210 CHAPTER SIX Reactions of alkenes Addition reactions FIGURE 6.1 A mechanisn Step 1: Hydrogen molecules react with Step 2: The alkene reacts with the metal for heterogeneous catalysis metal atoms at the catalyst surface n the hydrogenation of bond between the two carbons is replaced alkenes bond is broken and replaced by two weak by two relatively weak carbon-metal o bonds. 88 Step 3: A hy inferred from the catalyst surface carbons of the double bond 。e。 8 FIGURE 6.2 Heats plotted on a comme All energies are in m HC CH3 H3C H CH,=CHCH, CH3 1-Butene cis-2-Butene trans-2-Butene 119 115 △H° △HP △B° CHCH,CH,CH3 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website210 CHAPTER SIX Reactions of Alkenes: Addition Reactions Step 1: Hydrogen molecules react with metal atoms at the catalyst surface. The relatively strong hydrogen–hydrogen σ bond is broken and replaced by two weak metal–hydrogen bonds. Step 2: The alkene reacts with the metal catalyst. The π component of the double bond between the two carbons is replaced by two relatively weak carbon–metal σ bonds. Step 3: A hydrogen atom is transferred from the catalyst surface to one of the carbons of the double bond. Step 4: The second hydrogen atom is transferred, forming the alkane. The sites on the catalyst surface at which the reaction occurred are free to accept additional hydrogen and alkene molecules. FIGURE 6.1 A mechanism for heterogeneous catalysis in the hydrogenation of alkenes. 1-Butene cis-2-Butene trans-2-Butene Potential energy Alkene CH3CH2CH2CH3 CH2 CHCH2CH3 H3C CH3 C C H H 126 7 119 4 115
H2 H3C C C H CH3 H ∆H ∆H ∆H FIGURE 6.2 Heats of hydro￾genation of butene isomers plotted on a common scale. All energies are in kilojoules per mole. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website