Alkynes Are Less Reactive than Alkenes

The cloud of electrons completely surrounding the a bond makes an alkyne an electron-rich molecule. Alkynes therefore are nucleophiles, so they react with electrophiles. Thus alkynes, like alkenes, undergo electrophilic addition reactions because of their relatively weak tt bonds. The same electrophilic reagents that add to alkenes also add to alkynes. For example, the addition of hydrogen chloride to an alkyne forms a chloro-substituted alkene. 

Alkynes are less reactive than alkenes in electrophilic addition reactions. 

For an alkyne to be both less stable and less reactive than an alkene, the following two conditions must hold  

The transition state for the addition of an electrophile to an alkyne must be less stable than that for the addition of an electrophile to an alkene. 

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Problwn 8.12 In terms of the mechanism, explain why alkynes are less reactive than alkenes towards electrophilic addition of, e.g., HX or Br. ... 

It is well known that alkynes are less reactive than alkenes in electrophilic reactions 1117). The theoretical explanation is not obvious [118]. Although some theoretical calculations were also reported on the reactions of acetylene with [119], Si [120] and rare metal ions (Sc", , ... 

Clearly, the mechanism of the addition reaction is not completely understood. For now, then, we will assume that the reaction forms a rr-complex. It is more stable than a vinylic cation would be, but it is not as stable as an aUcyl cation. Therefore, the transition state leading to its formation is less stable than the transition state leading to formation of an alkyl cation, agreeing with the observation that alkynes are less reactive than alkenes (Figure 1.2). 

Alkynes are highly unsaturated and are reactive towards electrophiles and nucleophiles. They are less reactive than alkenes towards electrophiles and more reactive than alkenes towards nucleophiles. 

The addition proceeds in three discrete steps and the intermediates can be isolated. Simple alkenes are less reactive than alkynes and do not undergo the addition to aHylic boranes, but electron-rich alkyl vinyl ethers react at moderate temperatures to give 1,4-dienes or dienyl alcohols (440). 

Physical properties of alkynes [49, p. 251] are essentially similar to those of alkanes and alkenes. These compounds are weakly polar and are insoluble in water, but they are quite soluble in organic solvents of low polarity (e.g., ether, benzene, CCl ). Chemically, alkynes are more reactive than alkanes but behave like alkenes. The triple bond appears to be less reactive than the double bond in some reagents while more reactive in others. In a chemical reaction, the triple bond is usually broken into a double bond, which may eventually split into single bonds. 

Alkynes, although not as prevalent as alkenes, have a number of important uses in synthesis. In general, alkynes are somewhat less reactive than alkenes toward many electrophiles. A major reason for this difference in reactivity is the substantially higher energy of the vinyl cation intermediate that is formed by an electrophilic attack on an alkyne. It is estimated that vinyl cations are about lOkcal/mol less stable than an alkyl cation with similar substitution. The observed differences in rate of addition in direct comparisons between alkenes and alkynes depend upon the specific electrophile and the reaction conditions.111 112 Table 4.4 summarizes some illustrative rate comparisons. A more complete discussion of the mechanistic aspects of addition to alkynes can be found in Section 6.5 of Part A. 

In this ion some positive charge is dispersed to the C s, which, because of their 5p-like hybrid character, are less able to bear the positive charge than the sp C s in the alkene s bromonium ion. Such situations cause alkynes to be less reactive than alkenes toward Brj. 

As a C=C triple bond is shorter and stronger than a C=C double bond, the alkene 71 and n should lie between the alkyne 71 and 7t energy levels. Applying rule 2, we then draw the erroneous conclusion that alkynes are always less reactive than alkenes.44 Where is the mistake ... 

These addition reactions are similar to the addition reactions of alkenes. However, the reaction is much slower for an alkyne, because alkynes are less reactive. Alkynes can be expected to be more nucleophilic, as they are more electron rich in the vicinity of the multiple bond, that is, six electrons in a triple bond as compared to four in a double bond. However, electrophilic addition to an alkyne involves the formation of a vinylic carbocation (Following fig.). This carbocation is much less stable than the carbocation intermediate formed during electrophilic addition to an alkene. 

Aromatic hydrocarbons tend to be less reactive than alkenes or alkynes because they have no double bonds. Instead, electrons are shared evenly over the entire benzene ring. 

The carbometallation of allylboranes is known . Alkenes are less reactive than alkynes or allenes in reaction with allylboranes. Only highly reactive alkenes, such as vinyl ethers and cyclopropenes , are used. 

Alkynes are somewhat less reactive than alkenes. For example, 1-butene is 20 times more reactive than 1-butyne in 8.24 M112804. " Alkyne reactivity increases with the addition of ERG substituents. Solvent isotope effects are indicative of a ratedetermining protonation. " These reactions are believed to proceed by rate-determining proton transfer to give a vinyl cation. Reactions proceeding through a vinyl cation would not be expected to be stereospecific, since the cation will adopt sp hybridization (see Section 3.4.1). 

The copper-catalysed decomposition of ethyl diazoacetate in hex-l-en-4-yne has been reportedto give a 1 1 mixture of the products of addition at the double and triple bonds. This is a surprising result, as alkynes are generally much less reactive than alkenes towards carbene addition. 

We mentioned briefly in Section 7.6 that alkynes hehave similarly to alkenes in much of their chemistry. Thus, they undergo many addition reactions just as alkenes do. As a general rule, however, alkynes are somewhat less reactive than alkenes, so the various reactions can often be stopped at the monoaddition stage if only one molar equivalent of reagent is used. The additions typically show Markovnikov regiochemistry. Note that for the addition of 1 molar equivalent of H2 to an alkyne to give an alkene, a special hydrogenation catalyst called the Lindlar catalystis needed. The alkene that results has cis stereochemistry. 

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