o chem reading one 12.1-12.3 12.5
Overview of Chapter 12 Concepts
Focus on oxidation and reduction of alkynes and alkenes.
Comparison of various organic compounds: polar CX sigma bonds, alcohols, alkyl halides, and epoxides.
Discussion of mechanisms, often less certain than those of SN reactions.
Oxidation and Reduction
Definition:
Oxidation: Increase in carbon-electronegative bonds or decrease in C-H bonds.
Reduction: Decrease in carbon-electronegative bonds or increase in C-H bonds.
Example:
CH4 can be oxidized to CO by replacing C-H with C-O bonds.
Notation:
Oxidation: denoted as [ O ]
Reduction: denoted as [ H ]
Reducing Agents
Types:
Molecular H2:
Used with metal catalysts (e.g., Pd, Pt, Ni).
Dissolving Metal Reductions:
Uses Na and NH3; delivers 2 protons and electrons.
Hydride Reducing Agents:
Common agents include NaBH4 and LiAlH4; provide H-.
Reduction of Alkenes
Process:
Addition of H2 to an alkene leads to alkane formation; pi bond breaks.
Catalytic Hydrogenation:
Requires metal catalyst for H2 addition; results in syn addition.
Heat of Hydrogenation:
Exothermic; can indicate alkene stability (trans alkene more stable than cis).
Mechanism of Catalytic Hydrogenation
H2 binds to metal catalyst and approaches alkene.
Steps:
H2 attaches to catalyst; bond cleavage.
Alkene binds to catalyst; pi bond interacts.
Sequential H addition to alkene.
Alkane released from catalyst.
Reduction of Alkynes
Methods:
2 equivalents of H2 form alkenes or alkanes.
Cis Alkene Formation:
Syn addition with one equivalent of H2.
Trans Alkene Formation:
Achieved via dissolving metal reduction (e.g., Na and NH3).
Mechanism of Dissolving Metal Reduction
Steps for Trans Alkene Production:
Electron addition forms radical anion.
Protonation yields radical.
Further electron addition forms carbanion.
Protonation forms stable trans alkene.
More stable configurations formed preferentially due to sterics.
Summary of Triple Bond Reduction Methods
Various methods producing distinct alkene results (cis or trans depending on the approach).
When determining which compounds will react with hydrogen gas (H2) in the presence of palladium on carbon (Pd/C) but not with Lindlards catalyst, the key lies in understanding the differences between these two catalysts.
1. Pd/C (Palladium on Carbon):
Pd/C is a broad-spectrum hydrogenation catalyst. It generally reduces both alkynes and alkenes to alkanes.
Pd/C can fully hydrogenate compounds, meaning it will completely reduce alkynes (to alkanes) or alkenes (to alkanes) depending on the starting material.
2. Lindlars Catalyst:
Lindlarscatalyst is a poisoned form of palladium (usually Pd/C with lead acetate or quinoline), which slows down the hydrogenation process.
It is selective for the hydrogenation of alkynes to cis-alkenes (Z-alkenes) without further reducing them to alkanes. So, Lindlars catalyst will stop at the alkene stage, not fully reducing the compound.
Determining Reactivity:
Alkynes: Compounds with alkynes will react with Pd/C to give alkanes, but they will react with Lindlars catalyst to form cis-alkenes (not alkanes).
Alkenes: Compounds with alkenes are unlikely to react with Lindlars catalyst (since Lindlars catalyst is for alkynes), but will react with Pd/C to form alkanes.
Key Rule:
Compounds that will react with Pd/C but not Lindlars catalyst are typically alkenes or alkynes that would be reduced all the way to alkanes under Pd/C conditions but stop at cis-alkenes with Lindlars catalyst.
Alkenes will undergo complete hydrogenation with Pd/C to form alkanes, but wont be affected by Lindlars catalyst (which only affects alkynes).
In summary, alkenes and alkynes that are not conjugated (especially internal alkynes) will be selectively reduced with Pd/C, but will not react with Lindlars catalyst beyond the formation of a cis-alkene.