Alkenes hydration reactions

A number of useful reactions for the preparation of aldehydes and ketones, such as ozonization of alkenes and hydration of alkynes, have been considered in previous chapters. These and other methods of preparation are summarized in Tables 16-7 and 16-8 at the end of the chapter. Only a few rather general methods that we have not discussed will be taken up here. 

The rate of hydration is dependent on the substitution and hence the nucleophilicity of the C—C double bond.280 In general, trisubstituted alkenes are hydrated faster than terminal alkenes. Thus, the reaction is most commonly applied to the preparation of tertiary alcohols (equation 191).281... 

We ve already looked at several alcohol synthesis reactions hydration of an alkene, oxymeixruration/demeicu ration, hydroboration, and nucleophilic substitution. 

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. 

The reactions whereby alkenes are hydrated or alcohols are dehydrated are reactions in which the ultimate product is governed by the position of an equilibrium. Therefore, in the dehydration of an alcohol it is best to use a concentrated acid so that the concentration of water is low. (The water can be removed as it is formed, and it helps to use a high temperature.) In the hydration of an alkene it is best to use dilute acid so that the concentration of water is high. It also usually helps to use a lower temperature. 

Another important addition reaction of alkenes is hydration, the addition of a water molecule across the double bonding, yielding an alcohol. Here s the hydration of ethylene that gives ethyl alcohol (notice that 1 show the water molecule in a slightly different way so you can tell where the -OH ends up) ... 

The rates of bromination of dialkylacetylenes are roughly 100 times greater than for the corresponding monosubstituted alkenes. For hydration, however, the rates of reaction are less than 10 times greater for disubstituted derivatives. Account for this observation by comparison of the mechanisms for bromination and hydration. 

For reactions in H2O an analogous discussion applies as for reactions with the protonated sulfuric acid form. The reactive intermediate now, however, is H3O+. H3O+ has a much stronger OH bond than H3SO4 +, and hence its reactivity is much less. The species that is formed by reaction with alkene in an aqueous solution can best be compared with protonated alcohols forms typically known as alkyl oxonimn ionsl l Protonated alkene is hydrated in aqueous media. The formation of alkyloxonimn ions instead of the carbenium ions in H2O can be viewed as due to the basicity of water. The major difference between solid acids and acidic solutions arises because the hydrogen atoms in solid acids are part of strong covalent bonds and are not present as protons that are present in in the solution phase. In a solution there is a equilibrium between non-dissociated acid molecules and the... 

The net reaction from hydroboration and subsequent oxidation of an alkene is hydration of a carbon-carbon double bond. Because hydrogen is added to the more substituted carbon of the double bond and —OH to the less substituted carbon, we refer to... 

We have previously discussed the hydration of alkenes as a method for the synthesis of alcohols (Section 6.5). The reaction is easily reversible, and poor yields of product may result. Furthermore, even if the reaction is pushed by the selection of optimal reaction conditions, there is a competing reaction. Hydration is an electrophihc addition reaction that occurs by way of a carboca-tion, and can yield rearranged products. Therefore, direct hydration is not the preferred method of synthesis of most alcohols. In Section 15.8, we will examine alternate addition reactions that have no competing rearrangement reactions, and give good yields of alcohols. 

Pyridones carrying an N-alkenyl substituent such as 94 photodimerize (Figure 103.1) and photoi-somerize (Figure 103.5), but in water the alkene undergoes hydration to give 149. Performance of this reaction in the presence of perchloric acid leads to an isolable intermediate 148. When the alkene is a styrene, (E),(Z)-isomerization and stilbene-hke cyclization reactions are also observed. ... 

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

The mechanistic complexity of hydroboration-oxidation stands m contrast to the simplicity with which these reactions are carried out experimentally Both the hydrobo ration and oxidation steps are extremely rapid reactions and are performed at room tern perature with conventional laboratory equipment Ease of operation along with the fact that hydroboration-oxidation leads to syn hydration of alkenes and occurs with a regio selectivity opposite to Markovmkov s rule makes this procedure one of great value to the synthetic chemist... 

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... 

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. 

Strong acids also catalyze the addition of alcohols to alkenes to give ethers, and the mechanistic studies which have been done indicate that the reaction closely parallels the hydration process. ... 

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 rates of hydration of alkenes increase dramatically with increasing alkyl substitution (see table at left). This is usually attributed to the relative stabilities of carbocations formed as intermediates in the initial (and rate-hmiting) step of the reaction, e.g., for hydration of propene. 

Water adds to alkenes to yield alcohols, a process called hydration. The reaction takes place on treatment of the alkene with water and a strong acid catalyst (HA) by a mechanism similar to that of HX addition. Thus, protonation of an alkene double bond yields a carbocation intermediate, which reacts with water to yield a protonated alcohol product (ROH2+). Loss of H+ from this protonated alcohol gives the neutral alcohol and regenerates the acid catalyst (Figure 7.2). 

Acid-catalyzed alkene hydration is particularly suited to large-scale industrial procedures, and approximately 300,000 tons of ethanol are manufactured each year in the United States by hydration of ethylene. The reaction is of little value in the typical laboratory, however, because it requires high temperatures— 250 °C in the case of ethylene—and strongly acidic conditions. 

Reaction of 2-methylpropene with CH3OH in the presence of H2SO4 catalyst yields methyl tert-butyl ether, CP OQCHT, by a mechanism analogous to that of acid-catalyzed alkene hydration. Write the mechanism, using curved arrows for each step. 

Strategy What is an immediate precursor of a primary alcohol " Perhaps a terminal alkene, which could be hydrated with non-Markovnikov regiochemistiy by reaction with borane Followed by oxidation with H2O2-... 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Alcohols can be prepared by hydration of alkenes. Because the direct hydration of alkenes with aqueous acid is generally a poor reaction in the laboratory, two indirect methods are commonly used. Hydroboration/oxiclation yields the product of syn, non-Markovnikov hydration (Section 7.5), whereas... 

Chirality center, 292 detection of, 292-293 Eischer projections and, 975-978 R,S configuration of, 297-300 Chitin, structure of, 1002 Chloral hydrate, structure of, 707 Chloramphenicol, structure of, 304 Chlorine, reaction with alkanes, 91-92,335-338 reaction with alkenes, 215-218 reaction with alkynes, 262-263 reaction with aromatic compounds, 550 Chloro group, directing effect of, 567-568... 

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