Oxidation, Reduction, and Reactions of Hydrocarbons III: Alkanes, Alkenes, and Alkynes

Oxidation of Hydrocarbons

Baeyer's Test

Baeyer's test is a qualitative method in organic chemistry used to detect the presence of unsaturation (double or triple bonds) in organic compounds. The test utilizes an alkaline solution of potassium permanganate (KMnO4), a powerful oxidizing agent.

• Principle: KMnO4 reacts with double or triple bonds, causing the solution to change color from purplish-pink to brown due to the formation of manganese(IV) oxide (MnO2).

• Reaction: The double bond is replaced by hydroxyl groups (OH), resulting in syn-hydroxylation.

• Limitations: The test does not work on alkanes or aromatic compounds. Easily oxidized compounds (e.g., aldehydes, impurities in alcohols/alkanes) may give false positives.

Example Reaction:

• Alkene + KMnO4 + H2O → Glycol (1,2-diol) + MnO2 (brown solid)

• Alkyne + KMnO4 + H2O → Diketone or carboxylic acids + MnO2

Equation:

• $\ce{RCH=CHR + KMnO4 + H2O \rightarrow RCH(OH)-CH(OH)R + MnO2}$

Oxidation of Alkenes: Syn-Hydroxylation

Potassium permanganate oxidizes alkenes to produce 1,2-diols (glycols) via syn-hydroxylation, where both hydroxyl groups add to the same side of the double bond.

• Reagent: Cold, dilute, and basic KMnO4 is used to limit further oxidation.

• Mechanism: The reaction proceeds through a cyclic manganate ester intermediate.

• Product: 1,2-ethanediol (ethylene glycol) from ethene.

Equation:

• $\ce{H2C=CH2 + KMnO4 + H2O \rightarrow HOCH2CH2OH}$

Oxidative Cleavage of Alkenes and Alkynes

Strong oxidizing agents (hot, concentrated KMnO4 or ozone) can cleave double or triple bonds, breaking the unsaturation and forming highly oxidized products.

• Hot basic KMnO4: Cleaves alkenes to carboxylates, ketones, or CO2 depending on substitution.

• Ozonolysis: Cleavage with ozone followed by zinc/acetic acid workup yields aldehydes, ketones, or formaldehyde.

Product Table:

Type of Carbon	KMnO4 Cleavage Product	Ozonolysis Product
Unsubstituted	CO2	Formaldehyde
Monosubstituted	Carboxylate	Aldehyde
Disubstituted	Ketone	Ketone

Example:

• $\ce{CH3CH=CHCH3 + KMnO4, OH^-, heat \rightarrow 2 CH3COO^-}$ (Acetate ion)

• $\ce{CH3CH2C#CCH3 + KMnO4, OH^-, heat \rightarrow CH3COO^- + O=C=O + H2O}$

Ozonolysis of Alkenes and Alkynes

Ozonolysis is a method for cleaving double or triple bonds using ozone (O3), followed by reduction with zinc in acetic acid. This produces carbonyl compounds (aldehydes, ketones, or carboxylic acids).

• Alkenes: Ozone reacts to form unstable ozonides, which are reduced to carbonyl compounds.

• Alkynes: Internal alkynes yield two carboxylic acids; terminal alkynes yield a carboxylic acid and CO2.

Equation:

• $\ce{RCH=CHR' + O3 \rightarrow [Ozonide] \xrightarrow{Zn/HOAc} RCHO + R'CHO}$

• $\ce{RC#CR' + O3 \rightarrow RCOOH + R'COOH}$ (internal)

• $\ce{RC#CH + O3 \rightarrow RCOOH + CO2}$ (terminal)

Reduction of Alkynes

Complete Reduction to Alkanes

Alkynes can be fully reduced to alkanes by catalytic hydrogenation using a metal catalyst such as palladium on carbon (Pd/C).

• Reaction: Addition of H2 over Pd/C converts alkynes to alkanes.

