Hydration of Alkenes, Acid-Catalyzed

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 mechanism of this reaction and the structure of the intermediate carbocation were once the subject of intense controversy. When the double bond of norbornene is protonated, a carbocation is produced (Eq. 10.25). The question of whether this cation is 54a and is in rapid equilibrium with the isomeric cation 54b, or whether 54a and 54b are simply two contributing resonance structures to the resonance hybrid 55, has been the focus of the debate. Note that the contributing structures 54a and 54b dijfer in the location of an cr-bond, not in the location of Tr-bonds as is more commonly encountered in resonance structures. The delocalized structure 55 is thus referred to as a nonclassical carbocation, and most chemists now accept this formulation as the more likely representation of this intermediate. It is evident from empirical results that the sterically more accessible side of the carbocation is the one away from or exo to the bridging carbon atom as shown. Nucleophilic attack of water solely from this side leads to cxo-norborneol (53) rather than endo-norborneol (56). 

Purpose To study the acid-catalyzed addition of water to an alkene. 

Norbornene is a volatile solid. Perform all weighings in a hood if possible. 

If the acidic solution used in the first part of the experiment comes in contact with your skin, immediately flood the affected area with water and thoroughly rinse it with 5% sodium bicarbonate solution. 

Another method for the hydration of alkenes is by reaction with water under conditions of acid catalysis. 

Unlike the addition of concentrated snlfnric acid to form alkyl hydrogen sulfates, this reaction is carried ont in a dilute acid medinm. A 50% water/sulfuric acid solution is often nsed, yielding the alcohol directly without the necessity of a separate hydrolysis step. Markovnikov s rnle is followed  

This mechanism cannot be correct What is its fundamental flaw  

The notion that carbocation formation is rate-determining follows from our previous experience and by observing how the reaction rate is affected by the strncture of the alkene. Table 6.2 gives some data showing that alkenes that yield relatively stable carbo-cations react faster than those that yield less stable carbocations. Protonation of ethylene, the least reactive alkene in the table, yields a primary carbocation protonation of 2-methyl-propene, the most reactive in the table, yields a tertiary carbocation. As we have seen on other occasions, the more stable the carbocation, the faster is its rate of formation. 

Step 1 Protonation of the carbon-carbon donble bond in the direction that leads to the more stable carbocation  

Mechanism 6.3 extends the general principles of electrophilic addition to acid-catalyzed hydration. In the first step of the mechanism, proton transfer to 2-methylpropene forms the tert-h xiy cation. This is followed in step 2 by reaction of the carbocation with a molecule of water acting as a nucleophile. The alkyloxonium ion formed in this step is simply the conjugate acid of tert-h xiy alcohol. Deprotonation of the alkyloxonium ion in step 3 yields the alcohol and regenerates the acid catalyst. 

Instead of the three-step process of Mechanism 6.3, the following two-step mechanism might be considered  

Analogous to the conversion of alkenes to alkyl halides by electrophilic addition of hydrogen halides across the double bond, acid-catalyzed addition of water gives alcohols. 

The notion that carbocation formation is rate-determining follows from our previous experience with reactions that involve carbocation intermediates and by observing how the reaction rate is affected by the structure of the alkene. Alkenes that yield more stable carbocations react faster than those that yield less stable ones. 

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Acid catalyzed hydration of alkenes (Section 6 10) Water adds to the double bond in accordance with Markovnikov s rule... 

Oxymercuration/demercuration provides a milder alternative for the conventional acid-catalyzed hydration of alkenes. The reaction also provides the Markovnikov regiochemistry for unsymmetrical alkenes.33 Interestingly, an enantioselective/inverse phase-transfer catalysis (IPTC) reaction for the Markovnikov hydration of double bonds by an oxymercuration-demercuration reaction with cyclodextrins as catalysts was recently reported.34 Relative to the more common phase-transfer... 

The acid-catalyzed hydration of alkenes follows Markovnikov s rule => the reaction does not yield 1° alcohols except in the special case of the hydration of ethene. 

