Electrophilic addition reactions, alkynes

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. 

Quaternary ammonium tribromides can also be produced in situ from the quaternary ammonium bromide, sodium hypochlorite and sodium bromide and can be used, for example, in electrophilic addition reactions reaction with alkenes and alkynes. 

Alkenes and alkynes readily undergo electrophilic addition reactions. They are nucleophilic and commonly react with electrophiles. The n bonds of alkenes and alkynes are involved in the reaction, and reagents are added to the double or triple bonds. In the case of alkynes, two molecules of reagent are needed for each triple bond for the total addition. 

An alkyne is less reactive than an alkene. A vinyl cation is less able to accommodate a positive charge, as the hyperconjugation is less effective in stabilizing the positive charge on a vinyl cation than on an alkyl cation. The vinyl cation is more stable with positive charge on the more substituted carbon. Electrophilic addition reactions allow the conversion of alkenes and alkynes into a variety of other functional groups. 

Alkynes are somewhat less reactive than alkenes in electrophilic addition reactions... 

UV studies suggest that in 1-vinyl-imidazoles and -benzimidazoles there is conjugation with the heteroaromatic ring. Certainly the reactivities of such compounds would indicate that they are not normal alkenes, nor are the alkynyl compounds typical of other alkynes. They are very prone to polymerization processes, and electrophilic addition reactions are often quite difficult to accomplish. When 1-styrylimidazoles are prepared, the trans compound is much more likely to eventuate as the more stable isomer 73CJC3765, 76JCS(PD545). 

Because its weaker n bonds make an alkyne electron rich, alkynes undergo addition reactions with electrophiles (11.6). 

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... 

Such structure reactions have existed in organic chemistry since long. Wilson gives an example of classification of electrophilic addition reactions of alkenes and alkynes. Patterns in organometalhc chemistry with applications in organic synthesis have been discussed by Schwartz and Labinger. 

Key point. Alkenes and alkynes are unsaturated hydrocarbons, which possess a C=C double bond and a C=C triple bond, respectively. As (weak) re-bonds are more reactive than (strong) o-bonds, alkenes and alkynes are more reactive than alkanes. The electron-rich double or triple bond can act as a nucleophile, and most reactions of alkenes/ alkynes involve electrophilic addition reactions. In these reactions, the re-bond attacks an electrophile to generate a carbocation, which then reacts with a nucleophile. Overall, these reactions lead to the addition of two new substituents at the expense of the re-bond. 

Alkynes, like alkenes, act as nucleophiles and react with electrophiles in electrophilic addition reactions. The electrophiles generally add in the same way as they add to alkenes. 

Vinylic carbocations are generally less stable than alkyl carbocations, as there are fewer +1 alkyl groups to stabilise the positive charge. As a consequence, alkynes (which give vinylic carbocations) generally react more slowly than alkenes (which give alkyl carbocations) in electrophilic addition reactions. 

Thus alkynes, like alkenes, undergo electrophilic addition reactions. We will see that the same electrophilic reagents that add to alkenes also add to alkynes and that— again like alkenes—electrophilic addition to a terminal alkyne is regioselective When an electrophile adds to a terminal alkyne, it adds to the sp carbon that is bonded to the hydrogen. The addition reactions of alkynes, however, have a feature that alkenes do not have Because the product of the addition of an electrophilic reagent to an alkyne is an alkene, a second electrophilic addition reaction can occur. 

An alkyne is less reactive than an alkene. This might at first seem surprising because an alkyne is less stable than an alkene (Figure 6.2). However, reactivity depends on AG, which in turn depends on the stability of the reactant and the stability of the transition state (Section 3.7). For an alkyne to be both less stable and less reactive than an alkene, two conditions must hold The transition state for the first step (the rate-limiting step) of an electrophilic addition reaction for an alkyne must be less stable than the transition state for the first step of an electrophilic addition reaction for an alkene, and the difference in the stabilities of the transition states must be greater than the difference in the stabilities of the reactants so that AGli yne > alkene (Fi UTC 6.2). 

Why is the transition state for the first step of an electrophilic addition reaction for an alkyne less stable than that for an alkene The Hammond postulate predicts that the structure of the transition state will resemble the structure of the intermediate (Section 4.3). The intermediate formed when a proton adds to an alkyne is a vinylic... 

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. 

Dienes, like alkenes and alkynes, are nucleophiles because of the electron density of their tt bonds. Therefore, they react with electrophilic reagents. Like alkenes and alkynes, dienes undergo electrophilic addition reactions. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HCI to yield... 

Alkynes undergo addition reactions, especially electrophilic addition, with many of the same compounds that add to alkenes. 

Alkynes undergo electrophilic addition reactions, and under aqueous acidic conditions are hydrolysed to the ketone product. Protonation of the alkyne at the terminal position gives the more substituted w f7y//c cation, which Is Intercepted by water. The resulting enol tautomerises to the ketone product. 

Alkynes are very susceptible to electrophilic addition reactions, especially with strong electrophiles. 

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). 

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