Hydration of alkene

Markovnikov hydration, brought about by aqueous acid, involves a carbocation intermediate. The product has hydrogen attached to the less alkylated carbon. 

A key point is that in the first step the proton is bonded to the less alkylated carbon. This is because the carbocation 45 is tertiary and stable. The alternative possibility is to produce a primary carbocation Me2CHCH2+ that is significantly less stable. The order of relative stabilities of carbocations is tertiary secondary primary, and it is this relative sequence that dictates the site of proton attachment. Accordingly, one can state Markovnikov s rule, proposed in 1869 in the addition of H-X to a double bond, H becomes bonded to the carbon with fewer alkyl groups and X becomes attached to the more alkylated carbon. 

Another possibility is addition of the H and OH of water to an alkene so that H bonds to the carbon with more alkyl groups, and OH to the 

Anti-Markovnikov hydration is achieved by means of hydroboration with BH3. Hydrogen adds to the more alkylated carbon, and BH2 to the less alkylated, from the less hindered side in a reaction that proceeds via a four-centred transition state. 

All three hydrogens of BHa are involved in the addition, with resultant formation of a trialkylborane. In conversion of the resultant organoborane to alcohol (by OH, H2Q2). oxygen replaces boron with retention of configuration at carbon. 

There are a number of biological examples of halohydrin formation, particularly in marine organisms. As with halogenation (Section 8.2), halohydrin formation is carried out by haloperoxidases, which function by oxidizing Br or Cl ions to the corresponding HOBr or HOCl bonded to a metal atom in the enzyme. Electrophilic addition to the double bond of a substrate molecule then yields a bromonium or chloronium ion intermediate, and reaction with water gives the halohydrin. For example  

When an unsymmetrically substituted alkene such as propene is treated with Br2 and water, the major product has the bromine atom bonded to the less highly substituted carbon atom. Is this Markovnikov or non-Markovnikov orientation Explain. 

We saw in Section 7.6 that alkenes undergo an acid-catalyzed addition reaction with water to yield alcohols. The process 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. Unfortunately, the reaction is not of much use in the laboratory because of the high temperatures needed—250 °C in the case of ethylene. 

Acid-catalyzed hydration of isolated double bonds, although known, is also uncommon in biological pathways. More frequently, biological hydrations require that the double bond be adjacent to a carbonyl group for reaction to proceed. Fumarate, for instance, is hydrated to give malate as one step in the citric acid cycle of food metabolism. Note that the requirement for an adjacent carbonyl group in the addition of water is the same as that we saw in Section 8.1 for the elimination of water. WeTl see the reason for the requirement in Section 14.11, but might note for now that the reaction is not an 

The general theme of this text is to focus on reactions that have a direct relevance to the chemistry of living organisms. Every so often, though, we ll discuss a particularly useful laboratory reaction that has no biological counterpart. In the present case, alkenes are often hydrated in the laboratory by two nonbiological procedures, oxymercuration and hydroboration/oxidation, which give complementary results. 

Hydration of alkenes is the addition of the elements of water (H and OH) across the carbon-carbon double bond. There is substantial evidence that acid-catalyzed addition of water to an alkene involves a cationic intermediate. Rate constants for hydration increase with the electron-donating ability of the substituents on the double bond, and rate constants for hydration of im-symmetrical alkenes with the general formula RiR2C=CH2 give a good correlation with cr values. 

Acid-catalyzed hydration reactions occur with Markovnikov orientation. For example, hydration of 2-methyl-2-butene gives 2-methyl-2-butanol, consistent with the intermediacy of the 3° 2-methyl-2-butyl carbocation. Moreover, hydration of 2-methyl-l-butene was also found to produce 2-methyl-2-butanol, and there was no indication of isomerization to 2-methyl-2-butene during hydration. These results suggest, but do not confirm, that the cationic intermediate undergoes nucleophilic attack by water faster than it loses a proton to revert to starting material.  

With a generalized unsymmetrical alkene, RCH=CH2, the products from these reagents are RCHCICH2CCI3, RCHBrCHsCBrj, RCHjCHjCQj, and RCHBrCH2CCl3, respectively. Kharasch, M. S. Jensen, E. V. Urry, W. H. /. Am. Chem. Soc. 1947,69,1100. 

With a generalized unsymmetrical alkene, RCH=CH2, the product from this reagent is RCHICH2CF2I. Elsheimer, S. Dolbier, W. R., Jr. Murla, M. Seppelt, K. Paprott, G. /. Org. Chem. 1984,49, 205. 

At the beginning of this chapter, we noticed that water can add across the double bond of an alkene to give an alcohol. This is a hydration reaction. The reverse reaction is dehydration. 

Water adds to a 7t bond in an acidic medium such as aqueous sulfuric acid. A proton is transferred from to the 7t bond to give a carbocation, which then reacts with the nucleophilic oxygen atom of water to give an oxonium ion. 

The tert-butyl oxonium ion, which forms in the second step, is the conjugate acid of t t-butyl alcohol. The r rt-butyl oxonium ion transfers a proton to water, regenerating the hydronium ion, the catalyst for the reaction. 

The hydration reaction is regiospecific. It obeys Markovnikov s rule. r rr-Butyl alcohol forms rather than the isobutyl alcohol that would result from adding a proton to C-2 followed by reaction of water with a carbocation at C-1. Such a process would proceed via a much less stable primary carbocation (the isobutyl cation). 

The structure of the alkene affects the rate of the hydration reaction. The order of reactivity is 2-methylpropene propene ethene. This order reflects the effect of the stability of the carbocation intermediate. 2-Methylpropene reacts faster than propene or ethene because a 3° carbocation forms faster than a secondary or a primary carbocation. 

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Hydration of alkenes by this method however is limited to monosubstituted alkenes and disubstituted alkenes of the type RCH=CHR Disubstituted alkenes of the type R2C=CH2 along with trisubstituted and tetrasubstituted alkenes do not form alkyl hydrogen sulfates under these conditions but instead react m a more complicated way with concentrated sulfuric acid (to be discussed m Section 6 21)... 

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... 

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

By analogy to the hydration of alkenes hydration of an alkyne is expected to yield an alcohol The kind of alcohol however would be of a special kind one m which the hydroxyl group is a substituent on a carbon-carbon double bond This type of alcohol IS called an enol (the double bond suffix ene plus the alcohol suffix ol) An important property of enols is their rapid isomerization to aldehydes or ketones under the condi tions of their formation... 

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

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. 

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. 

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... 

Oxygen nucleophiles can be added to double bonds under strongly acidic conditions. A fundamental example is the hydration of alkenes in acidic aqueous solution. 

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 most common cation is H , adding to unsaturated linkages, i.e. protonation, in for example the acid-catalysed hydration of alkenes (p. 187) ... 

Anri-Markownikov hydration of alkenes may be effected indirectly by addition of B2H6 (hydroboratiori), followed by oxidation of the... 

The acids most commonly used to catalyze the hydration of alkenes are dilute solutions of sulfuric acid and phosphoric acid. 

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 ultimate products for the hydration of alkenes or dehydration of alcohols are governed by the position of an equilibrium. 

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

Values of Kadd for the addition of water (hydration) of alkenes to give the corresponding alcohols. These equilibrium constants were obtained directly by determining the relative concentrations of the alcohol and alkene at chemical equilibrium. The acidity constants pATaik for deprotonation of the carbocations by solvent are not reported in Table 1. However, these may be calculated from data in Table 1 using the relationship pA ik = pATR + logA dd (Scheme 7). 

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