Alkyne vs. Alkene Reactivity

A simple-minded picture suggests that the CC K bonds in aUcynes and alkenes ought to be similar. Are they Consider the thermodynamics of reduction of phenylacetylene to first give styrene and then phenylethane. (The energy for H2 is given at right.) 

Which addition is more favorable thermodynamically Assuming that the difference is entirely due to different 7t-bond energies, then which contains the stronger % bond, the alkyne or the alkene What flaws might there be in the basic assumption  

Metals that catalyze reaction (2) generally catalyze reaction (1) as well, and the rate for (1) is usually somewhat higher. Given such a catalyst, what product(s) would be obtained were phenylacetylene to be combined with one equivalent of H2 What would happen if the catalyst accelerated reaction (2) more effectively than reaction (1)  

Thermodynamics and kinetics need not go hand in hand. Consider all possible products resulting from addition of one equivalent of bromine to phenylacetylene (phenylacetylene+Br2) and to styrene (styrene+Br2). Calculate the heat of reaction for each addition. (The energy of Br2 is given at right.) Is addition to the alkyne or to the alkene more favorable  

Bromination usually follows a two-step mechanism, the rate-limiting step involving formation of an adduct with Br. Calculate energies for Br addition to phenylacetylene and styrene, leading to phenylacetylene+Br+ and styrene+Br+, respectively. (The energy of Br+ is given at right.) Which reaction is more favorable Is this the same preference as seen for Br2 addition  

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At infrequent intervals, different chemists have juxtaposed properties of alkynes and alkenes. The purpose, of course, was to enrich their understanding of both families In so doing, a number of misconceptions of alkyne vs. alkene reactivity were clarified. The major conclusion that evolved was simple nucleophiles react faster with alkynes than with alkenes electrophiles (including radicals ) react slower with alkynes than with alkenes. Since new kinds of data are available, we believe that the comparisons are especially illuminating now. 

Alkenes and alkynes destabilized by such distortions display increased reactivity in reactions that alleviate this penalty for example in different variations of catalyst-free click chemistry for biological applications. For example, the calculated activation barrier for the cycloaddition of methyl azide to cyclooctyne is half that for cycloaddition to 2-butyne (10.5 vs. 21 kcal - Figure 2.22). ... 

When compared with C2 symmetric ligands, Ci symmetric and bidentate NHCs induced the highest ee values (<99% ee) with unsaturated C-C bonds and [B(pin)]2 in MeOH. For instance, in the p-boration of a- and p-unsatu-rated aldehydes and esters, which are more reactive than other C-C unsaturated substrates, a 213-derived system is more enantioselective than the 214-derived complex (74% ee vs. 59% ee, respectively) (Scheme 13.8). Improved results were obtained by Hoveyda and co-workers, using bidentate 217-220 with acyclic and cyclic internal alkenes, alkynes, 1,6-dienes and various allenes ee s of <98% were achieved (Scheme 13.8). For the regio-and enantioselective hydroboration of a,p-unsaturated carbonyl compounds... 

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