Alkynes..........

[center]Addition Reactions of Alkynes

A carbon-carbon triple bond may be located at any unbranchedsite within a carbon chain or at the end of a chain, in which case itis called terminal. Because of its linear configuration ( thebond angle of a sp-hybridized carbon is 180º ), a ten-memberedcarbon ring is the smallest that can accommodate this function withoutexcessive strain. Since the most common chemical transformation of acarbon-carbon double bond is an addition reaction, we might expect thesame to be true for carbon-carbon triple bonds. Indeed, most of the alkene addition reactions discussed earlier also take place with alkynes, and with similar regio- and stereoselectivity.
1. Catalytic Hydrogenation

The catalytic addition of hydrogen to 2-butyne not only serves asan example of such an addition reaction, but also provides heat ofreaction data that reflect the relative thermodynamic stabilities ofthese hydrocarbons, as shown in the diagram to the right. From the heats of hydrogenation, shown in blue in units of kcal/mole,it would appear that alkynes are thermodynamically less stable thanalkenes to a greater degree than alkenes are less stable than alkanes.The standard bond energiesfor carbon-carbon bonds confirm this conclusion. Thus, a double bond isstronger than a single bond, but not twice as strong. The difference (63 kcal/mole ) may be regarded as the strength of the π-bond component.Similarly, a triple bond is stronger than a double bond, but not 50%stronger. Here the difference ( 54 kcal/mole ) may be taken as thestrength of the second π-bond. The 9 kcal/mole weakening of this secondπ-bond is reflected in the heat of hydrogenation numbers ( 36.7 - 28.3= 8.4 ).
Since alkynes are thermodynamically less stable thanalkenes, we might expect addition reactions of the former to be moreexothermic and relatively faster than *****alent reactions of thelatter. In the case of catalytic hydrogenation, the usual Pt and Pdhydrogenation catalysts are so effective in promoting addition ofhydrogen to both double and triple carbon-carbon bonds that the alkeneintermediate formed by hydrogen addition to an alkyne cannot beisolated. A less efficient catalyst, Lindlar's catalyst,prepared by deactivating (or poisoning) a conventional palladiumcatalyst by treating it with lead acetate and quinoline, permitsalkynes to be converted to alkenes without further reduction to analkane. The addition of hydrogen is stereoselectively syn (e.g.2-butyne gives cis-2-butene). A complementary stereoselective reductionin the anti mode may be accomplished by a solution of sodium in liquidammonia. This reaction will be discussed later in this section.
R-C≡C-R + H2 & Lindlar catalyst [size=16]——> cis R-CH=CH-R R-C≡C-R + 2 Na in NH3 (liq) ——> trans R-CH=CH-R + 2 NaNH2Alkenes and alkynes show a curious difference in behavior towardcatalytic hydrogenation. Independent studies of hydrogenation rates foreach class indicate that alkenes react more rapidly than alkynes.However, careful hydrogenation of an alkyne proceeds exclusively to thealkene until the former is consumed, at which point the product alkeneis very rapidly hydrogenated to an alkane. This behavior is nicelyexplained by differences in the stages of the hydrogenation reaction.Before hydrogen can add to a multiple bond the alkene or alkyne must beadsorbed on the catalyst surface. In this respect, the formation ofstable platinum (and palladium) complexes with alkenes has beendescribed earlier.Since alkynes adsorb more strongly to such catalytic surfaces than doalkenes, they preferentially occupy reactive sites on the catalyst.Subsequent transfer of hydrogen to the adsorbed alkyne proceeds slowly,relative to the corresponding hydrogen transfer to an adsorbed alkenemolecule. Consequently, reduction of triple bonds occurs selectively ata moderate rate, followed by rapid addition of hydrogen to the alkeneproduct. The Lindlar catalyst permits adsorption and reduction ofalkynes, but does not adsorb alkenes sufficiently to allow theirreduction.
2. Addition by Electrophilic Reagents

When the addition reactions of electrophilic reagents, such asstrong Brønsted acids and halogens, to alkynes are studied wefind a curious paradox. The reactions are even more exothermic than theadditions to alkenes, and yet the rate of addition to alkynes is slowerby a factor of 100 to 1000 than addition to *****alently substitutedalkenes. The reaction of one *****alent of bromine with 1-penten-4-yne,for example, gave 4,5-dibromo-1-pentyne as the chief product.

HC≡C-CH2-CH=CH2 + Br2 ——> HC≡C-CH2-CHBrCH2BrAlthough these electrophilic additions to alkynes are sluggish, they dotake place and generally display Markovnikov Rule regioselectivity andanti-stereoselectivity. One problem, of course, is that the products ofthese additions are themselves substituted alkenes and can thereforeundergo further addition. Because of their high electronegativity,halogen substituents on a double bond act to reduce itsnucleophilicity, and thereby decrease the rate of electrophilicaddition reactions. Consequently, there is a delicate balance as towhether the product of an initial addition to an alkyne will sufferfurther addition to a saturated product. Although the initial alkeneproducts can often be isolated and identified, they are commonlypresent in mixtures of products and may not be obtained in high yield.The following reactions illustrate many of these features. In the lastexample, 1,2-diodoethene does not suffer further addition inasmuch asvicinal-diiodoalkanes are relatively unstable.
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