Hydrogenation of 1-alkynes

Hydrogenation of the carbon-carbon triple bonds, particularly to the olefinic bonds, has been the subject of numerous investigations since the very early stage of the study on catalytic hydrogenation, not only in terms of its synthetic utility but also with respect to the selectivity of catalytic metals for the semihydrogenation.1-6 

An addition that places the parts of the adding reagent on the same side (or face) of the reactant is called syn addition. We have just seen that the platinum-catalyzed addition of 

The opposite of a syn addition is an anti addition. An anti addition places the parts of the adding reagent on opposite faces of the reactant. 

In Chapter 8 we shall study a number of important syn and anti additions. 

Depending on the conditions and the catalyst employed, one or two molar eqnivalents of hydrogen will add to a carbon-carbon triple bond. When a platinnm catalyst is used, the alkyne generally reacts with two molar equivalents of hydrogen to give an alkane  

However, hydrogenation of an alkyne to an alkene can be accomplished throngh the nse of special catalysts or reagents. Moreover, these special methods allow the preparation of either ( )- or (Z)-aIkenes from disubstituted alkynes. 

Half-hydrogenations of alkynes are usually accomplished over poisoned Pd catalysts, but other catalysts also may also be used.13 Unpoisoned Pd sometimes does not give high stereoselectivity.14 One of the most mentioned Pd cat- 

Other poisons (modifiers) used to create such selective Pd catalysts may be metals 23 Zn, Cd, Zr, Ru, Au, Cu, Fe, Hg, Ag, Pb, Sb, and Sn or solvents (organic modifiers) 24 pyridine, quinoline, piperidine, aniline, diethylamine, other amines, chlorobenzene, and sulfur compounds. Hydroxides have also been used to increase the half-hydrogenation selectivity of Pd. 

The mechanism of half-hydrogenation of alkynes is not fully understood, but some details are recognized. For example, it has long been recognized that alkynes adsorb more strongly than alkenes. During the half-hydrogenation of 

TABLE 2.1 Order of ris-Alkene Selectivity from Hydrogenation of Alkynes25 

FIGURE 2.2 Mechanism of half-hydrogenation of alkynes showing two possible adsorption sites. 

Catalytic hydrogenation of alkynes on a metal surface provides cis alkenes (see Chapter 7, Problem 13), while treatment with sodium in liquid ammonia nearly always leads to trans alkenes, e.g., hydrogenation of 2-butyne. 

Several intermediates are involved in the latter reaction. The first is a radical anion resulting from electron transfer from sodium to the alkyne. This then deprotonates ammonia leading to a vinyl radical. The process repeats (electron transfer and deprotonation), and involves a vinyl anion intermediate. 

Where is the trans stereochemistry of the overall reduction decided Which is the first intermediate that shows a preference for a trans geometry Is this preference maintained for all successive intermediates  

Energy minima have all real frequencies, while transition states have one imaginary frequency. 

Spin density surface for trans radical anion shows location of unpaired electron. 

The C—H acid dissociation constant. A, is related to the hybridization of the carbon atom. It increases for carbon atoms in the order sp sp sp. This order of acidities parallels the percent s character of the hybrid orbitals. Because an sp hybrid orbital has more s character than an sp or sp5 orbital, its electrons are located closer to the nucleus, and a hydrogen atom bonded to an sp-hybridized carbon atom can be more easily removed as a proton. 

Although ethyne and terminal alkynes are much stronger acids than other hydrocarbons, they are still very weak acids. Hydroxide ion is not a strong enough base to convert a terminal alkyne to its conjugate base to any significant degree. 

In fact, the conjugate base of an alkyne rapidly and quantitatively converts to the alkyne whenever it reacts with compounds that contain hydroxyl groups (such as water, alcohols, and carboxylic acids). 

