Oxidation Reactions of Alkenes and Alkynes

1. Introduction to Oxidation Reactions

Oxidation reactions of alkenes and alkynes involve the addition of oxygen atoms or the removal of hydrogen atoms. These reactions are fundamental in organic chemistry and have significant synthetic applications.

Definition: Oxidation in organic chemistry typically involves:

Addition of oxygen atoms
Removal of hydrogen atoms
Increase in oxidation state of carbon

Types of Oxidation Reactions

Type	Substrate	Product	Common Reagents
Mild Oxidation	Alkenes	Diols, Epoxides	OsO₄, mCPBA
Strong Oxidation	Alkenes	Ketones, Carboxylic acids	KMnO₄, K₂Cr₂O₇
Ozonolysis	Alkenes/Alkynes	Aldehydes, Ketones	O₃, Zn/CH₃COOH

2. Oxidation of Alkenes

2.1 Combustion (Complete Oxidation)

General Reaction:
C_nH_2n + (3n/2)O₂ → [Heat] nCO₂ + nH₂O + Heat

Example 1: Ethene Combustion
CH₂=CH₂ + 3O₂ → [Heat] 2CO₂ + 2H₂O + 1411 kJ/mol

Example 2: Propene Combustion
CH₃CH=CH₂ + (9/2)O₂ → [Heat] 3CO₂ + 3H₂O

2.2 Oxidation with Potassium Permanganate (KMnO₄)

2.2.1 Cold, Dilute KMnO₄ (Baeyer's Test)

Reaction:
R-CH=CH-R' + KMnO₄ + H₂O → [Cold, dilute] R-CH(OH)-CH(OH)-R' + MnO₂ + KOH

Example: Ethene with cold KMnO₄
3CH₂=CH₂ + 2KMnO₄ + 4H₂O → [0°C] 3CH₂(OH)-CH₂(OH) + 2MnO₂ + 2KOH
Product: Ethylene glycol (1,2-ethanediol)

2.2.2 Hot, Concentrated KMnO₄ (Oxidative Cleavage)

Mechanism: The reaction proceeds through formation of diol intermediate, followed by cleavage of C-C bond.

Example 1: But-2-ene oxidation
CH₃CH=CHCH₃ + 4KMnO₄ + 6H₂SO₄ → [Heat] 2CH₃COOH + 4MnSO₄ + 2K₂SO₄ + 6H₂O
Product: Acetic acid

Example 2: Propene oxidation
CH₃CH=CH₂ + 2KMnO₄ + 3H₂SO₄ → [Heat] CH₃COOH + CO₂ + 2MnSO₄ + K₂SO₄ + 4H₂O

2.3 Oxidation with Potassium Dichromate (K₂Cr₂O₇)

General Reaction:
3R-CH=CH-R' + 2K₂Cr₂O₇ + 8H₂SO₄ → [Heat] 3R-COOH + 3R'-COOH + 2Cr₂(SO₄)₃ + 2K₂SO₄ + 8H₂O

Example: Cyclohexene oxidation
3C₆H₁₀ + 8K₂Cr₂O₇ + 32H₂SO₄ → [Heat] 3HOOC-(CH₂)₄-COOH + 8Cr₂(SO₄)₃ + 8K₂SO₄ + 32H₂O
Product: Adipic acid

3. Ozonolysis

Ozonolysis is the cleavage of alkenes and alkynes using ozone (O₃), followed by reduction to produce carbonyl compounds.

3.1 Ozonolysis of Alkenes

3.1.1 Mechanism

Step 1: Formation of Molozonide (1,2,3-trioxolane)
R₂C=CR₂ + O₃ → R₂C-O-O-O-CR₂ (Molozonide)

Step 2: Rearrangement to Ozonide (1,2,4-trioxolane)
Molozonide → R₂C-O-CR₂ (with O-O bridge)

Step 3: Reduction
Ozonide + Zn/CH₃COOH → R₂C=O + R₂C=O

3.1.2 Reductive Workup

With Zinc/Acetic acid:
R₂C=CR₂ + O₃ → [Zn/CH₃COOH] R₂C=O + R₂C=O

3.1.3 Oxidative Workup

With Hydrogen Peroxide:
R₂C=CHR + O₃ → [H₂O₂] R₂C=O + RCOOH

Example 1: But-2-ene ozonolysis
CH₃CH=CHCH₃ + O₃ → [Zn/CH₃COOH] 2CH₃CHO
Product: Acetaldehyde

