Hydration—Electrophilic Addition of Water

Hydration is the addition of water to an alkene to form an alcohol. HiO itself is too weak an acid to protonate an alkene, but with added H2SO4, H30 is formed and addition readily occurs. 

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. 

Hydration is simply another example of electrophilic addition. The first two steps of the mechanism are similar to those of electrophilic addition of HX—that is, addition of (from HgO ) to generate a carbocation, followed by nucleophilic attack of H2O. Mechanism 10.2 illustrates the addition of H2O to cyclohexene to form cyclohexanol. 

There are three consequences to the formation of carbocation intermediates  

Alcohols add to alkenes, forming ethers, using the same mechanism. Addition of CH3OH to 2-methylpropene, for example, forms tert-butyl methyl ether (MTBE), a high octane fuel additive described in Section 3.4C. 

Hydration is the addition of water to an aUcene to form an alcohol. H2O itself is too weak an 

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The procedures presented in this chapter represent basic reactions of alkynes. That involving hydration of 2-methyl-3-butyn-2-ol through electrophilic addition of water to the tr-system is a reaction analogous to the conversion of acetylene to acetaldehyde, a precursor to acetic acid and acetone. In addition, the formation of an alkyne via an elimination reaction illustrates an alternate approach to forming a carbon-carbon triple bond, although one that is not nearly so easy experimentally as adding water to calcium carbide to make acetylene ... 

ELECTROPHILIC ADDITION OF WATER TO ALKENES AND ALKYNES HYDRATION... 

Electrophilic Addition of Water to Alkenes and Alkynes Hydration... 

Bell, R. P. "The Reversible Hydration of Carbonyl Compounds." Adv. Phys. Org. Chem., 4,1-29 (1966). Electrophilic Addition of Water to Alkenes and Alkynes... 

Besides the addition of halides and hydrogen-halide adds to alkenes or alkynes, other industrially relevant electrophilic addition reactions involve hydratization reactions (addition of water to alkenes and alkynes, forming alcohols), cationic polymerization (addition of carbocation to an alkene), hydrogenation (addition of hydrogen to alkenes to form alkanes), and Diels-Alder reactions (addition of an alkene to a conjugated diene to form complex, unsaturated hydrocarbon structures). 

Acid-catalyzed hydration of isolated double bonds 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 7.1 for the elimination of water. We ll see the reason for the requirement in Section 19.13, but might note for now that the reaction is not an electrophilic addition but instead occurs... 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

The acid-catalvzed hydration reaction begins with protonation of the carbonyl oxygen atom, which places a positive charge on oxygen and makes the carbonyl group more electrophilic. Subsequent nucleophilic addition of water to the protonated aldehyde ot ketone then yields a protonated gem diol, which loses H+ to give the neutral product (Figure 19.5). 

With an electrophilic transition metal complex, it is believed that the hydration of an alkyne occurs through a trans-addition of water to an 72-alkyne metal complex (Scheme 15, path A),380 although the m-pathway via hydroxymetallation has also been proposed (path B).381,382 However, distinguishing between the two pathways is difficult due to the rapid keto-enol tautomerization that renders isolation of the initial water adduct challenging. 

The mercuric ion-catalyzed hydration of alkynes probably proceeds in a similar manner to the oxymercuration of alkenes (see Section 5.1). Electrophilic addition of Hg to the triple bond leads to a vinylic cation, which is trapped by water to give an vinylic organomercury intermediate. Unlike the alkene oxymercuration, which requires reductive removal of the mercury by NaBH4, the vinylic mercury intermediate is cleaved under the acidic reaction conditions to give the enol, which tautomerizes to the ketone. Hydration of terminal alkynes follows the Mai kovnikov rule to furnish methyl ketones. ° ... 

As showm in Figure 8.3, the mechanism of the mercury ID-catalyzed alkyne hydration reaction is analogous to the oxymercuration reaction of alkenes (Section 7.4). Electrophilic addition of mercury(Il) ion to the alkyne gives a vinylic cation, which reacts with water and loses a proton to yield a mercury-containing enol intermediate. In contrast with alkene oxymercuration, however, no treatment with NaBl-14 is necessary to remove the mercury. The acidic reaction conditions alone are sufficient to effect replacement of mercury by hydrogen. 

The ketone group in trialkyl phosphonoglyoxylates is highly electrophilic, being quantitatively converted to the hydrate by addition of one equivalent of water, and there is no evidence for rapid exchange on the NMR time scale. The hemi-acetal from methanol also forms quantitatively and showed no sign of conversion to hydrate when kept 48 h in water at room temperature. However, one equivalent of a more hindered tert-amyl) alcohol gave only about 5% hemiac-etal. 

If, however, an acid (e.g., H2SO4 or HCl) is added to the solution, a reaction will occur because the acid provides an electrophile. The product of the reaction is an alcohol. The addition of water to a molecule is called hydration, so we can say that an alkene will be hydrated in the presence of water and acid. 

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a cyclic mercurinium ion. (Two of the electrons in mercury s filled 5d atomic orbital are shown.) This should remind you of the cyclic bromonium and mercurinium ions formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7 and 4.8). In the second step of the reaction, water attacks the most substituted carbon of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion forms an enol, which rearranges to a ketone. Notice that the overall addition of water follows both the general rule for electrophilic addition reactions and Markovnikov s rule The electrophile (H in the case of Markovnikov s rule) adds to the sp carbon bonded to the greater number of hydrogens. 

In the oxymercuration process, the electrophilic addition of the mercuric species occurs resulting in a mercurinium ion which is a three-membered ring. This is followed by the nucleophilic attack of water and as the proton leaves, an organomercuric alcohol (addition product) is formed. The next step, demercuration, occurs when sodium borohydride (NaBH ) substitutes the mercuric acetate substituent with hydrogen. If an alkene is unsymmetric, Oxymercuration-demercuration results in Markovnikov addition. The addition of mercuric species and water follows an anti (opposite side) addition pattern. This reaction has good yield, since there is no possibility of rearrangement unlike acid-catalyzed hydration of alkenes. 

The mechanism of nitrile hydrolysis involves acid or base promoted addition of water across the triple bond. This gives an intermediate imidate that tautomerizes to an amide. The amide is then hydrolyzed to the carboxylic acid. The addition of water to the nitrile resembles the hydration of an alkyne (eq. 3.52). The oxygen of water behaves as a nucleophile and bonds to the electrophilic carbon of the nitrile. Amide hydrolysis will be discussed in Section 10.20. 

Allynes can be hydrated in the presence of acid and HgS04 by electrophilic addition of a molecule of water to the triple bond. The reaction proceeds by way of a carbocation intermediate. Hydration of acetylene (ethyne) produces acetaldehyde (ethanal). Outline the steps that occur in this transformation. 

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