Electrophilic addition reactions, alkynes hydration

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). 

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. 

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. 

Alkynes undergo electrophilic addition reactions with hydrogen halides and bromine, but these reactions have limited synthetic utility. However, one reaction of alkynes that is commonly used in organic chemistry is hydration of the carbon-carbon triple bond to give a ketone, a transformation that is catalyzed by mercuric ion in the presence of sulfuric acid (Eq. 11.10). 

We expect the reactions complementary to equations (1) and (2), namely electrophilic attacks, to be faster for alkenes than for alkynes. Thus, reactivity ratios (/-ii and rj2) for corresponding alkynes and alkenes (PhC CH, PhCH=CH and BuC CH, BuCH=CH2) in radical copolymerizations favour the alkene over the alkyne . Electrophilic additions of Br, CI2, ArSCl and H3O+ to alkenes are usually much faster than those to alkynes . However, A (C=C)/A (C=C) can vary from 10 to < 1 for the different electrophilic processes and by 10 for one process (Br2 addition) when the solvent is changed from HjO to HOAc . This unexpected trend in reactivity continues undiminished in the rates of acid-catalysed hydration... 

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 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 ... 

The standard mechanism written for electrophilic addition of HX to an alkene or alkyne is similar to acid-catalyzed hydration. As shown in Scheme 10.2, protonation is the first step, followed by nucleophilic attack of the halide on the resulting electrophilic carbenium ion. Just as with hydration, electrostatic effects guide our understanding of both steps of the reaction. As weTl show below, this standard electron pushing implies a mechanism that is much simpler than that often foimd experimentally. 

As a center of high electron density, the triple bond is readily attacked by electrophiles. This section describes the resnlts of three such processes addition of hydrogen halides, reaction with halogens, and hydration. The hydration is catalyzed by mercury(II) ions. As is the case in electrophilic additions to unsymmetrical alkenes (Section 12-3), the Markovnikov rule is followed in transformations of terminal alkynes The electrophile adds to the terminal (less snbstituted) carbon atom. 

Usually, the hydroxy cyclization is much faster than the direct nucleophilic addition of water to the alkyne to form the corresponding methyl ketone. However, hydration of the alkyne takes place with 1,6-enynes in which the alkene bears electron withdrawing substituents that reduce its reactivity in the electrophilic addition [227]. Thus, enyne 88a reacts with MeOH in the presence of gold(I)-catalyst to give 89 in excellent yield, whereas 88b with a p-nitrophenyl group at the terminal carbon of the alkene gives ketone 90 (Scheme 48). Formation of ketone 90 is the result of addition of water contained in the solvent, since addition of 4 A molecular sieves inhibits the hydration reaction [50, 51]. 

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