangements, acid-catalyzed hydration is an inefficient method for adding water across the alkene. Many other methods can achieve a Markovnikov addition of water across an alkene without carbocation rearrangements. One of the oldest and perhaps best known methods is called oxymercuration-demercuration ... 

Mechanism 6.3 extends the general principles of electrophilic addition to acid-catalyzed hydration. In the first step of the mechanism, proton transfer converts the alkene to a carbocation, which then reacts with a molecule of water in step 2. The alkyloxonium ion formed in this step is the conjugate acid of the ultimate alcohol and yields it in step 3 while regenerating the acid catalyst. 

The mechanism of the mercurydD-catalyzed alkyne hydration reactioi is analogous to the oxymercuration reaction of alkenes (Section 7.4). Elec trophilic addition of mercury(II) ion to the alkyne gives a vinylic cation which reacts with water and loses a proton to yield a mercury-containii enol intermediate. In contrast to alkene oxymercuration, no treatment widi NaBH4 is necessary to remove the mercury the acidic reaction conditions alone are sufficient to effect replacement of mercury by hydrogen (Figure 8.3). 

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