Example 2: 2-Methylpropene ozonolysis
(CH₃)₂C=CH₂ + O₃ → [Zn/CH₃COOH] (CH₃)₂C=O + CH₂O
Products: Acetone + Formaldehyde

Example 3: Styrene ozonolysis (Oxidative workup)
C₆H₅CH=CH₂ + O₃ → [H₂O₂] C₆H₅CHO + HCOOH
Products: Benzaldehyde + Formic acid

3.2 Ozonolysis of Alkynes

General Reaction:
R-C≡C-R' + 2O₃ → [H₂O] RCOOH + R'COOH

Example 1: But-2-yne ozonolysis
CH₃C≡CCH₃ + 2O₃ → [H₂O] 2CH₃COOH
Product: Acetic acid

Example 2: Phenylacetylene ozonolysis
C₆H₅C≡CH + 2O₃ → [H₂O] C₆H₅COOH + HCOOH
Products: Benzoic acid + Formic acid

4. Dihydroxylation

Dihydroxylation is the addition of two hydroxyl groups across a double bond to form vicinal diols (glycols).

4.1 Syn-Dihydroxylation with Osmium Tetroxide (OsO₄)

4.1.1 Mechanism

Step 1: Formation of Cyclic Osmate Ester
R₂C=CR₂ + OsO₄ → Cyclic Osmate Ester

Step 2: Hydrolysis
Osmate Ester + H₂O → R₂C(OH)-C(OH)R₂ + OsO₂(OH)₂

Catalytic Process (Upjohn Process):
R₂C=CR₂ + OsO₄ (cat.) + NMO + H₂O → [syn addition] R₂C(OH)-C(OH)R₂ + NMO
NMO = N-Methylmorpholine N-oxide

Example 1: Cyclohexene dihydroxylation
C₆H₁₀ + OsO₄ + NMO + H₂O → [syn] cis-1,2-cyclohexanediol

Example 2: But-2-ene dihydroxylation
CH₃CH=CHCH₃ + OsO₄ + H₂O₂ → [syn] CH₃CH(OH)CH(OH)CH₃
Product: 2,3-butanediol

4.2 Anti-Dihydroxylation

4.2.1 Via Epoxide Formation and Ring Opening

Step 1: Epoxidation
R₂C=CR₂ + mCPBA → Epoxide

Step 2: Acid-catalyzed ring opening
Epoxide + H₂O/H⁺ → [anti addition] R₂C(OH)-C(OH)R₂

Example: Cyclohexene anti-dihydroxylation
Step 1: C₆H₁₀ + mCPBA → Cyclohexene oxide
Step 2: Cyclohexene oxide + H₂O/H⁺ → [anti] trans-1,2-cyclohexanediol

5. Epoxidation

Epoxidation is the formation of three-membered cyclic ethers (oxiranes) from alkenes through oxidation.

5.1 Epoxidation with Peroxyacids

5.1.1 Mechanism

Concerted Mechanism:
The peroxyacid transfers oxygen to the alkene in a single step through a cyclic transition state.

R₂C=CR₂ + R'COOOH → R₂C-CR₂ (with O bridge) + R'COOH

5.1.2 Common Peroxyacids

Peroxyacid	Abbreviation	Structure	Notes
meta-Chloroperoxybenzoic acid	mCPBA	m-ClC₆H₄COOOH	Most common, stable
Peroxyacetic acid	CH₃COOOH	CH₃COOOH	Less stable
Peroxyformic acid	HCOOOH	HCOOOH	Very reactive

Example 1: Ethene epoxidation
CH₂=CH₂ + mCPBA → [CH₂Cl₂] CH₂-CH₂ (with O bridge) + m-ClC₆H₄COOH
Product: Ethylene oxide (oxirane)

Example 2: Cyclohexene epoxidation
C₆H₁₀ + CH₃COOOH → [CHCl₃] Cyclohexene oxide + CH₃COOH

