Acid-Catalyzed Hydration of an Alkene

Acid-Catalyzed Hydration of an Alkene Step 1 Protonation of the double bond forms a carbocation. 

Step 2 Nucleophilic attack by water gives a protonated alcohol. 

Step 1 of the hydration mechanism is similar to the first step in the addition of HBr. The proton adds to the less substituted end of the double bond to form the more substituted carbocation. Water attacks the carbocation to give (after loss of a proton) the alcohol with the —OH group on the more substituted carbon. Like the addition of hydrogen halides, hydration is regioselective It follows Markovnikov s rule, giving a product in which the new hydrogen has added to the less substituted end of the double bond. Consider the hydration of 2-methyl-2-butene  

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You may have noticed that the acid catalyzed hydration of an alkene and the acid catalyzed dehydration of an alcohol are the reverse of each other... 

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. 

Fig. 2 Free energy reaction coordinate profiles for the stepwise acid-catalyzed hydration of an alkene through a carbocation intermediate (Scheme 5). (a) Reaction profile for the case where alkene protonation is rate determining (ks kp). This profile shows a change in rate-determining step as a result of Bronsted catalysis of protonation of the alkene. (b) Reaction profile for the case where addition of solvent to the carbocation is rate determining (ks fcp). This profile shows a change in rate-determining step as a result of trapping of the carbocation by an added nucleophilic reagent.

The mechanism of the formation of the tetrahydropyranyl ether (see Figure 23.1) is an acid-catalyzed addition of the alcohol to the double bond of the dihydropyran and is quite similar to the acid-catalyzed hydration of an alkene described in Section 11.3. Dihydropyran is especially reactive toward such an addition because the oxygen helps stabilize the carbocation that is initially produced in the reaction. The tetrahydropyranyl ether is inert toward bases and nucleophiles and serves to protect the alcohol from reagents with these properties. Although normal ethers are difficult to cleave, a tetrahydropyranyl ether is actually an acetal, and as such, it is readily cleaved under acidic conditions. (The mechanism for this cleavage is the reverse of that for acetal formation, shown in Figure 18.5 on page 776.)... 

Mechanism 8-4 Acid-Catalyzed Hydration of an Alkene 338 8-5 Hydration by Oxymercuration-Demercuration 340 Mechanism 8-5 Oxymercuration of an Alkene 340 8-6 Alkoxymercuration-Demercuration 342 8-7 Hydroboration of Alkenes 343... 

CHAPTER 8 Ionic Addition of HX to an Alkene 332 Free-Radical Addition of HBr to Alkenes 334 Acid-Catalyzed Hydration of an Alkene 338 Oxymercuration of an Alkene 340 Hydroboration of an Alkene 345 Addition of Halogens to Alkenes 350 Formation of Halohydrins 352 Epoxidation of Alkenes 360 Acid-Catalyzed Opening of Epoxides 362 Olefin Metathesis 376... 

For synthesis of an alcohol, acid-catalyzed hydration of an alkene is useful in all of the following instances except ... 

Hydration of an internal alkyne with strong acid forms an enol by a mechanism similar to that of the acid-catalyzed hydration of an alkene (Section 10.12). Mechanism 11.4 illustrates the hydration of 2-butyne with H2O and H2SO4. Once formed, the enol then tautomerizes to the more stable keto form by protonation followed by deprotonation. 

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). 

Propose a mechanism similar to that proposed for the acid-catalyzed hydration of an alkene involving proton transfer from the acid catalyst to form a carbocation intermediate, rearrangement of the carbocation intermediate to a more stable intermediate, reaction of the more stable carbocation with water to form an oxonium ion, and finally proton transfer from the oxonium ion to water to give the product and regenerate the acid catalyst. Lest you be tempted to use to initiate the reaction, remember that ionization of a strong acid in water generates a hydronium ion and an anion. Hydronium ion and not H is the true catalyst in this reaction. 

Acid-catalyzed hydration of an alkene is the addition of OH and H across the carbon-carbon double bond. The reaction occurs with Markovnikov regioselectivity. 

According to the mechanism given in the text for acid-catalyzed hydration of an alkene, the —H and —OH groups added to the double bond both arise from the same molecule of H2O. (5.3)... 

In the second step of the cycle, citrate is converted to the constitutional isomer isocitrate. This isomerization occurs in two steps, both catalyzed by aconitase. First, in a reaction analogous to the acid-catalyzed dehydration of an alcohol (Section 8.2E), citrate undergoes enzyme-catalyzed dehydration of aconitate. Then, in a reaction analogous to acid-catalyzed hydration of an alkene (Section 5.3B), aconitate undergoes enzyme-catalyzed hydration to give isocitrate. 

Zaitsev s rule is followed that is, the predominant product is the most stable (usually the most highly substituted) alkene. The mechanism is the reverse of acid-catalyzed hydration of an alkene. 

Ethers can be prepared through the acid-catalyzed addition of methyl or primary Problems 11.3,11.4,11.15, alcohols to alkenes that can form a stable carbocation upon protonation, via a 11 -16,11.41,11.42,11.43 mechanism analogous to acid-catalyzed hydration of an alkene. 

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