Addition reaction of alkynes

A triple bond consists of the end-on overlap of two sp-hybrid orbitals to form a a bond and the lateral overlap of two sets of parallel-oriented p orbitals to form two mutually perpendicular ir bonds. 

The resulting carbon-carbon triple bond, with a hydrogen atom attached to each remaining sp bond. (The orbitals involved in the C—H bonds are omitted for clarity.) 

In the first step, the addition occurs mainly trans. 

With an ordinary nickel or platinum catalyst, alkynes are hydrogenated all the way to alkanes (eq. 3.1). However, a special palladium catalyst (called Lindlar s catalyst) can control hydrogen addition so that only 1 mole of hydrogen adds. In this case, the 

Lindlar s catalyst limits addition of hydrogen to an alkyne to 1 mole and produces a c/s alkene. 

The regiochemistry of the electrophilic addition of unsymmetrical reagents to terminal alkynes follows Markovnikov s rule in the same fashion as observed for alkenes (Sec. 10.4). Thus, the Lewis acid component, E, of the reagent E-Nu adds to the electron-rich triple bond to form the more stable, or more substituted, of the two possible intermediate vinyl carbocations. This carbocation then undergoes nucleophilic attack by the Lewis base component, Nu , of the reagent (Eq. 11.9). The addition of the second mole of E-Nu to the resulting alkene occurs with the same regiochemistry as the first (Eqs. 11.8 and 11.9). 

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). 

The role that the mercuric ion, a polarizable Lewis acid, plays in this reaction is not completely understood. However, a reasonable hypothesis is that it first coordinates with the triple bond to generate a vinyl cation that may be stabilized by conversion to a cyclic mercurinium ion (Eq. 11.11). Attack by water on the cyclic ion then occurs at the more substituted carbon. Following several proton transfers, mercuric ion is lost to give a substituted vinyl alcohol, which is 

In the experiment that follows, the hydration of a terminal alkyne is illustrated by the conversion of 2-methyl-3-butyn-2-ol (3) to 3-hydroxy-3-methyl-2-butanone (4), as shown in Equation 11.13. The presence of a hydroxyl group in 3 has little effect on the chemical properties of the carbon arbon triple bond. Rather, the main effect of the polar hydroxyl group is on the physical properties of the molecule, with the boiling point of 3 being considerably higher than those of other acetylenic hydrocarbons having the same molecular weight. 

This experiment provides an opportunity to verify experimentally that the hydration of a terminal alkyne occurs in accordance with Markovnikov s rule (see Historical Highlight in Chap. 10). For example, that the structure of the product of the hydration of 3 is 4 rather than 5 may be confirmed by comparing the IR, H, and NMR spectra of the product of the reaction with known standard samples of 4 and 5, the spectra of which are clearly distinct. When all of the spectral properties of a compound are superimposable with those of a standard sample, the two are identical. 

We have already discussed some of the most important reactions of alkynes. The nucleophilic attack of acetylide ions on electrophiles, for example, is one of the best methods for making more complicated alkynes (Section 9-7). Now we consider reactions that involve transformations of the carbon-carbon triple bond itself. 

Many of the reactions of alkynes are similar to the corresponding reactions of alkenes because both involve pi bonds between two carbon atoms. Like the pi bond of an alkene, the pi bonds of an alkyne are electron-rich, and they readily undergo addition reactions. Table 9-3 shows how the energy differences between the kinds of carbon-carbon bonds can be used to estimate how much energy it takes to break a particular bond. The bond energy of the alkyne triple bond is only about 226 kJ (54 kcal) more than the bond energy of an alkene double bond. This is the energy needed to break one of the pi bonds of an alkyne. 

Reagents add across the triple bonds of alkynes just as they add across the double bonds of alkenes. In effect, this reaction converts a pi bond and a sigma bond into 

TABLE 9-3 Approximate Bond Energies of Carbon-Carbon Bonds 

We must consider the possibility of a double addition whenever a reagent adds across the triple bond of an alkyne. Some conditions may allow the reaction to stop after a single addition, while other conditions give double addition. 

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Some of the most synthetically useful addition reactions of alkynes are with organometallic reagents, and these reactions, which can lead to carbon-carbon bond formation, are discussed in Chapter 8. 

Addition Reactions of Alkynes Catalyzed by Gold Complexes 266... 

A review which covered the different known methods for the preparation of chiral amines and analysis of the different chiral catalysts used, correlating them according to their efficiency, selectivity, and flexibility, has been presented.279 Reduction reactions of alkenes, arenes, alkynes and allenes resulting in the formation of two or more C-H bonds280 and reduction and addition reactions of alkynes to alkenes to form one or more C=C bonds281 have been reviewed. 

Predicting the Products, Including Regiochemistry and Stereochemistry, Resulting from Addition Reactions of Alkynes... 

Introduction 392 9-2 Nomenclature of Alkynes 393 9-3 Physical Properties of Alkynes 394 9-4 Commercial Importance of Alkynes 395 9-5 Electronic Structure of Alkynes 396 9-6 Acidity of Alkynes Formation of Acetylide Ions 397 9-7 Synthesis of Alkynes from Acetylides 399 9-8 Synthesis of Alkynes by Elimination Reactions 403 Summary Syntheses of Alkynes 404 9-9 Addition Reactions of Alkynes 405... 

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. 

The addition reactions of alkynes resemble those of alkenes, as shown in these examples ... 

Addition Reactions of Alkynes A WORD ABOUT... Petroleum, Gasoline, and Octane Number... 

Grignard reagents are also synthesized by addition reactions of alkynes, addition reactions of unsaturated compounds having C=S groups, reactions of cycloalkanes, reactions with organolithium compounds or organomercury compounds, etc. [4,15,27,28],... 

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