Example 3: Styrene epoxidation
C₆H₅CH=CH₂ + mCPBA → [CH₂Cl₂, 0°C] C₆H₅CH-CH₂ (with O bridge)
Product: Styrene oxide

5.2 Asymmetric Epoxidation (Sharpless Epoxidation)

Sharpless Asymmetric Epoxidation:
Allylic alcohol + tBuOOH + Ti(OiPr)₄ + (+)-DET → [CH₂Cl₂, -20°C] Epoxy alcohol (high ee)
DET = Diethyl tartrate, ee = enantiomeric excess

5.3 Industrial Epoxidation

Silver-Catalyzed Ethylene Epoxidation:
CH₂=CH₂ + ½O₂ → [Ag catalyst, 250°C] CH₂-CH₂ (with O bridge)

6. Oxidation of Alkynes

6.1 Oxidative Cleavage with KMnO₄

General Reaction:
R-C≡C-R' + 4KMnO₄ + 6H₂SO₄ → [Heat] RCOOH + R'COOH + 4MnSO₄ + 2K₂SO₄ + 6H₂O

Example 1: Hex-3-yne oxidation
CH₃CH₂C≡CCH₂CH₃ + 4KMnO₄ + 6H₂SO₄ → [Heat] 2CH₃CH₂COOH + 4MnSO₄ + 2K₂SO₄ + 6H₂O
Product: Propionic acid

Example 2: Terminal alkyne oxidation
CH₃CH₂C≡CH + 2KMnO₄ + 3H₂SO₄ → [Heat] CH₃CH₂COOH + CO₂ + 2MnSO₄ + K₂SO₄ + 3H₂O
Products: Propionic acid + CO₂

6.2 Oxidation with K₂Cr₂O₇

Reaction:
3R-C≡C-R' + 4K₂Cr₂O₇ + 16H₂SO₄ → [Heat] 3RCOOH + 3R'COOH + 4Cr₂(SO₄)₃ + 4K₂SO₄ + 16H₂O

6.3 Partial Oxidation - Formation of α-Diketones

With mild oxidizing agents:
R-C≡C-R' + KMnO₄ → [Cold, dilute] R-CO-CO-R'
α-Diketone formation

Example: Diphenylacetylene oxidation
C₆H₅C≡CC₆H₅ + KMnO₄ → [Cold, dilute] C₆H₅COCOC₆H₅
Product: Benzil (diphenyl α-diketone)

6.4 Ruthenium-Catalyzed Oxidation

Selective oxidation to ketones:
R-C≡C-H + H₂O → [RuCl₃/NaIO₄] R-CO-CH₃

Example: 1-Hexyne to 2-hexanone
CH₃(CH₂)₃C≡CH + H₂O → [RuCl₃/NaIO₄, CH₃CN/H₂O] CH₃(CH₂)₃COCH₃
Product: 2-Hexanone

7. Comparison and Summary

7.1 Oxidation Methods Comparison

Reagent	Conditions	Alkene Products	Alkyne Products	Selectivity
KMnO₄ (cold, dilute)	0°C, pH ~8	Vicinal diols	α-Diketones	Syn addition
KMnO₄ (hot, conc.)	Heat, acidic	Carboxylic acids/ketones	Carboxylic acids	Oxidative cleavage
K₂Cr₂O₇	Heat, H₂SO₄	Carboxylic acids/ketones	Carboxylic acids	Oxidative cleavage
O₃/Zn	-78°C, then Zn/AcOH	Aldehydes/ketones	Carboxylic acids	Reductive workup
O₃/H₂O₂	-78°C, then H₂O₂	Ketones/carboxylic acids	Carboxylic acids	Oxidative workup
OsO₄	Room temp, with co-oxidant	Vicinal diols	Not applicable	Syn dihydroxylation
mCPBA	Room temp, CH₂Cl₂	Epoxides	Not applicable	Stereospecific

7.2 Product Prediction Rules

For Alkenes:

Terminal alkenes (RCH=CH₂): Give RCOOH + CO₂ (or HCHO) on strong oxidation
Internal alkenes (R₂C=CR₂): Give two carbonyl compounds on ozonolysis
Cyclic alkenes: Give dicarboxylic acids on strong oxidation
Disubstituted alkenes: Stereochemistry is retained in syn additions (OsO₄, KMnO₄)

