11.4 Unsaturated Hydrocarbons—Alkenes & Alkynes

Alkenes:

• Alkenes are hydrocarbons with at least one carbon-carbon double bond (C=C).

• General Formula: CnH2n

• Alkenes are sometimes called olefins (from the Greek word “Olefiant,” meaning oil-forming) because they produce oily substances when reacting with halogens (like Cl2, Br2).

Naming of Alkenes (IUPAC System):

• Alkenes are named by replacing the “ane” suffix of alkanes with “ene”.

• Examples:

• Ethane (C2H6) → Ethene (C2H4)

• Propane (C3H8) → Propene (C3H6)

Preparation of Alkenes:

1. From Ethyl Chloride:

• Ethene, sodium chloride, and water are produced when sodium hydroxide (NaOH) solution reacts with ethyl chloride (C2H5Cl).

• Reaction:

• C2H5Cl + NaOH → C2H4 + NaCl + H2O (Ethene, Sodium Chloride, Water)

2. From Ethanol:

• Ethene and water are produced when ethanol (C2H5OH) reacts with concentrated sulfuric acid (H2SO4) at high temperatures.

• Reaction:

• C2H5OH → C2H4 + H2O (Ethene, Water)

Chemical Properties of Alkenes:

• Alkenes are chemically reactive due to the carbon-carbon double bond.

• One of the bonds in the double bond is weaker and more reactive, leading to addition reactions.

1. Addition of Hydrogen (Hydrogenation):

• Ethene reacts with hydrogen (H2) at 180-200°C in the presence of a nickel catalyst to form ethane.

• Reaction:

• C2H4 + H2 → C2H6 (Ethene + Hydrogen → Ethane)

2. Addition of Water (Hydration):

• Ethene reacts with water (H2O) in the presence of phosphoric acid (H3PO4) at high temperatures and pressures to form ethanol.

• Reaction:

• C2H4 + H2O → C2H5OH (Ethene + Water → Ethanol)

• Significance: Ethanol is used as an environmentally friendly fuel and a solvent in the petroleum industry.

3. Addition of Bromine (Halogenation):

• Ethene reacts with bromine (Br2) solution, resulting in the formation of dibromoethane and the disappearance of the red color of bromine, proving that ethene is unsaturated.

• Reaction:

• C2H4 + Br2 → C2H4Br2 (Ethene + Bromine → Dibromoethane)

4. Oxidation by Potassium Permanganate (KMnO4):

• Ethene reacts with potassium permanganate (KMnO4) and potassium hydroxide (KOH), producing ethylene glycol (C2H6O2) and causing the pink color of KMnO4 to disappear, which indicates that ethene is unsaturated.

• Reaction:

• C2H4 + KMnO4 → C2H6O2 (Ethene + KMnO4 → Ethylene Glycol)

5. Polymerization of Ethene:

• Ethene can undergo polymerization, where many ethene molecules (monomers) combine to form polyethene (a polymer) under high pressure and temperature (1000 atm, 200°C) with a small amount of oxygen.

• Reaction:

• nC2H4 → (-C2H4-)n (Ethene → Polyethene)

Alkynes:

• Alkynes are hydrocarbons with at least one carbon-carbon triple bond (C≡C).

• General Formula: CnH2n-2

• Example: Acetylene (C2H2) is the smallest member of the alkyne group.

Naming of Alkynes (IUPAC System):

• Alkynes are named by replacing the “ane” suffix of alkanes with “yne”.

• Examples:

• Ethane (C2H6) → Ethyne (C2H2)

• Propane (C3H8) → Propyne (C3H6)

• Butane (C4H10) → Butyne (C4H6)

Preparation of Alkynes:

1. From Calcium Carbide:

• Ethyne (C2H2) and calcium hydroxide (Ca(OH)2) are produced when water reacts with calcium carbide (CaC2).

• Reaction:

• CaC2 + 2H2O → C2H2 + Ca(OH)2 (Calcium Carbide + Water → Ethyne + Calcium Hydroxide)

Chemical Properties of Alkynes:

• Alkynes contain carbon-carbon triple bonds, where one bond is strong, and the other two are weaker, making alkynes highly reactive.

• The weaker bonds undergo addition reactions.

1. Addition of Hydrogen (Hydrogenation):

• Ethyne reacts with hydrogen (H2) at 180-200°C in the presence of a nickel catalyst to form ethane.

• Reaction:

• C2H2 + 2H2 → C2H6 (Ethyne + Hydrogen → Ethane)

2. Addition of Bromine (Halogenation):

• Ethyne reacts with bromine (Br2) solution, resulting in the formation of tetra-bromoethane and the disappearance of the red color of bromine, indicating that ethyne is unsaturated.

• Reaction:

• C2H2 + 2Br2 → C2H2Br4 (Ethyne + Bromine → Tetra-bromoethane)

3. Addition of Water (Hydration):

• Ethyne reacts with water in the presence of 20% sulfuric acid (H2SO4) and 2% mercuric sulfate (HgSO4) at 80°C to form ethanal (acetaldehyde).

• Reaction:

• C2H2 + H2O → CH3CHO (Ethyne + Water → Ethanal)