Chapter 6 - Reactions of Alkenes Addition Reactions Chapter...

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Unformatted text preview: Reactions of Alkenes Addition Reactions Chapter 6 Reactions with Alkenes Addition and Elimination Addition to Alkenes • Alkenes are electron rich , with the electron density of the π bond concentrated above and below the plane of the molecule. • Alkenes react with electrophiles . Hydrogenation of Alkenes Addition of H to a C=C double bond • Addition of H 2 to a C=C double bond • C=C double bonds b C-C single bonds • Requires a transition metal catalyst (w/o catalyst, H-H bond too strong to be broken) • Exothermic (can be used to estimate stability of alkenes (see Section 6.2)) • Reduction (increase in the number of hydrogens bonded to carbon) Hydrogenation Catalysts • Transition metals – Typically: Pt , Pd, Ni, or Rh – Often adsorbed onto charcoal (carbon) • In this case, the catalyst is often abbreviated Pt/C, Pd/C, etc. • Catalyst generally not soluble in reaction solvent – “Heterogeneous reactions” An adsorbed H H-H broken into 2 H atoms bonded to metal atom adds to each carbon of the C=C π bond of alkene interacts with empty orbitals on metal Hydrogenation of Alkenes • Hydrogenation occurs by “syn addition” – H atoms transferred from catalyst surface to the same face (side) of the double bond Hydrogenation of Alkenes O O O O H 2 Pt O O O O H H • In some cases, the addition is also stereoselective due to steric effects of surface during alkene approach. • Important note on heterogeneous reactions: Must consider the physical constraints imparted by the surface. Hydrogenation of Alkenes Other Addition Reactions to Alkenes Hydrohalogenation Electrophilic Addition of Hydrogen Halides to Alkenes Hydrohalogenation Electrophilic Addition of Hydrogen Halides to Alkenes Mechanism: Hydrohalogenation Electrophilic Addition of Hydrogen Halides to Alkenes Regioselectivity of Hydrohalogenation Markovnikov’s Rule • Markovnikov’s rule: • With an unsymmetrical alkene, the H atom adds to the less substituted carbon atom- that is, the carbon that has the greater number of H atoms to begin with....
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This note was uploaded on 12/22/2011 for the course CHEM 3371 taught by Professor Anderson,k during the Fall '08 term at Southern Methodist.

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Chapter 6 - Reactions of Alkenes Addition Reactions Chapter...

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