16.6 Chemical Reactions of Alkenes | Hydrocarbons | Chemistry 11

Alkenes are characterized by their reactivity, which is centered around the carbon-carbon double bond. They primarily undergo addition reactions, where a molecule adds across the double bond. This process converts the $π$ -bond into two new $σ$ -bonds, resulting in a saturated product.

1. Hydrogenation

Hydrogenation (catalytic reduction) is the addition of hydrogen gas ( $H_{2}$ ) to an alkene in the presence of a metal catalyst to form the corresponding alkane.

Conditions: Finely divided Ni at $250$ – $30 0^{\circ} C$ , or Pt/Pd at room temperature.
Reaction: $Ethene C H_{2} = C H_{2} + H_{2} N i / P t heat Ethane C H_{3} - C H_{3}$
Industrial Application: Used to convert unsaturated vegetable oils (containing C=C bonds) into saturated solid fats (margarine/ghee).

2. Hydrohalogenation

Hydrohalogenation is the addition of a hydrogen halide ( $H X$ , e.g., $H B r$ or $H C l$ ) to an alkene to form a halogenoalkane (alkyl halide).

Reaction with propene: $Propene C H_{3} - C H = C H_{2} + H B r \to 2-Bromopropane (Major) C H_{3} - C H B r - C H_{3} + 1-Bromopropane (Minor) C H_{3} - C H_{2} - C H_{2} B r$

Markovnikov's Rule

Markovnikov's Rule: When an unsymmetrical reagent (like $H$ – $X$ ) adds to an unsymmetrical alkene, the negative part ( $X^{-}$ ) attaches to the carbon of the double bond that has the fewer hydrogen atoms (the more substituted carbon).

Mechanism of Electrophilic Addition

Electrophilic Attack: $H^{+}$ from $H B r$ attacks the $π$ -bond of propene, forming two possible carbocation intermediates:
- Path A: More stable secondary (2°) carbocation → leads to 2-bromopropane (major product)
- Path B: Less stable primary (1°) carbocation → leads to 1-bromopropane (minor product)

Figure 16.6.1: Electrophilic addition of HBr to propene showing two possible carbocation intermediates — Electrophilic addition of HBr to propene showing two possible carbocation intermediates.

Carbocation Stability Order: $Tertiary (3°) > Secondary (2°) > Primary (1°)$ Stability increases with more alkyl groups due to the positive inductive effect — electron-donating alkyl groups help disperse the positive charge.
Nucleophilic Attack: $B r^{-}$ attacks the more stable secondary carbocation → 2-bromopropane (major product).

3. Hydration of Alkenes

Hydration is the addition of water ( $H_{2} O$ ) across the double bond to produce an alcohol, requiring a strong acid catalyst (e.g., $H_{2} S O_{4}$ ).

Industrial Application: Primary method for industrial production of ethanol.
Mechanism (Two Steps):

Step 1 — Formation of Ethyl Hydrogen Sulphate: $Ethene C H_{2} = C H_{2} + H_{2} S O_{4} \to Ethyl hydrogen sulphate C H_{3} - C H_{2} - O S O_{3} H$

Step 2 — Hydrolysis: $Ethyl hydrogen sulphate C H_{3} - C H_{2} - O S O_{3} H + H_{2} O heat Ethanol C H_{3} - C H_{2} - O H + H_{2} S O_{4}$

The $H_{2} S O_{4}$ is regenerated, confirming its role as a catalyst.

4. Halogenation

Halogenation is the addition of a halogen molecule ( $X_{2}$ , e.g., $C l_{2}$ , $B r_{2}$ ) across the double bond to form a dihalogenoalkane.

Reaction: Ethene reacts with bromine in $C C l_{4}$ : $Ethene C H_{2} = C H_{2} + B r_{2} C C l_{4} 1,2-Dibromoethane B r - C H_{2} - C H_{2} - B r$
Test for Unsaturation:
- Bromine water test: The reddish-brown colour of $B r_{2} / H_{2} O$ is rapidly decolorized by alkenes.
- Baeyer's Test: Cold dilute alkaline $K M n O_{4}$ (purple) is decolorized, forming a colourless vicinal diol (glycol). This is the more specific test for unsaturation.

5. Halohydration

Halohydration is the addition of a halogen and a hydroxyl group across a double bond using a hypohalous acid ( $H O X$ ).

Reaction: Ethene reacts with hypobromous acid ( $H O B r$ ): $Ethene C H_{2} = C H_{2} + H O B r \to 2-Bromoethanol H O - C H_{2} - C H_{2} - B r$

6. Epoxidation

Epoxidation is the reaction of an alkene with a peracid (e.g., peroxyacetic acid, $C H_{3} C O O O H$ ) to form an epoxide — a three-membered ring containing an oxygen atom.

Reaction: Ethene reacts with peroxyacetic acid: $Ethene C H_{2} = C H_{2} + C H_{3} C O O O H \to Epoxyethane (Ethylene oxide) C H_{2} - C H_{2} O + C H_{3} C O O H$

Epoxides are highly reactive three-membered ring compounds used as intermediates in organic synthesis.

7. Ozonolysis

Ozonolysis is the cleavage of the C=C double bond using ozone ( $O_{3}$ ), followed by workup to give carbonyl compounds.

