Alcohols acid-catalyzed hydration

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

Three of the six reactions are well suited for the synthesis of alcohols acid-catalyzed hydration, oxymercuration (after reduction of the C-Hg bond by NaBHi), and hydroboration (by oxidation of the C-B bond by H2O2). Compare the alcohol from hydration (shown earlier) with that from demer-curation of the oxymercuration product ... 

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... 

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... 

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... 

Acid catalyzed hydration converts alkenes to alcohols with regioselectivity according to Markovnikov s rule Frequently however one needs an alcohol having a structure that corresponds to hydration of an alkene with a regioselectivity opposite to that of Markovnikov s rule The conversion of 1 decene to 1 decanol is an example of such a transformation... 

Oxidation of tridecylborane gives 1 decanol The net result is the conversion of an alkene to an alcohol with a regioselectivity opposite to that of acid catalyzed hydration... 

Although 2 methylpropene undergoes acid catalyzed hydration m dilute sulfuric acid to form tert butyl alcohol (Section 6 10) a different reaction occurs m more concentrated solutions of sulfuric acid Rather than form the expected alkyl hydrogen sulfate (see Sec tion 6 9) 2 methylpropene is converted to a mixture of two isomeric C Hig alkenes... 

Epoxidation of an alkene followed by lithium aluminum hydride reduction of the result mg epoxide gives the same alcohol that would be obtained by acid catalyzed hydration (Section 610) of the alkene... 

Synthetic pine oil is produced by the acid-catalyzed hydration of a-pinene (Fig. 1). Mineral acids, usually phosphoric acid, are used in concentrations of 20—40 wt % and at temperatures varying from 30—100°C. Depending on the conditions used, alcohols, chiefly a-terpineol (9), are produced along with /)-menthadienes and cineoles, mainly limonene, terpinolene, and 1,4- and 1,8-cineole (46—48). Various grades of pine oil can be produced by fractionation of the cmde products. Formation of terpin hydrate (10) from a-terpineol gives P-terpineol (11) and y-terpineol (12) as a consequence of the reversible... 

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 ... 

The acid-catalyzed hydration of isobutylene produces ter-butyl alcohol. The reaction occurs in the liquid phase in the presence of 50-65% H2SO4 at mild temperatures (10-30°C). The yield is approximately 95% ... 

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. 

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. 

The products from the acid-catalyzed hydration of a-tertiary alcohols 30 (Meyer-Schuster and Rupe rearrangements) are formed via the mesomeric propargyl-allenyl cation (equation 9) and have been extensively investigated28. 

The first hydration step was promoted by Bronsted acids containing weakly or noncoordinating anions. In the second step, an intramolecular hydrogen transfer in the secondary alcohol was catalyzed by ruthenium(III) salts with chelating bipyridyl-type ligands. The possible complexation of the latter with the diene did not inhibit its catalytic activity in the allylic rearrangements, under acid-catalyzed hydration conditions. 

Terpineols are unsaturated monocyclic terpene alcohols and are formed by acid-catalyzed hydration of pinenes a-, (3-, 7- and 6-isomers exist ... 

Synthetic pine oil is produced by the acid-catalyzed hydration of mainly a-pinene derived from sulfate turpentine, followed by distillation of the crude mixture of hydrocarbons and alcohols. The predominant alcohol obtained is a-terpineol, although under the usual conditions of the reaction, reversible and dehydration reactions lead to multiple hydrocarbon and alcohol components (Fig. 1). 

The use of silica-supported Zn(BH4)2 is a useful procedure for the hydration of unactivated alkenes and alkynes.559 The main products are usually formed as a result of anti-Markovnikov addition. In contrast to acid-catalyzed hydration (see Section 6.1.2), this procedure allows the transformation of alkynes to alcohols. 

The other significant industrial route to /-butyl alcohol is the acid catalyzed hydration of isobutylene (24), a process no longer practiced in the United States. Raffinate I, C-4 refinery streams containing isobutylene [115-11-7], -butenes and saturated C-4s or C-4 fluid catalytic cracker (FCC) feedstocks (23)... 

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... 

Terpin hydrate [2451-01-6] (10), one of the most well-known expectorants, is isolated from cmde pine rosin left after the distillation of volatile terpene hydrocarbons and alcohols. It is also manufactured from turpentine (a-pinene) by acid-catalyzed hydration. Terpin hydrate may exist as cis and trans isomers, but only the cis isomer forms a stable, crystalline monohydrate. Terpin hydrate is available in the United States only in prescription products. 

The boron adds to the less substituted carbon of the double bond, and, in the oxidation, the boron is replaced by an OH group. Note that the acid-catalyzed hydration of the same alkene would give the alcohol (CH3)2C(OH)CH2CH2CH2CH3 instead, according to Markovnikov s rule. 

For these reasons, acid-catalyzed hydration is often not the method of choice for preparing alcohols from alkenes. Another reaction that accomplishes this same transformation, often in higher yield, is described in Section 11.6. 

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.)... 

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