Heats of Hydrogenation The bonds in the product are stronger than the bonds in the reactants; two C-H o bonds of an alkane are formed at the expense of the H-H o bond and the T component of the alkene s double bond. The overall reaction is exothermic, and the heat evolved on hydrogenation of one mole of an alkene is its heat of hydrogenation Heat of hydro- genation is a positive quantity equal to-AHo for the reaction tain finely divided metal catalysts. Platinum is the hydrogenation catalyst most often used, although palladium, nickel, and rhodium are also effective. Metal-catalyzed addi- The french chemist paul tion of hydrogen is normally rapid at room temperature, and the alkane is produced in Sabatier received the 1912 high yield, usually as the only product. his discovery that finely di- vided nickel is an effective CH3)2C=CHCH3+ H (CH3)2CHCH,CH3 hydrogenation catalyst. 2-Methyl-2-butene Hydroger 2-Methylbutane(100%) CH3 H3C 5.5-Dimethyl( methylene)cyclononane Hydrogen 1, 1, 5-Trimethylcyclononane(73%) PROBLEM 6.1 What three alkenes yield 2-methylbutane on catalytic hydro- genation? The solvent used in catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol, hexane, or acetic acid. The metal catalysts are insoluble in these solvents(or, indeed, in any solvent). Two phases, the solution and the metal, are present, and the reaction takes place at the interface between them. Reactions involving a substance in one phase with a different substance in a second phase are called eterogeneous reactions. Catalytic hydrogenation of an alkene is believed to proceed by the series of steps shown in Figure 6.1. As already noted, addition of hydrogen to the alkene is very slow in the absence of a metal catalyst, meaning that any uncatalyzed mechanism must have a very high activation energy. The metal catalyst accelerates the rate of hydrogenation by providing an alternative pathway that involves a sequence of several low activation energy steps 6.2 HEATS OF HYDROGENATION Heats of hydrogenation are used to compare the relative stabilities of alkenes in much Remember that a ca the same way as heats of combustion. Both methods measure the differences in the fe rects the rate of a reaction energy of isomers by converting them to a product or products common to all. Catalytic but not the energy relation hydrogenation of 1-butene, cis-2-butene, or trans-2-butene yields the same product- oducts Thus. the heat of butane. As Figure 6.2 shows, the measured heats of hydrogenation reveal that trans-2- hydrogenation of a particu- butene is 4 J/mol (1.0 kcal/mol) lower in energy than cis-2-butene and that cis-2-butene lar alkene is the same irre is 7 k/mol(1.7 kcal/mol) lower in energy than 1-butene spective of what catalyst is Heats of hydrogenation can be used to estimate the stability of double bonds as structural units, even in alkenes that are not isomers. Table 6. 1 lists the heats of hydro- genation for a representative collection of alkenes Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteThe bonds in the product are stronger than the bonds in the reactants; two C±H bonds of an alkane are formed at the expense of the H±H bond and the component of the alkene’s double bond. The overall reaction is exothermic, and the heat evolved on hydrogenation of one mole of an alkene is its heat of hydrogenation. Heat of hydro￾genation is a positive quantity equal to H° for the reaction. The uncatalyzed addition of hydrogen to an alkene, although exothermic, is very slow. The rate of hydrogenation increases dramatically, however, in the presence of cer￾tain finely divided metal catalysts. Platinum is the hydrogenation catalyst most often used, although palladium, nickel, and rhodium are also effective. Metal-catalyzed addi￾tion of hydrogen is normally rapid at room temperature, and the alkane is produced in high yield, usually as the only product. PROBLEM 6.1 What three alkenes yield 2-methylbutane on catalytic hydro￾genation? The solvent used in catalytic hydrogenation is chosen for its ability to dissolve the alkene and is typically ethanol, hexane, or acetic acid. The metal catalysts are insoluble in these solvents (or, indeed, in any solvent). Two phases, the solution and the metal, are present, and the reaction takes place at the interface between them. Reactions involving a substance in one phase with a different substance in a second phase are called heterogeneous reactions. Catalytic hydrogenation of an alkene is believed to proceed by the series of steps shown in Figure 6.1. As already noted, addition of hydrogen to the alkene is very slow in the absence of a metal catalyst, meaning that any uncatalyzed mechanism must have a very high activation energy. The metal catalyst accelerates the rate of hydrogenation by providing an alternative pathway that involves a sequence of several low activation energy steps. 6.2 HEATS OF HYDROGENATION Heats of hydrogenation are used to compare the relative stabilities of alkenes in much the same way as heats of combustion. Both methods measure the differences in the energy of isomers by converting them to a product or products common to all. Catalytic hydrogenation of 1-butene, cis-2-butene, or trans-2-butene yields the same product— butane. As Figure 6.2 shows, the measured heats of hydrogenation reveal that trans-2- butene is 4 kJ/mol (1.0 kcal/mol) lower in energy than cis-2-butene and that cis-2-butene is 7 kJ/mol (1.7 kcal/mol) lower in energy than 1-butene. Heats of hydrogenation can be used to estimate the stability of double bonds as structural units, even in alkenes that are not isomers. Table 6.1 lists the heats of hydro￾genation for a representative collection of alkenes. (CH3)2C CHCH3 2-Methyl-2-butene  H2 Hydrogen (CH3)2CHCH2CH3 2-Methylbutane (100%) Pt Pt CH3 H3C CH3 H 1,1,5-Trimethylcyclononane (73%) H2 Hydrogen CH3 H3C CH2 5,5-Dimethyl(methylene)cyclononane  6.2 Heats of Hydrogenation 209 The French chemist Paul Sabatier received the 1912 Nobel Prize in chemistry for his discovery that finely di￾vided nickel is an effective hydrogenation catalyst. Remember that a catalyst af￾fects the rate of a reaction but not the energy relation￾ships between reactants and products. Thus, the heat of hydrogenation of a particu￾lar alkene is the same irre￾spective of what catalyst is used. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website