Water acid-catalyzed addition

The formation of alcohols by acid-catalyzed addition of water to alkenes is a fundamental organic reaction. At the most rudimentary mechanistic level, it can be viewed as involving a carbocation intermediate. The alkene is protonated, and the carbocation is then captured by water. 

A Alkenes can be converted into alcohols by acid-catalyzed addition of water. Assuming that Vlarkovnikov s rule is valid, predict the major alcohol product from each of the following alkenes. 

Triple bonds can be monohydroborated to give vinylic boranes, which can be reduced with carboxylic acids to cis alkenes or oxidized and hydrolyzed to aldehydes or ketones. Terminal alkynes give aldehydes by this method, in contrast to the mercuric or acid-catalyzed addition of water discussed at 15-4. However, terminal alkynes give vinylic boranes (and hence aldehydes) only when treated with a hindered borane such as 47, 48, or catecholborane (p. 798)," or with BHBr2—SMe2. The reaction between terminal alkynes and BH3 produces 1,1-... 

Formamides can be prepared by the acid-catalyzed addition of water to isocyanides. The mechanism is probably ... 

The acid-catalyzed addition of water to the double bond of an alkene is a method for the preparation of low molecular weight alcohols that has its greatest utility in large-scale industrial processes. 

In aqueous solutions, aldehydes [RHC=0] undergo general acid-catalyzed addition of water to yield the hydrate [RHC(0H)2], and the equilibrium position lies in favor of the hydrate. Jencks summarized the most likely mechanism for the hydration reaction. 

Acid-catalyzed addition of water and alcohols to 4/f-chromenes gives the expected products as predicted by Markovnikov s rule (56JCS4785) an anti-Markovnikov addition of methanol followed by the reintroduction of a double bond in the alternative position gives an overall effect of substitution of hydrogen by methoxy and this is effected by treating methyl 2if-chromene-3-carboxylate (166) with triphenylmethyl perchlorate and addition of methanol to the resulting benzopyrylium salt (167) (72CR(C)(274)650). 

The acid-catalyzed addition of water to a double bond is kinetically second-order, first-order each in olefin and in H30 +. 2 This fact is equally consistent with the concerted addition of a proton and a water molecule from the same H30+ and with initial attack of a proton to one side of the double bond followed by... 

The 2-butanol feedstock is conventionally obtained by the sulfuric acid-catalyzed addition of water to -butenes. This is a two-step reaction involving sulfation and hydrolysis in which the conversion of -butenes is 90% and selectivity to 2-butanol is 95% (15). During operation the sulfuric acid becomes diluted and must be reconcentrated before reuse. In 1983 Deutsche Texaco commercialized a single-step route in which 2-butanol is formed by the hydration of -butenes in the presence of a strongly acidic ion-exchange resin containing sulfonic acid groups (16—18). The direct reaction is carried out at 150—160°C and 7 MPa. Virtually anhydrous 2-butanol is recovered in this process (19). Direct hydration requires lower utilities and investment costs, operates at 99% selectivity to 2-butanol, but is hindered by low (5—15%) -butene conversion per pass. 

Addition of Water Alkenes don t react with pure water, but in the presence of a strong acid catalyst such as sulfuric acid, a hydration reaction takes place to yield an alcohol. An -H from water adds to one carbon, and an -OH adds to the other. For example, nearly 300 million gallons of ethyl alcohol (ethanol) are produced each year in the United States by the acid-catalyzed addition of water to ethylene ... 

Only about 5% of the ethanol produced industrially comes from fermentation. Most is obtained by acid-catalyzed addition of water to ethylene (Section 23.10). 

Reactive intermediates which are planar can also produce enantiomers. The acid-catalyzed addition of water to 1-pentene proceeds via a secondary carboca-tion. Because the carbocation is a trigonal, planar intermediate, water can add to either face to give the R or S enantiomers. 

Very different solvent deuterium isotope effects (SDIE) of (/ch)/( d) = 0.42 and 1.66, respectively, are observed for acid-catalyzed addition of solvent to o-l and 81.50,58 The inverse SDIE for hydration of o-1 is consistent with initial fast and reversible protonation of substrate followed by rate-determining addition of solvent to the protonated benzylic carbocation o-H-l +. The normal primary SDIE on acid-catalyzed hydration of 81 is consistent with a change in mechanism from stepwise, for acid-catalyzed addition of water o-l,50 to concerted for the acid-catalyzed reaction of 81, where the addition of water and hydron occurs at a single reaction stage (kcon, Scheme 47). 

