Ch_322a_7.02

Ch_322a_7.02 - 2 =CH 2 H H H H CH 2 =CH 2 hydrogen atom...

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Hydrogenation of Alkenes: An Addition Reaction The addition of H 2 to an alkene to produce an alkane is exothermic ( ! H o ~ -30 kcal/mol), but very slow in the absence of a metal catalyst (Pt, Pd, Ni). CH 2 =CH 2(g) + H 2(g) CH 3 CH 3(g) ! H o f (12.50) (0) (-20.2) kcal/mol ! H o = " ! H o prod - " ! H o react = -32.7 kcal/mol In the presence of a metal catalyst , these hydrogenations proceed rapidly at room temperature in common solvents. CH 3 CH=CH 2 + H 2 Pt CH 3 CH 2 CH 3 C=C CH 3 CH 3 H H + H 2 Pt CH 3 CH 2 CH 2 CH 3 CH 3 C=CH 2 CH 3 + H 2 Pt CH 3 CHCH 3 CH 3
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Catalytic Hydrogenation In the absence of a metal catalyst, hydrogenation is slow because of a large energy of activation (energy barrier). The metal catalyst provides a low energy reaction pathway between the reactant and product states. Proposed Mechanism for Catalytic Hydrogenation Hydrogen molecules reversibly adsorb on the surface of the metal. metal + H 2 - H 2 metal H H H H hydrogen adsorbed and activated The alkene also adsorbs to the metal surface. + CH 2 =CH 2 - CH
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Unformatted text preview: 2 =CH 2 H H H H CH 2 =CH 2 hydrogen atom adds to one carbon CH 2-C H H H H H H H H C C H H H H H H hydrogen atom adds to second carbon H H C C H H H H H H and alkane desorbs Energetics of Catalytic Hydrogenation Catalysts change the rates of chemical reactions but do not change the overall thermodynamics. Enthalpy Reaction Coordinate CH 2 =CH 2 + H 2 CH 3 CH 3 ! H o = -32.7 kcal/mol large energy barrier without catalyst low energy barrier with catalyst Stereochemistry of Additions to Alkenes There are two modes of addition to an alkene: C=C X-Y C C X Y syn addition addition of X-Y to the same face of alkene C=C X-Y anti addition C C X Y addition of X-Y to the opposite faces of alkene Catalytic hydrogenation is a syn addition. However, this outcome is only noticed in hydrogenation reactions that yield two stereocenters....
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_7.02 - 2 =CH 2 H H H H CH 2 =CH 2 hydrogen atom...

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