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Hydrogenation of Alkenes

Hydrogenation of Alkenes. H. H. H. H. C. C. H. H. C. C. H. H. H. H. Hydrogenation of Ethylene. exothermic D H ° = –136 kJ/mol catalyzed by finely divided Pt, Pd, Rh, Ni. s. s. s. p. + H—H. CH 2. H 3 C. H 3 C. CH 3. H 3 C. H. H 3 C. Example. H 2 , Pt. (73%).

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Hydrogenation of Alkenes

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  1. Hydrogenation of Alkenes

  2. H H H H C C H H C C H H H H Hydrogenation of Ethylene • exothermic DH° = –136 kJ/mol • catalyzed by finely divided Pt, Pd, Rh, Ni s s s p + H—H

  3. CH2 H3C H3C CH3 H3C H H3C Example H2, Pt (73%)

  4. Problem 1 • What three alkenes yield 2-methylbutane on catalytic hydrogenation?

  5. Problem 6.1 • What three alkenes yield 2-methylbutane on catalytic hydrogenation? H2, Pt

  6. H H B Y H H C C A X Mechanism of Catalytic Hydrogenation:

  7. H H H H Mechanism of Catalytic Hydrogenation: B Y C C A X

  8. B Y X A H H H C C H Mechanism of Catalytic Hydrogenation:

  9. H H H H Mechanism of Catalytic Hydrogenation: B Y X A C C

  10. B Y X A H H C C H H Mechanism of Catalytic Hydrogenation:

  11. B Y X A C C H H H H Mechanism of Catalytic Hydrogenation:

  12. Heats of Hydrogenation • can be used to measure relative stability of isomeric alkenes • correlation with structure is same as when heats of combustion are measured

  13. Heats of Hydrogenation of Isomers 126 119 115 CH3CH2CH2CH3

  14. Heats of Hydrogenation (kJ/mol) • Ethylene 136 • Monosubstituted 125-126 • cis-Disubstituted 117-119 • trans-Disubstituted 114-115 • Terminally disubstituted 116-117 • Trisubstituted 112 • Tetrasubstituted 110

  15. 126 kJ/mol 118 kJ/mol 112 kJ/mol Problem 2 Match each alkene of Problem 1 with its correctheat of hydrogenation.

  16. 126 kJ/mol 118 kJ/mol 112 kJ/mol Problem 2 Match each alkene of Problem 1 with its correctheat of hydrogenation. highest heat ofhydrogenation;least stable isomer lowest heat ofhydrogenation;most stable isomer

  17. Stereochemistry of Alkene Hydrogenation

  18. Two spatial (stereochemical) aspects ofalkene hydrogenation: • syn addition of both H atoms to double bond • hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond

  19. syn-Additon versus anti-Addition syn addition anti addition

  20. H CO2CH3 H2, Pt CO2CH3 H Example of Syn Addition CO2CH3 CO2CH3 (100%)

  21. Stereoselectivity • A reaction in which a single starting materialcan give two or more stereoisomeric productsbut yields one of them in greater amounts thanthe other (or even to the exclusion of the other)is said to be stereoselective.

  22. H3C CH3 H H3C H3C H3C CH3 CH3 H H H H H3C H H3C H Example of Stereoselective Reaction H2, cat Both productscorrespond tosyn additionof H2.

  23. H3C CH3 H H3C H3C CH3 H H H3C H Example of Stereoselective Reaction H2, cat But only thisone is formed.

  24. H3C CH3 H H3C H3C CH3 H H H3C H Example of Stereoselective Reaction H2, cat Top face of doublebond blocked bythis methyl group

  25. H3C CH3 H H3C H3C CH3 H H H3C H Example of Stereoselective Reaction H2, cat H2 adds to bottom face of double bond.

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