Alkene Reactions and Synthesis

Alkene Reactions and Synthesis

Alkene Addition Reactions

  • Addition reactions are classified as reduction, oxidation, or neither.
  • Reduction: Adding hydrogen across a double bond is a net reduction.
    • Adding two atoms (A and B) to an alkene, where both A and B are less electronegative than carbon.
  • Oxidation: Adding two atoms to an alkene, where both atoms are more electronegative than carbon (e.g., addition of bromine).
  • Neither: Adding two different atoms, one less electronegative and one more electronegative than carbon (e.g., addition of water).

Oxidation States of Carbon

  • Qualitatively, oxidation replaces bonds to less electronegative atoms with bonds to more electronegative atoms.
  • Formal oxidation numbers help determine oxidation states.
  • If an atom is less electronegative than carbon, carbon gets a -1 charge; if it's more electronegative, carbon gets a +1 charge.

Examples of Oxidation States

  • Methane (CH_4): Most reduced form of carbon.
    • Oxidation state: -4 (carbon has four bonds to hydrogen, each contributing -1).
  • Methanol (CH_3OH): More oxidized than methane.
    • Oxidation state: -2 (three bonds to hydrogen at -1 each, one bond to oxygen at +1).
  • Formaldehyde (CH_2O):
    • Oxidation state: 0 (two bonds to hydrogen at -1 each, two bonds to oxygen at +1 each).
  • Formic acid (HCOOH):
  • Carbon Dioxide (CO_2): Most oxidized form of carbon.
    • Oxidation state: +4 (four bonds to oxygen, each contributing +1).
  • Carbon Tetrachloride (CCl_4): Same oxidation state as carbon dioxide.

Flammability and Oxidation State

  • More oxidized compounds are less flammable because they are already partially oxidized.
  • Reduced forms of carbon are higher in energy and more flammable (e.g., methane).

Two-Carbon Examples

  • Carbons in methane are slightly more oxidized than in ethane.
  • 2 CH4 \rightarrow C2H6 + H2 has a negative \Delta G (oxidation).
    • Any time hydrogen is a product, oxidation occurs.
    • Any time hydrogen is a reactant, reduction occurs.
    • Ethane: Each carbon has an oxidation state of -3.

Comparison of Ethane and Ethanol

  • Ethanol is partially oxidized compared to ethane.
  • Ethanol has less energy density than ethane.
  • Addition or elimination of water is neither oxidation nor reduction.

Hydroxylation Reactions

  • Hydration: Acid-catalyzed addition of water.
  • Hydroxylation: Addition of two hydroxyl groups (OH).
    • Neither oxidation nor reduction.

Permanganate Hydroxylation

  • Reagent: Potassium permanganate (KMnO_4).
  • Conditions: Basic conditions.
  • Syn hydroxylation (both OH groups add to the same side).
  • Manganese is reduced from +7 to manganese oxide, a brown solid precipitate.
  • Bayer oxidation: Test for alkenes where purple solution turns colorless with brown solid formation.

Osmium Tetroxide Hydroxylation

  • Reagent: Osmium tetroxide (OsO_4).
  • Co-oxidant: Hydrogen peroxide (H2O2).
  • Milder and more selective than potassium permanganate.
  • Also results in syn hydroxylation.

Epoxidation with Peroxyacids

  • Peroxyacids (RCO3H) are strong oxidizers.
  • Metachloroperbenzoic acid (mCPBA) is a common peroxyacid.
  • Reaction with alkenes yields epoxides (oxiranes).
  • Syn addition.

Epoxide Formation Mechanism

  • Similar to Simmons-Smith reaction.
  • Electrophilic oxygen is added to the alkene.

Reactions of Epoxides

  • Protonation makes epoxides more reactive electrophiles.
  • Epoxides react with nucleophiles predictably.

Stereochemistry of Epoxidation

  • Cis-alkenes give meso epoxides.
  • Trans-alkenes give racemic mixtures.

Hydrolysis of Epoxides

  • Acid-catalyzed hydrolysis yields trans-diols.
  • Water acts as a nucleophile to open the epoxide ring in an SN2 fashion.

Comparison of Hydroxylation Reactions

  • Osmium tetroxide/peroxide: Syn addition of hydroxyl groups.
  • mCPBA followed by acidified water: Anti addition of hydroxyl groups via epoxide intermediate.

Ozonolysis

  • Ozone (O_3) cleaves carbon-carbon double bonds to form carbonyl compounds.
  • Done in alcohol solvent (e.g., methanol) at low temperatures.
  • Two-step process: Ozonation followed by reduction.
  • Reducing agents: Dimethyl sulfide (DMS) or zinc and water.
  • Net result: Replacement of C=C with C=O bonds.

Mechanism & outcome of Ozonolysis

  • The first step involves the addition of ozone across the double bond to form an ozonide intermediate (a five-membered ring containing three oxygen atoms).
  • The ozonide is unstable and undergoes rearrangement and cleavage to form carbonyl compounds.
  • The reducing agent is added in the second step to control the final products and prevent over-oxidation.

Synthetic Applications of Ozonolysis

  • Determining the structure of unknown alkenes (historically).
  • Synthesizing carbonyl compounds that are difficult to obtain otherwise.

Ozonolysis of Cyclic Alkenes

  • Yields a single dicarbonyl compound.

Importance

  • High-yielding and clean reaction.

Synthesis Strategies

  • Classify reactions as functional group transformations or carbon-carbon bond formations.

Functional Group Transformation

  • Conversion of one functional group into another (e.g., alcohol to halide) without changing the number of carbons.

Carbon-Carbon Bond Formation

  • Reactions that create new carbon-carbon bonds to build larger molecules.

Retrosynthetic Analysis

  • Thinking backwards from the target molecule to identify suitable starting materials and reactions.

Example Synthesis Problems

  • Converting an alcohol to a selectively deuterated compound.
    • Two-step synthesis: Dehydration to form an alkene, followed by hydroboration with deuterated borane and protonation with carboxylic acid.
  • Converting cyclohexanol to a trans-chlorocyclohexanol.
    • Two-step synthesis: Dehydration to form cyclohexene, followed by addition of chlorine in water to form a halohydrin.

Alkynes

  • Alkynes have two pi bonds.