For Alkynes:

Terminal alkynes (R-C≡C-H): Give RCOOH + CO₂ on oxidation
Internal alkynes (R-C≡C-R'): Give two carboxylic acids (RCOOH + R'COOH)
Symmetrical alkynes: Give identical products on both sides
Aromatic alkynes: Aromatic ring usually survives mild oxidation

7.3 Synthetic Applications

Industrial Application 1: Ethylene Glycol Production
CH₂=CH₂ → [Ag, O₂] CH₂-CH₂-O → [H₂O, H⁺] HOCH₂CH₂OH
Used in antifreeze, polyester production

Industrial Application 2: Adipic Acid Synthesis
Cyclohexene → [KMnO₄, heat] HOOC(CH₂)₄COOH
Used in nylon-6,6 production

Pharmaceutical Application: Epoxide Intermediates
Alkene → [mCPBA] Epoxide → [Nucleophile] Drug intermediates

7.4 Environmental Considerations

Green Chemistry Aspects:

OsO₄: Toxic and expensive; use catalytic amounts with co-oxidants
KMnO₄: Produces Mn waste; can be recovered and recycled
Ozone: Environmentally benign; decomposes to O₂
H₂O₂: Green oxidant; produces only water as byproduct
Enzyme-catalyzed oxidations: Emerging green alternatives

7.5 Troubleshooting Common Issues

Problem	Cause	Solution
Low yield in epoxidation	Competitive ring opening	Use dry solvents, low temperature
Over-oxidation with KMnO₄	Too concentrated/hot	Use cold, dilute conditions
Ozonide explosion	Concentration, heating	Keep cold, immediate workup
Poor stereoselectivity	Wrong reagent choice	Choose syn (OsO₄) vs anti (mCPBA→hydrolysis)

7.6 Practice Problems

Problem 1: Predict the products of the following reactions:

a) CH₃CH=CHCH₃ + KMnO₄/OH⁻ (cold) → ?
b) (CH₃)₂C=CH₂ + O₃, then Zn/AcOH → ?
c) C₆H₅C≡CH + KMnO₄/H₂SO₄ (hot) → ?

Solutions:
a) CH₃CH(OH)CH(OH)CH₃ (2,3-butanediol)
b) (CH₃)₂C=O + CH₂=O (acetone + formaldehyde)
c) C₆H₅COOH + CO₂ (benzoic acid + carbon dioxide)

Problem 2: Design a synthesis:
Convert cyclohexene to trans-1,2-cyclohexanediol

Solution:
Step 1: Cyclohexene + mCPBA → Cyclohexene oxide
Step 2: Cyclohexene oxide + H₂O/H⁺ → trans-1,2-cyclohexanediol

Key Points to Remember:

Always consider stereochemistry in oxidation reactions
Terminal carbons in ozonolysis give CO₂ or HCHO
Ozonolysis is excellent for structure determination
OsO₄ gives syn-diols, mCPBA→hydrolysis gives anti-diols
Strong oxidants (KMnO₄, K₂Cr₂O₇) cause C-C bond cleavage
Safety: Handle OsO₄ and ozonides with extreme care
Workup conditions determine final products in ozonolysis

8. Summary and Conclusion

Oxidation reactions of alkenes and alkynes represent fundamental transformations in organic chemistry, providing access to a wide variety of functional groups including diols, epoxides, carbonyl compounds, and carboxylic acids. The choice of oxidizing agent and reaction conditions determines both the product distribution and stereochemical outcome.

Strategic Importance:

Structure Determination: Ozonolysis helps identify alkene/alkyne substitution patterns
Synthetic Utility: Provides access to diverse functional groups from simple starting materials
Industrial Relevance: Large-scale production of important chemicals (ethylene glycol, adipic acid)
Stereochemical Control: Allows for both syn and anti addition patterns

Understanding these reactions is crucial for organic synthesis, pharmaceutical development, and industrial chemical production. The mechanisms and selectivity patterns provide the foundation for rational synthetic design and problem-solving in organic chemistry.