Hydration, alkenes reversibility

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... 

Addition and elimination processes are the reverse of one another in a formal sense. There is also a close mechanistic relationship between the two reactions, and in many systems reaction can occur in either direction. For example, hydration of alkenes and dehydration of alcohols are both familiar reactions that are related as an addition-elimination pair. 

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 ... 

Acid-catalysed hydration of an alkene is the reversal of the similarly acid-catalysed dehydration (by the El pathway, cf. p. 248) of alcohols to alkenes ... 

Addition of water is known as a hydration reaction. The hydration reaction occurs when alkenes are treated with aqueous acids, most commonly H2SO4, to form alcohols. This is called acid-catalysed hydration of alkenes, which is the reverse of the acid-catalysed dehydration of an alcohol. 

The reactions are reversible and the equilibrium is less favourable for the decomposition of a thiol to an alkene and H2S than of the corresponding alcohol to an alkene and H20 under the same conditions data on equilibria in the reactions of propene with water [245] and with hydrogen sulphide [246] indicate that the equilibrium constant of propene hydration is smaller than that for propene sulphidation by approximately two orders of magnitude. 

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 ... 

Furthermore, cis isomers are more easily hydrated than their trans counterparts, although exceptions are known. Thus, the hydration of cis-1,2-dicyclopropylethylene is 2.5 times faster than its trans isomer.282 Strain introduced into a ring by the incorporation of a trans double bond reverses the trend, thereby making the hydration of franr-cyclooctene more rapid than the cis compound.283 Smaller ring alkenes are also rather sluggish towards hydration.284... 

In so far as values of pATn2o for the hydration of alkenes are known or can be estimated,47 values of pATR can be derived by combining rate constants for protonation of alkenes with the reverse deprotonation reactions of the carbocations. The protonation reactions seem much less likely to be concerted with attack of water on the alkene than the corresponding substitutions. Indeed arguments have been presented that even protonation of ethylene in strongly acidic media involves the intermediacy of the ethyl carbocation.97,98... 

Hydration of an alkene also follows Markovnikov s rule. Hydration takes place when water is added to an alkene in the presence of an acid. Tlxis reaction is the reverse of dehydration of an alcohol. Low temperatures and dilute add drive this reaction toward alcohol formation high temperatures and concentrated acid drive the reaction toward alkene formation. 

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.)... 

Dehydration is reversible, and in most cases the equilibrium constant is not large. In fact, the reverse reaction (hydration) is a method for converting alkenes to alcohols (see Section 8-4). Dehydration can be forced to completion by removing the products from the reaction mixture as they form. The alkene boils at a lower temperature than the alcohol because the alcohol is hydrogen bonded. A carefully controlled distillation removes the alkene while leaving the alcohol in the reaction mixture. 

An alkene may react with water in the presence of a strongly acidic catalyst to form an alcohol. Formally, this reaction is a hydration (the addition of water), with a hydrogen atom adding to one carbon and a hydroxyl group adding to the other. Hydration of an alkene is the reverse of the dehydration of alcohols we studied in Section 7-10. 

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. 

The synthetic value of the reaction lies in the modification of these organoboranes. The commonest reaction involves the decomposition of the borane by alkaline hydrogen peroxide. The highly nucleophilic hydroperoxide anion attacks the electron-deficient boron with the formation of an ate complex. Rearrangement of this leads to the formation of a borate ester which then undergoes hydrolysis to an alcohol in which an oxygen atom has replaced the boron (Scheme 3.15). The overall outcome of this reaction is the anti-Markownikoff hydration of the double bond. The regiochemistry is the reverse of the acid-catalysed hydration of an alkene. The overall addition of water takes place in a cis manner on the less-hindered face of the double bond. 

Hydration of an alkene to form an alcohol is the reverse of the dehydration of an alcohol to form an alkene, a reaction discussed in detail in Section 9.8. 

Although alcohols can be relatively easily dehydrated to form an alkene, the reverse is not as easily done alcohols cannot be synthesised in a single simple step in an ordinary laboratory by the addition of water to an alkene. However, in industry it is very different. Most ethanol these days is manufactured by hydration of ethene using a catalyst at high temperature and pressure ... 

Problem 5.8 According to the principle of microscopic reversibility, a reaction and its reverse follow exactly the same path but in opposite directions. On this basis wfite a detailed mechanism for the hydration of alkenes, a reaction that is the exact reverse of the dehydration of alcohols. (Check your answer in Sec. 6.10.)... 

We notice that the carbonium ion combines with water to form not the alcohol but the protonated alcohol in a subsequent reaction this protonated alcohol releases a hydrogen ion to another base to form the alcohol. This sequence of reactions, we can see, is just the reverse of that proposed for the dehydration of alcohols (Sec. 5.20). In dehydration, the equilibria are shifted in favor of the alkene chiefly by the removal of the alkene from the reaction mixture by distillation in hydration, the equilibria are shifted in favor of the alcohol partly by the high concentration of water. 

A related reaction is dehydration, in which an alcohol is converted into an alkene and water by the elimination of H— and —OH from adjacent carbon atoms. The dehydration of an alcohol to form an alkene can be considered the reverse of the hydration of an alkene to form an alcohol (Section 27-17). Dehydration reactions are catalyzed by acids. 

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. 

This reaction is reversible and the mechanism for the hydration of an alkene is simply the reverse of that of the dehydration of an alcohol. 

In Section 4.5, we saw that an alkene is hydrated (adds water) in the presence of an acid catalyst, thereby forming an alcohol. The hydration of an alkene is the reverse of the acid-catalyzed dehydration of an alcohol. 

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