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  Alkynes Addition Reactions Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Addition Reactions of Alkynes

In a carbon chain, a carbon-carbon triple bond might be situated at any unbranched site or at the end of a chain, in which case it is called as terminal. Due to its linear configuration (bond angle of a sp-hybridized carbon is 180º), ten-membered carbon ring is the smallest that can contain this function with no excessive strain. Because the most general chemical transformation of a carbon-carbon double bond is an addition reaction, we may be supposed the same to be true for carbon-carbon triple bonds. Certainly, most of the alkene additions reactions that are explained earlier also take place with alkynes and with the identical region and Stereoselectivity.

1. Catalytic Hydrogenation:

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The catalytic addition of hydrogen to 2-butyne not only provides as an instance of such type of an addition reaction but also exhibits heat of reaction data which reflect the relative thermodynamic stabilities of these hydrocarbons, as displayed in the diagram to the right. From the heats of hydrogenation, displayed in blue color in units of kcal/mole, it would show that alkynes are thermodynamically less stable than alkenes to a greater degree than alkenes are less stable than alkanes. The standard bond energies (Bond Energy because reactions of organic compounds involve the breaking and making of bonds, the strength of bonds, or their resistance to breaking, becomes a major consideration. For an instance, the chlorination of methane, depicted earlier, was induced by breaking a relatively weak Cl-Cl covalent bond.

The energy required to break a covalent bond homolytically (into neutral fragments) is known as bond energy. Bond energies are generally specified in units of kcal/mol or kJ/mol, and are usually called bond dissociation energies when given for particular bonds, or average bond energies when concise for a given type of bond over many types of compounds. Such type of average values is often considered as standard bond energies, and is given in units of kcal/mole.) For carbon-carbon bonds validate this conclusion. So, than a single bond, the double bond is stronger, but not two times as strong. The variation (63 kcal/mole) may be considered as the strength of the π-bond component. Likewise, than a double bond, a triple bond is more stronger, but not 50% stronger. Here the variation (54 kcal/mole) might be taken like the strength of the second π-bond. The 9 kcal/mole weakening of that second π-bond is reflected in the heat of hydrogenation numbers (36.7 - 28.3 = 8.4).

Because than alkenes, alkynes are thermodynamically less stable, we may expect addition reactions of the former to be more exothermic and comparatively faster than equivalent reactions of the latter. In the case of catalytic hydrogenation, the general Pd and Pt hydrogenation catalysts are so effective in promoting addition of hydrogen to both double and triple carbon-carbon bonds that the alkene intermediate created by hydrogen addition to an alkyne cannot be isolated. A less capable catalyst, Lindlar's catalyst, which is prepared by deactivating a conventional palladium catalyst by treating it with lead quinoline and acetate, allows alkynes to be converted to alkenes without additional reduction to an alkane. Addition of hydrogen is stereoselectively syn (for example 2-butyne gives cis-2-butene). Within the anti mode a complementary stereoselective reduction might be accomplished by a solution of sodium in liquid ammonia.

R-C≡C-R   +   H2   &   Lindlar catalyst   -->  cis R-CH=CH-R

R-C≡C-R   +   2 Na   in   NH3 (liq)   --> trans R-CH=CH-R   +   2 NaNH2

Alkynes and Alkenes display a curious dissimilarity in behavior toward catalytic hydrogenation. The Independent studies of hydrogenation rates for each class point out that the alkenes react more quickly than alkynes. Though, careful hydrogenation of an alkyne proceeds completely to the alkene until the former is consumed, at which point the product alkene is very quickly hydrogenated to an alkane. This behaviour is well elaborated by differences in the stages of the hydrogenation reaction. Before hydrogen can add to a multiple bond the alkyne or alkene or might be adsorbed on the catalyst surface. In this way, formation of stable platinum (and palladium) complexes with alkenes has been illustrated earlier. Because alkynes adsorb more strongly to such type of catalytic surfaces than do alkenes, they preferentially occupy reactive sites on the catalyst. Consequent transfer of hydrogen to the adsorbed alkyne proceeds slowly, relative to the subsequent hydrogen transfer to an adsorbed alkene molecule. Accordingly, reduction of triple bonds comes into existence selectively at a moderate rate, which is followed by rapid addition of hydrogen to the alkene product. The Lindlar catalyst allows reduction and adsorption of alkynes, but does not adsorb alkenes adequately to allow their reduction.

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