• Equation: $\ce{HC#CH + 2 H2 \xrightarrow{Pd/C} CH3CH3}$

Partial Reduction: Formation of Cis- and Trans-Alkenes

• Cis-Alkene Formation (Lindlar's Catalyst): Hydrogenation using Lindlar's catalyst (deactivated palladium on calcium carbonate) produces cis-alkenes via syn addition.

• Trans-Alkene Formation (Dissolving Metal Reduction): Sodium or lithium in liquid ammonia reduces alkynes to trans-alkenes via a radical anion intermediate.

Equations:

• $\ce{RC#CR' + H2 \xrightarrow{Lindlar} cis-RCH=CHR'}$

• $\ce{RC#CR' + Na/NH3 \rightarrow trans-RCH=CHR'}$

Example:

• 3-hexyne + Lindlar's catalyst + H2 → cis-3-hexene

• 3-hexyne + Na/NH3 → trans-3-hexene

Acetylide Ions

Formation and Properties

Terminal alkynes can be deprotonated by strong bases to form acetylide ions, which are strong carbon nucleophiles.

• Acidity: Alkynes are more acidic than alkenes and alkanes due to increased s-character of the sp-hybridized carbon.

• Base Used: Sodium amide (NaNH2) is commonly used to deprotonate terminal alkynes.

Acidity Table (pKa values):

Compound	pKa
HCl	-8.0
CH3COOH	4.76
H2O	15.7
HC≡CH	24
HC=CH	44
CH3CH3	50

Equation:

• $\ce{RC#CH + NaNH2 \rightarrow RC#C^- Na^+ + NH3}$

Reactions of Acetylide Ions

Acetylide ions act as nucleophiles and react with alkyl halides to form new carbon-carbon bonds via alkylation.

• Alkylation: Reaction with 1° alkyl halides (RCH2X) leads to chain extension.

• Elimination: 2° and 3° alkyl halides undergo elimination (E2 mechanism) due to steric hindrance.

Equation:

• $\ce{RC#C^- + R'CH2X \rightarrow RC#CR' + X^-}$

Example:

• 1-pentyne + NaNH2 → acetylide ion; acetylide ion + 1° alkyl halide → extended alkyne

Hydrocarbon Combustion

Complete Combustion

Alkanes undergo combustion in the presence of oxygen to produce carbon dioxide and water, releasing energy.

• Equation: $\ce{CH4 + 2 O2 \rightarrow CO2 + 2 H2O}$

• Every C-H and C-C bond in the starting material is converted to a C-O bond in the product.

Incomplete Combustion

In the absence of sufficient oxygen, incomplete combustion occurs, forming carbon monoxide (CO) and water.

• General Equation: $\ce{C_nH_{2n+2} + \frac{n}{2} O2 \rightarrow (n+1) H2O + n CO}$

• For methane: $\ce{2 CH4 + 3 O2 \rightarrow 2 CO + 4 H2O}$

• Oxidation of saturated hydrocarbons is the basis for their use as energy sources for heat.

Summary Table

Topic	Key Reagents/Conditions	Main Products
Oxidation (Baeyer's Test)	KMnO4 (alkaline)	Diols, brown MnO2
Oxidative Cleavage	Hot KMnO4, Ozone/Zn/HOAc	Carboxylates, aldehydes, ketones, CO2
Reduction	Pd/C, Lindlar's catalyst, Na/NH3	Alkanes, cis-alkenes, trans-alkenes
Acetylide Ion Formation	NaNH2, LDA, tBuLi	Acetylide ions
Combustion	O2 (excess or limited)	CO2, H2O, CO

Knowledge Check Examples

• Which reaction conditions best convert 3-hexyne to cis-3-hexene? Answer: Lindlar's catalyst and H2

• Classify the following transformation as oxidation or reduction: Alkene to alkane: Reduction Aldehyde to alcohol: Reduction

References

• Carey, F.A. (2008) Organic Chemistry 7th ed. McGraw Hill

• McMurry, J. (2008) Organic Chemistry 7th ed. Thomson Brooks Cole

• Bruice, P.Y. (2017) Organic Chemistry 8th ed. Prentice Hall International

• Brown W.H., Poon, T. (2016) Introduction to Organic Chemistry 6th ed. Wiley