Because rearrangements often occur, the acid-catalyzed hydration of alkenes... 

This is the reverse of acid-catalyzed hydration of alkenes discussed previously (Section 10-3E) and goes to completion if the alkene is allowed to distill out of the reaction mixture as it is formed. One mechanism of dehydration involves proton transfer from sulfuric acid to the alcohol, followed by an E2 reaction of hydrogen sulfate ion or water with the oxonium salt of the alcohol ... 

In view of the many limitations inherent in the direct acid-catalyzed hydration of alkenes, indirect hydration via hydroxymercuration-demeicuration has become a very valuable method for the preparation of alcohols. This process has recently been thoroughly reviewed.311... 

The first step in the mechanism of acid-catalyzed hydration of alkenes is protonation of the double bond to give a carbocation intermediate. 

Acid catalyzed hydration of alkenes is not well suited for laboratory preparation of alcohols. Since the reaction proceeds via carbocation intermediates, mixtures of alcohols may be formed. However, oxymercuration-demercuration of alkenes provides a simple tool for regioselective hydration of alkenes whereby rearrangements are seldom observed. 

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. 

Acid-catalyzed hydration of alkenes (Section 6.10) The elements of water add to the double bond In accordance with Markovnikov s rule. R2C = CR2 + H2O R2CHCR2 OH Alkene Water Alcohol CH3 (CH3)2C=CHCH3 CH3CCH2CH3 M2SU4 OH 2-Methyl-2-butene 2-Methyl-2-butanol (90%)... 

Addition and elimination processes are the formal reverse of one another, and in some cases the reaction can occur in either direction. For example, acid-catalyzed hydration of alkenes and dehydration of alcohols are both familiar reactions that constitute an addition-elimination pair. 

In the oxymercuration process, the electrophilic addition of the mercuric species occurs resulting in a mercurinium ion which is a three-membered ring. This is followed by the nucleophilic attack of water and as the proton leaves, an organomercuric alcohol (addition product) is formed. The next step, demercuration, occurs when sodium borohydride (NaBH ) substitutes the mercuric acetate substituent with hydrogen. If an alkene is unsymmetric, Oxymercuration-demercuration results in Markovnikov addition. The addition of mercuric species and water follows an anti (opposite side) addition pattern. This reaction has good yield, since there is no possibility of rearrangement unlike acid-catalyzed hydration of alkenes. 

Acid-Catalyzed Hydration of Alkenes 240 Industrial Organic Chemicals 263... 

As a consequence, hydroboration-oxidation gives us a method for the preparation of alcohols that cannot normally be obtained through the acid-catalyzed hydration of alkenes or by oxymercuration—demercuration. 

Acid-catalyzed hydration of alkenes takes place with Markovmkov regiochemistry but may lead to a mixture of constitutional isomers if the carbocation intermediate in the reaction undergoes rearrangement to a more stable carbocation. 

We have already studied the acid-catalyzed hydration of alkenes, oxymercutadon— demercuration, and hydroboration-oxidation as methods for the synthesis of alcohols from alkenes (see Sections 8.4, 8.5, and 8.6, respectively). Below, we briefly summarize these methods. 

Acid-Catalyzed Hydration of Alkenes Alkenes add water in the presence of an acid catalyst to yield alcohols (Section 8.5). The addition takes place with Markovnikov regioselectivity. The reaction is reversible, and the mechanism for the acid-catalyzed hydration of an alkene is simply the reverse of that for the dehydration of an alcohol (Section 7.7). 

Acid-catalyzed hydration of alkenes has limited synthetic utility, however, because the carbocation intermediate may rearrange if a more stable or isoenergetic carbocation is possible by hydride or alkanide migration. Thus, a mixture of isomeric alcohol products may result. 

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 mechanism for the acid-catalyzed hydration of alkenes is quite similar to what we have already proposed for the addition of HCl, HBr, and HI to alkenes and is illustrated by the hydration of propene to 2-propanol. This mechanism is consistent with the fact that acid is a catalyst. An is consumed in Step 1, but another is generated in Step 3. 

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