These data could also be rationalized if there were a change in the ratedetermining step for a stepwise reaction, from k s for acid-catalyzed addition of solvent to o-l to k for the reaction of 81. However, it is unlikely that k will be rate determining for the stepwise addition to 81. This would require k s > k for Scheme 47. A value of k i > 1010 s 1 is expected for the strongly favorable deprotonation of H-81+ by water,162 and there is evidence for a significant barrier to k s for addition of water to -SR-substituted benzylic carbocations. For example, a value of k s = 5 x I07s 1 has been determined for addition of an aqueous solvent to the l-4-(thiomethylphenyl)ethyl carbocation. The concerted mechanism is favored because it avoids formation of the unstable acidic intermediate H-81+. It is possible, but not proven, that the concerted mechanism is enforced by the absence of a vibrational barrier to the step k i for deprotonation of this very strongly acid reaction intermediate by water.163,164... 

Two pathways are observed for nucleophile addition to 48 in water (Scheme 49) (i) uncatalyzed nucleophile addition to form the oxygen anion 48 that undergoes rapid protonation (ii) specific acid-catalyzed nucleophile addition. The SDIE on the specific acid-catalyzed addition of solvent and bromide anion to 48 are kH/kD = 0.68 and 1.0, respectively, for reactions in 50/50 (v/v) water trifluoroethanol,67 but a smaller SDIE of kH/kD = 0.41 is observed for the specific acid-catalyzed addition of an aqueous solvent to l.52 The larger SDIE for acid-catalyzed addition of Br to 48 is consistent with a concerted reaction mechanism, in which protonation of oxygen and carbon-bromine bond formation occur in a single step with a rate constant kHBr (Scheme 49). 

Acid-catalyzed addition of water to an amide, though itself a slow reaction, is inherently faster than acid-catalyzed addition of water to a nitrile, even though the equilibrium constant for the latter reaction is much less unfavorable. This is again a result of the difference in angular distortion see Fig. 9. For the amide reaction, the distortions are the same as for the ester example above three times... 

Some additional examples are provided by the following equations. Note the excellent yields in all of the examples. Also note that the last example proceeds without rearrangement because the intermediate is a mercurinium ion rather than a carbocation. Attempts to prepare this alcohol by acid-catalyzed addition of water result in completely rearranged product. 

Q In the presence of acid the double bond of the enol gets proton-ated. This step of the mechanism is identical to the first step of the mechanism for addition of the hydrogen halides and the acid-catalyzed addition of water to alkenes. The addition occurs so that the positive charge is located on the carbon that is bonded to the hydroxy group because this carbocation is stabilized by resonance. 

Section 11.6 discussed the acid-catalyzed addition of water to alkynes. The initial product of this reaction, called an enol, has a hydroxy group attached to one of the carbons of a CC double bond. The enol is unstable and rapidly converts to its tautomer, a carbonyl compound. The carbonyl and enol tautomers of acetone are shown in the following equation ... 

Mercuric Ion-Catalyzed Hydration Alkynes undergo acid-catalyzed addition of water across the triple bond in the presence of mercuric ion as a catalyst. A mixture of mercuric sulfate in aqueous sulfuric acid is commonly used as the reagent. The hydration of alkynes is similar to the hydration of alkenes, and it also goes with Markovnikov orientation. The products are not the alcohols we might expect, however. 

The acid-catalyzed hydration is a typical acid-catalyzed addition to the carbonyl group. Protonation, followed by addition of water, gives a protonated product. Deprotonation gives the hydrate. 

First half Acid-catalyzed addition of the nucleophile (water). 

Although promiscuous in their choice of guest-compounds, the only well-defined catalytic system based upon the calixarenes, which has been documented, is the one described by Shinkai [61]. He has studied the acid-catalyzed addition of water (Scheme 6) to lV-benzyl-l,4-dihydronicotinamide in the presence of a para-sulfonatocalix[6]arene and its methoxycarbonyl derivative. The polyanionic nature of the upper rim, containing the sulfonate groups, provides a site for the stabilization of the cationic intermediate. The lower rim, which carries the phenolic... 

Scheme 6. In spite of being good host molecules, there is only one well-documented [61] catalytic system based upon the calixarenes. The acid-catalyzed addition of water to yV-benzyl-l,4-dihydronicotinamide 22 to give 24 via 23 is accelerated by the calix[6]arene derivative 25...

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