Relative reactivity of alkenes and alkynes

Ratio of second-order rate constants (alkene/alkyne) 

A major factor in determining the magnitude of relative rates is the solvent used. Thus, high alkene alkyne reactivity ratios are found in organic solvents of low dielectric constant (acetic acid), but the rates of reaction are comparable when water is the solvent. This is true not only for the hydration but also for bromination. Polar solvents, water in particular, are evidently capable of minimizing the energy difference between the two transition states. Whether this is accomplished by very strong solvation or by some other mechanism is not entirely clear. Addition reactions of alkenes and alkynes with trifluoroacetic acid also take place at comparable rates.  

Acid-catalyzed additions to alkynes follow the Markownikoff rule. The initial addition products are not always stable, however. Addition of acetic acid, for example, results in the formation of enol acetates, which are easily converted to the corresponding carbonyl compound  

In aqueous solution, enols are formed and rapidly converted to the carbonyl compound. 

The mercuric-ion-catalyzed hydration of terminal acetylenes has been a significant method for synthesis of methyl ketones. The reaction can also be used with internal acetylenes if they are symmetrical. Unsymmetrical alkynes would be 

Addition of chlorine to alkynes is slow in the absence of light. When the addition reaction is initiated by light, the major product when butyne is in large excess is fran5-l,2-dichlorobutene  

The requirement for photoinitiation indicates that a radical-chain mechanism must be involved. Chlorination of 1-pentyne carried out in a gas phase reactor at higher temperatures and with higher chlorine alkyne ratios gives both the trans dichloroalkene and the saturated tetrachloro compound derived from addition of a second mole of chlorine.Fair yields of dichloroalkenes, predominantly the cis dihalide, can be obtained from acetylenes by reaction with antimony penta-chloride 

Bromination of alkynes occurs via an electrophilic mechanism. A bridged intermediate is postulated for alkyl substituted acetylenes, while vinyl cations are 

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As part of their study of the relative reactivity of alkenes and alkynes, Modena and coworkers determined the reactivity ratio toward carbocations. These cations were generated by interaction of either diphenylmethyl chloride or 1-phenylethyl chloride with zinc chloride. Absolute rate constants were not determined. In general, the reactivities of alkynes and alkenes are of the same order of magnitude (see also Section II. A). 

Further, when double and triple bonds are both present in the same substrate (vide infra), their relative reactivity appears to depend upon what is being added. Thus, generalizations regarding the relative reactivities of alkenes and alkynes can be made only within narrow Omits. 

The double and triple bonds of alkenes and alkynes are composed of strong a bonds and relatively weak tt bonds. We have seen that the reactivity of alkenes and alkynes is the result of an electrophile being attracted to the cloud of electrons that constitutes the TT bond. 

The structure of Os3(/r-H)2(CO)10 has been established by X-ray8 and neutron diffraction.9 The 46-electron complex displays a relatively high reactivity under mild conditions, associated with a stable triosmium framework and has been extensively studied as a model for the chemisorption of alkenes and alkynes on surfaces and in the catalytic isomerization and hydrogenation of alkenes.10 When supported onto alumina it is a catalyst for the methanation of CO and C02 slightly less efficient than NiOs3(/r-H)3(CO)9(,5-CsH5)>... 

As the reactions of carbocations with alkynes have higher activation enthalpies and less negative activation entropies than analogous reactions with alkenes, the relative reactivities of styrene and phenylacetylene have been found to strongly depend on temperature (Table 8) [213],... 

In contrast to the relatively unreactive alkanes, alkenes and alkynes are chemically reactive because they have double and triple carbon-carbon bonds as functional groups. In the lUPAC nomenclature the names of alkenes end with the suffix -ene and the position of the double bond is labeled by number of the C-atom on which this double bond begins. In analogy with alkanes with branched chains, the root of the word is the name of the longest chain of C-atoms and the atom numbering is arranged... 

The chemistry of benzene and its derivatives is quite different from that of alkenes and alkynes, but even though we do not study the chemistry of arenes until Chapters 21 and 22, we will show structural formulas of compounds containing aryl groups before then. The three double bonds in a six-membered ring create a special stabilization called aromaticity, which lowers the reactivity of benzene relative to other alkenes. What you need to remember at this point is that an aryl group is not chemically reactive under any of the conditions we describe in Chapters 6 through 20. 

The reactions initiated by the addition of a carbon-carbon multiple bond to the cation pool bring in unique one-pot transformations otherwise difficult to realize, because the reactive carbocations are existing in the solution in relatively high concentration. For example, a sequential one-pot three-component coupling reactions have been developed (Scheme 5) [9, 10]. [4 -f 2] cyclo addition reactions in which an N-acyliminium ion was used as a hetero diene [11], and cationic carbohydroxylation of alkenes and alkynes using the cation pool method [12] were also accomplished. 

A related selectivity problem arises in addition reactions of unsaturated alkyl derivatives. The example shown on the bottom of Figure 1 illustrates that the 1 1 product 6 can only be generated in reasonable yield, if the nucleophilicity of 5 is higher than that of 4 and 6. An estimate for the relative nucleophilicities of olefinic TT systems in such reactions can be derived from the recently determined reactivity ratios of alkenes and alkynes towards arylcarbenium ions. An additional selectivity problem encountered in the reaction of 4 with 5 - compound 6 does not only incorporate a nucleophilic but also an electrophilic center will be discussed in the next section. 

Reactive Enophile in [4 + 2] Cycloadditions. Vinylketenes are not effective as dienes in Diels-Alder reactions because they undergo only [2 + 2] cycloaddition with alkenes, as predicted by frontier molecular orbital theory. However, silylketenes exhibit dramatically different properties from those found for most ketenes. (Trimethylsilyl)vinylketene (1) is a relatively stable isolable compound which does not enter into typical [2 + 2] cy do additions with electron-rich alkenes. Instead, (1) participates in Diels-Alder reactions with a variety of alkenic and alkynic dienophiles. The directing effect of the carhonyl group dominates in controlling the regiochemical course of cycloadditions using this diene. For example, reaction of (1) with methyl propiolate produced methyl 3-(trimethylsilyl)sahcylate with the expected regiochemical orientation. ProtodesUylation of this adduct with trifluoroacetic acid in chloroform (25 °C, 24 h) afforded methyl salicylate in 78% yield (eq 2). 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

New catalysts have been described,646 and ab initio MO calculations have shown that the transformation takes place through a four-center transition state.647 In addition, the anomalous relative reactivities of substrates, specifically, the higher reactivity of alkynes compared to those of alkenes, can be explained by considering the reaction to essentially be a nucleophilic attack by an alkyl anion, rather than an electrophilic one. 

The sluggish reaction of disiamylborane with tri- and tetrasubstituted alkenes as compared to terminal and most internal disubstituted alkenes, makes it one of the most versatile hydroborating agents. The relative reactivity of disiamylborane with alkenes, dienes, and alkynes varies over a range of 104 (Table 2)26), whereas that of diborane varies in the range of 20-30. 

Table 2. Relative Reactivities of the Hydroboration of Various Alkenes, Dienes, and Alkynes with Disiamylborane in THF at 0 °C 261...

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