From the periodic trends of acidity we discussed earlier, we know that an N—H bond is a weaker acid than an O—H bond. Therefore NH , the conjugate base of ammonia, is a stronger base than OH , the conjugate base of water. The A of ammonia is Thus, amide ion quantitatively removes a proton from a terminal alkyne, whose pK value is about 10 25. 

Like alkenes, alkynes react with hydrogen gas to give more saturated compounds. Alkynes are quantitatively reduced to alkanes by reaction with two molar equivalents of hydrogen gas in the presence of a palladium catalyst. 

The conditions for hydrogenation of alkynes are similar to those employed for alkenes. In the presence of hnely divided platinnm, palladinm, nickel, or rhodinm, two molar equivalents of hydrogen add to the triple bond of an alkyne to yield an alkane. 

Substituents affect the heats of hydrogenation of alkynes in the same way they affect alkenes. Alkyl groups release electrons to 5p-hybridized carbon, stabilizing the alkyne and decreasing the heat of hydrogenation. 

The heat of hydrogenation of an alkyne is greater than twice the heat of hydrogenation of the derived alkene. The hrst hydrogenation step of an alkyne is therefore more exothermic than the second. 

Write a series of equations showing how you could prepare octane from acetylene and any necessary organic and inorganic reagents. 

Similarly to olefinic double bonds, the carbon-carbon triple bond can be hydrogenated with ease over a wide variety of catalysts. Bulky groups, however, may hinder hydrogenation. 

The complete hydrogenation of alkynes to alkanes occurs quite readily under the usual alkene hydrogenation conditions. This may or may not involve an intermediate alkene 81 

A more challenging task is the selective partial hydrogenation (semihydrogenation) of alkynes to yield alkenes. This is a selectivity problem similar to the hydrogenation of dienes in that that the alkyne is hydrogenated preferentially in the presence of an alkene. The possibility of the formation of geometric isomers from nonterminal acetylenes raises the problem of stereoselective semihydrogenation. 

The high selectivity of alkene formation is not the result of a large rate difference in the hydrogenation of the triple and double bonds. Rather, it is ensured by the strong adsorption of alkynes compared with that of alkenes. The alkyne displaces the alkene from the surface or blocks its readsorption. 

A simple alternative to the Lindlar reduction process has been recognized.95 The palladium(II) acetate-catalyzed polymerization of triethoxysilane in water produces finely divided palladium metal dispersed on a polysiloxane matrix with concomitant hydrogen evolution. Addition of (EtO)3SiH to the mixture of an alkyne and all other necessary constituents allows facile and selective reduction of the alkyne without an external hydrogen source  

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Like the hydrogenation of alkenes hydrogenation of alkynes is a syn addition CIS alkenes are intermediates in the hydrogenation of alkynes to alkanes... 

Hydrogenation of alkynes with internal triple bonds gives cis alkenes... 

The stereochemistry of metal-ammonia reduction of alkynes differs from that of catalytic hydrogenation because the mechanisms of the two reactions are different The mechanism of hydrogenation of alkynes is similar to that of catalytic hydrogenation of alkenes (Sections 6 1-6 3) A mechanism for metal-ammonia reduction of alkynes is outlined m Figure 9 4... 

Hydrogenation of alkynes may be halted at the alkene stage by using special catalysts Lindlar palladium is the metal catalyst employed most often Hydrogenation occurs with syn stereochemistry and yields a cis alkene... 

Lindlar catalyst (Section 9 9) A catalyst for the hydrogenation of alkynes to as alkenes It is composed of palladium which has been poisoned with lead(II) acetate and quino line supported on calcium carbonate... 

Noting that cis alkenes are intennediates in the hydrogenation of alkynes leads us to consider the possibility of halting hydrogenation at the cis alkene stage. If partial hydrogenation of an alkyne could be achieved, it would provide us with methods for preparing ... 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

Hydrogenation of alkynes to alkanes (Section 9.9) Alkynes are completely hydrogenated, yielding alkanes, in the presence of the customary metal hydrogenation catalysts. 

Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of... 

The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. 

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