HC≡C-CH 2-CH=CH 2 + Br 2 - > HC≡C-CH 2-CH BrCH 2 BrĪlthough these electrophilic additions to alkynes are sluggish, they do take place and generally display Markovnikov Rule regioselectivity and anti-stereoselectivity. The reaction of one equivalent of bromine with 1-penten-4-yne, for example, gave 4,5-dibromo-1-pentyne as the chief product. The reactions are even more exothermic than the additions to alkenes, and yet the rate of addition to alkynes is slower by a factor of 100 to 1000 than addition to equivalently substituted alkenes. When the addition reactions of electrophilic reagents, such as strong Brønsted acids and halogens, to alkynes are studied we find a curious paradox. The Lindlar catalyst permits adsorption and reduction of alkynes, but does not adsorb alkenes sufficiently to allow their reduction. Consequently, reduction of triple bonds occurs selectively at a moderate rate, followed by rapid addition of hydrogen to the alkene product. Subsequent transfer of hydrogen to the adsorbed alkyne proceeds slowly, relative to the corresponding hydrogen transfer to an adsorbed alkene molecule. Since alkynes adsorb more strongly to such catalytic surfaces than do alkenes, they preferentially occupy reactive sites on the catalyst. In this respect, the formation of stable platinum (and palladium) complexes with alkenes has been described earlier. Before hydrogen can add to a multiple bond the alkene or alkyne must be adsorbed on the catalyst surface. This behavior is nicely explained by differences in the stages of the hydrogenation reaction. However, careful hydrogenation of an alkyne proceeds exclusively to the alkene until the former is consumed, at which point the product alkene is very rapidly hydrogenated to an alkane. Independent studies of hydrogenation rates for each class indicate that alkenes react more rapidly than alkynes.
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R-C≡C-R + 2 Na in N H 3 (liq) - > trans R-C H=C H-R + 2 NaN H 2Īlkenes and alkynes show a curious difference in behavior toward catalytic hydrogenation. R-C≡C-R + H 2 & Lindlar catalyst - > cis R-C H=C H-R This reaction will be discussed later in this section. A complementary stereoselective reduction in the anti mode may be accomplished by a solution of sodium in liquid ammonia. The addition of hydrogen is stereoselectively syn (e.g. A less efficient catalyst, Lindlar's catalyst, prepared by deactivating (or poisoning) a conventional palladium catalyst by treating it with lead acetate and quinoline, permits alkynes to be converted to alkenes without further reduction to an alkane. In the case of catalytic hydrogenation, the usual Pt and Pd hydrogenation catalysts are so effective in promoting addition of hydrogen to both double and triple carbon-carbon bonds that the alkene intermediate formed by hydrogen addition to an alkyne cannot be isolated. Since alkynes are thermodynamically less stable than alkenes, we might expect addition reactions of the former to be more exothermic and relatively faster than equivalent reactions of the latter. The 9 kcal/mole weakening of this second π-bond is reflected in the heat of hydrogenation numbers ( 36.7 - 28.3 = 8.4 ). Here the difference ( 54 kcal/mole ) may be taken as the strength of the second π-bond. Similarly, a triple bond is stronger than a double bond, but not 50% stronger. The difference ( 63 kcal/mole ) may be regarded as the strength of the π-bond component.
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Thus, a double bond is stronger than a single bond, but not twice as strong. The standard bond energies for carbon-carbon bonds confirm this conclusion. From the heats of hydrogenation, shown in blue in units of kcal/mole, it would appear that alkynes are thermodynamically less stable than alkenes to a greater degree than alkenes are less stable than alkanes. The catalytic addition of hydrogen to 2-butyne not only serves as an example of such an addition reaction, but also provides heat of reaction data that reflect the relative thermodynamic stabilities of these hydrocarbons, as shown in the diagram to the right.
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Indeed, most of the alkene addition reactions discussed earlier also take place with alkynes, and with similar regio- and stereoselectivity. Since the most common chemical transformation of a carbon-carbon double bond is an addition reaction, we might expect the same to be true for carbon-carbon triple bonds. Because of its linear configuration ( the bond angle of a sp-hybridized carbon is 180º ), a ten-membered carbon ring is the smallest that can accommodate this function without excessive strain. A carbon-carbon triple bond may be located at any unbranched site within a carbon chain or at the end of a chain, in which case it is called terminal.