Mechanism:
1. Ozone adds across the double bond to form an unstable ozonide intermediate.
2. The ozonide is cleaved by workup:
  - Reductive workup ( $Z n / H_{2} O$ ): gives aldehydes or ketones (no further oxidation).
  - Oxidative workup ( $H_{2} O_{2}$ ): gives carboxylic acids from terminal alkenes.
Example (Reductive workup): $Ethene C H_{2} = C H_{2} 1. O_{3} 2. Z n / H_{2} O 2 Formaldehyde H C H O$
Application: Ozonolysis is used to determine the position of the double bond in an unknown alkene by identifying the carbonyl fragments produced.

1. Hydrogenation

Hydrogenation (catalytic reduction) is the addition of hydrogen gas ( $H_{2}$ ) to an alkene in the presence of a metal catalyst to form the corresponding alkane.

Conditions: Finely divided Ni at $250$ – $30 0^{\circ} C$ , or Pt/Pd at room temperature.
Reaction: $Ethene C H_{2} = C H_{2} + H_{2} N i / P t heat Ethane C H_{3} - C H_{3}$
Industrial Application: Used to convert unsaturated vegetable oils (containing C=C bonds) into saturated solid fats (margarine/ghee).

2. Hydrohalogenation

Hydrohalogenation is the addition of a hydrogen halide ( $H X$ , e.g., $H B r$ or $H C l$ ) to an alkene to form a halogenoalkane (alkyl halide).

Reaction with propene: $Propene C H_{3} - C H = C H_{2} + H B r \to 2-Bromopropane (Major) C H_{3} - C H B r - C H_{3} + 1-Bromopropane (Minor) C H_{3} - C H_{2} - C H_{2} B r$

Markovnikov's Rule

Mechanism of Electrophilic Addition

Electrophilic Attack: $H^{+}$ from $H B r$ attacks the $π$ -bond of propene, forming two possible carbocation intermediates:
- Path A: More stable secondary (2°) carbocation → leads to 2-bromopropane (major product)
- Path B: Less stable primary (1°) carbocation → leads to 1-bromopropane (minor product)

Carbocation Stability Order: $Tertiary (3°) > Secondary (2°) > Primary (1°)$ Stability increases with more alkyl groups due to the positive inductive effect — electron-donating alkyl groups help disperse the positive charge.
Nucleophilic Attack: $B r^{-}$ attacks the more stable secondary carbocation → 2-bromopropane (major product).

3. Hydration of Alkenes

Hydration is the addition of water ( $H_{2} O$ ) across the double bond to produce an alcohol, requiring a strong acid catalyst (e.g., $H_{2} S O_{4}$ ).

Industrial Application: Primary method for industrial production of ethanol.
Mechanism (Two Steps):

Step 1 — Formation of Ethyl Hydrogen Sulphate: $Ethene C H_{2} = C H_{2} + H_{2} S O_{4} \to Ethyl hydrogen sulphate C H_{3} - C H_{2} - O S O_{3} H$

Step 2 — Hydrolysis: $Ethyl hydrogen sulphate C H_{3} - C H_{2} - O S O_{3} H + H_{2} O heat Ethanol C H_{3} - C H_{2} - O H + H_{2} S O_{4}$

The $H_{2} S O_{4}$ is regenerated, confirming its role as a catalyst.

4. Halogenation

Halogenation is the addition of a halogen molecule ( $X_{2}$ , e.g., $C l_{2}$ , $B r_{2}$ ) across the double bond to form a dihalogenoalkane.

Reaction: Ethene reacts with bromine in $C C l_{4}$ : $Ethene C H_{2} = C H_{2} + B r_{2} C C l_{4} 1,2-Dibromoethane B r - C H_{2} - C H_{2} - B r$
Test for Unsaturation:
- Bromine water test: The reddish-brown colour of $B r_{2} / H_{2} O$ is rapidly decolorized by alkenes.
- Baeyer's Test: Cold dilute alkaline $K M n O_{4}$ (purple) is decolorized, forming a colourless vicinal diol (glycol). This is the more specific test for unsaturation.

5. Halohydration

Halohydration is the addition of a halogen and a hydroxyl group across a double bond using a hypohalous acid ( $H O X$ ).

Reaction: Ethene reacts with hypobromous acid ( $H O B r$ ): $Ethene C H_{2} = C H_{2} + H O B r \to 2-Bromoethanol H O - C H_{2} - C H_{2} - B r$

6. Epoxidation

Epoxidation is the reaction of an alkene with a peracid (e.g., peroxyacetic acid, $C H_{3} C O O O H$ ) to form an epoxide — a three-membered ring containing an oxygen atom.

Reaction: Ethene reacts with peroxyacetic acid: $Ethene C H_{2} = C H_{2} + C H_{3} C O O O H \to Epoxyethane (Ethylene oxide) C H_{2} - C H_{2} O + C H_{3} C O O H$

Epoxides are highly reactive three-membered ring compounds used as intermediates in organic synthesis.

7. Ozonolysis

Ozonolysis is the cleavage of the C=C double bond using ozone ( $O_{3}$ ), followed by workup to give carbonyl compounds.

Mechanism:
1. Ozone adds across the double bond to form an unstable ozonide intermediate.
2. The ozonide is cleaved by workup:
  - Reductive workup ( $Z n / H_{2} O$ ): gives aldehydes or ketones (no further oxidation).
  - Oxidative workup ( $H_{2} O_{2}$ ): gives carboxylic acids from terminal alkenes.
Example (Reductive workup): $Ethene C H_{2} = C H_{2} 1. O_{3} 2. Z n / H_{2} O 2 Formaldehyde H C H O$
Application: Ozonolysis is used to determine the position of the double bond in an unknown alkene by identifying the carbonyl fragments produced.