(1249) Topic 6# Reactions of Alkenes Part 6

Hydrogenation Reactions of Alkenes

  • Definition: Hydrogenation reactions involve the addition of two hydrogen atoms to the carbon-carbon double bonds in alkenes.

    • General Reaction: Alkene + H2 in the presence of a catalyst (e.g., palladium on carbon) at room temperature leads to an alkane.

  • Catalytic Hydrogenation:

    • Common catalysts include palladium (Pd), platinum (Pt), and nickel (Ni), which lower the activation energy needed for the reaction.

    • The presence of these catalysts allows the reaction to occur at lower temperatures (e.g., 25°C) and under normal atmospheric conditions.

    • The reaction results in the formation of cis products, meaning the two hydrogens added to the alkene are oriented in the same direction.

  • Stereochemistry:

    • The addition of hydrogen is stereospecific, always producing cis isomers of the product due to both hydrogens being added from the same face of the double bond.

  • Mechanism Overview:

    • The metal catalyst activates hydrogen molecules, weakening H-H bonds, which facilitates the reaction with the alkene's pi system.

    • Despite the mechanism not being crucial for memorization, understanding the catalytic role of metals is important.

  • Example Problem:

    • Unsymmetrical alkene + H2 yields two possible products with a mixture of cis hydrogens, resulting in a 50/50 mixture.

Relative Stabilities of Alkenes

  • Trans-2-butene vs. Cis-2-butene:

    • Trans-2-butene has methyl groups that face opposite directions, reducing steric strain, leading to lower heat of hydrogenation (-27.6 kcal/mol).

    • Cis-2-butene, with methyl groups facing each other, experiences steric interactions, resulting in higher heat of hydrogenation (-28.6 kcal/mol).

    • Conclusion: Trans-2-butene is more stable than cis-2-butene based on heat of hydrogenation values.

  • Substituted Alkenes:

    • The more substituted the alkene, the more stable it is, indicated by lower heats of hydrogenation.

    • Stability order: Least substituted < More substituted < Heavily substituted

  • Molecular Orbital Considerations:

    • Trans-2-butene displays minimized steric interactions among substituents compared to cis-2-butene due to spatial orientation.

Dihydroxylation Reactions of Alkenes

  • Conversion to Diols:

    • Hydroxylation reactions add two hydroxyl (OH) groups to the carbon-carbon double bonds, forming diols or glycols.

    • Common Reagents:

      • Potassium Permanganate (KMnO4): Cold conditions yield dihydroxy products (cis).

        • Heat causes complete oxidation, breaking the carbon-carbon bonds.

      • Osmium Tetroxide (OsO4): Forms diols; reaction occurs in two steps using an organic solvent (CH2Cl2) and a reducing agent (NaHSO3) to convert the osmium ester to diols.

  • Stereochemistry:

    • Both methods produce cis-diols due to the syn addition from the same face of the alkene.

  • Mechanistic Insight:

    • Alkene reacts with OsO4, forming an osmium ester, which is reduced to diols in the subsequent step by sodium bisulfite. This ensures both OH groups are introduced from the same side of the alkene.