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.
• $2 CH<em>4 \rightarrow C</em>2H<em>6 + H</em>2$ 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 ($H<em>2O</em>2$).
• 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.