9.2.2 Properties of Alkenes

a. Physical Properties

• Alkenes are unsaturated hydrocarbons.
• The first three members, i.e., ethene, propene, and butene, are gases.
• C₅ - C₁₅ members are liquids.
• The higher members are solids.
• They are insoluble in water but soluble in organic solvents such as alcohol, etc.

b. Chemical Properties

The two carbon atoms forming the double bond is the site where other molecules can attack, giving addition reactions.

1. Reaction with Halogens

• Chlorine and bromine are added to the double bond C=C.
• One bromine atom becomes attached to one carbon and another to the other carbon atom.
• Such a type of reaction is called an addition reaction.
• Alkenes react with bromine water in the same way.

Example:

Ethene (colourless) + Br₂ (reddish-brown) → 1,2-Dibromoethane (colourless)

Key observation: Bromine is a reddish-brown liquid and the product is colourless. When bromine water is added to an alkene, the red-brown colour disappears. The decolourization of bromine solution is frequently used as a simple test for the detection of unsaturation. Alkynes also give this reaction.

Key point: Alkenes decolourize bromine water — a test for unsaturation.

2. Reaction with KMnO₄

• When an alkene is treated with a dilute alkaline aqueous solution of KMnO₄ (1%), addition of two hydroxyl groups occurs across the double bond.
• The pink colour of the KMnO₄ solution is discharged during the reaction.
• This reaction is used as a test for the presence of an unsaturated compound like alkene and is known as Baeyer's test.
• This test is used to distinguish saturated and unsaturated hydrocarbons.
• Saturated hydrocarbons do not give this reaction.

Example:

3CH₂ = CH₂ + 2KMnO₄ + 4H₂O → 3CH₂ - CH₂ + 2MnO₂ + 2KOH (with OH groups on each CH₂)

Product: Ethylene glycol

Uses: Ethylene glycol is used as an anti-freeze.

Key point: Alkenes decolourize KMnO₄ solution — Baeyer's test for unsaturation.

Summary Table

Intermediate Level

Preparation of alkenes:

• Cracking: Large alkanes break into smaller alkanes and alkenes.
• Dehydration of alcohols: Alcohol → alkene + water.
• Dehydrohalogenation: Halogenoalkane + KOH → alkene + KX + H₂O.

Properties of alkenes:

• Physical: Gases (C₂–C₄), liquids (C₅–C₁₅), solids (higher); insoluble in water.
• Chemical: Addition reactions with halogens, KMnO₄, hydrogen, steam.

Tests for unsaturation:

• Bromine water test: Reddish-brown colour disappears.
• Baeyer's test (KMnO₄): Pink colour disappears.

Key point: Alkenes undergo addition reactions and can be detected by the bromine water test and Baeyer's test.

Solved Example

Example: Complete the Following Reactions

1. CH₃ - CH = CH₂ + Br₂ → ?

Answer:

CH₃ - CH = CH₂ + Br₂ → CH₃ - CHBr - CH₂Br

Product: 1,2-dibromopropane

2. CH₃ - CH = CH₂ + KMnO₄ + H₂O → ?

Answer:

3CH₃ - CH = CH₂ + 2KMnO₄ + 4H₂O → 3CH₃ - CH(OH) - CH₂(OH) + 2MnO₂ + 2KOH

Product: Propylene glycol

3. CH₃ - CH₃ + KMnO₄ + H₂O → ?

Answer:

No reaction. Saturated hydrocarbons do not react with KMnO₄.

Concept Assessment Exercise 9.2

Complete the following reactions:

1. CH₃ - CH₂ - CH₂ - OH Conc. H₂SO₄→ ?

Answer:

CH₃ - CH = CH₂ + H₂O

Product: Propene

2. CH₃ - CH₂ - CH₂ - Br + KOH Alcohol→ ?

Answer:

CH₃ - CH = CH₂ + KBr + H₂O

Product: Propene

3. Which alkene is formed by the dehydration of the following alcohol? CH₃ - CH₂ - CH₂ - OH

Answer:

CH₃ - CH = CH₂

Product: Propene

Concept Assessment Exercise 9.3

Complete the following reactions:

1. CH₃ - CH = CH₂ + Br₂ → ?

Answer:

CH₃ - CHBr - CH₂Br

2. CH₃ - CH = CH₂ + KMnO₄ + H₂O → ?

Answer:

3CH₃ - CH = CH₂ + 2KMnO₄ + 4H₂O → 3CH₃ - CH(OH) - CH₂(OH) + 2MnO₂ + 2KOH

3. CH₃ - CH₃ + KMnO₄ + H₂O → ?

Answer:

No reaction.

Important Points

• - Alkenes = unsaturated hydrocarbons with at least one carbon-carbon double bond.
• - General formula: C_nH_2n.
• - Simplest alkene: Ethene (CH₂ = CH₂).
• - Preparation:
• 1. Cracking (thermal and catalytic)
• 2. Dehydration of alcohols
• 3. Dehydrohalogenation of halogenoalkanes
• - Physical properties: Gases (C₂–C₄), liquids (C₅–C₁₅), solids (higher); insoluble in water.
• - Chemical properties:
• - Bromine water test: Decolourizes (test for unsaturation).
• - Baeyer's test (KMnO₄): Decolourizes (test for unsaturation).
• - Addition reactions with halogens, KMnO₄, hydrogen, steam.
• - Science titbits: Ethene is a natural plant hormone that promotes ripening.

Remember

• - Alkenes = unsaturated hydrocarbons with a double bond.
• - General formula: C_nH_2n.
• - Simplest alkene: Ethene (CH₂ = CH₂).
• - Preparation: Cracking, dehydration, dehydrohalogenation.
• - Bromine water test → decolourizes.
• - Baeyer's test (KMnO₄) → decolourizes.
• - Addition reactions with halogens, KMnO₄, hydrogen, steam.
• - Ethene is a natural plant hormone.

Common Mistakes

Quick Revision

Alkenes:

Alkenes have one or more double bonds between carbon atoms. They have the general formula C_nH_2n. When two carbon atoms share two pairs of electrons, they form a double bond between the carbon atoms. How many electrons are left on each carbon atom? Doubly bonded carbon atoms form a single bond with two other atoms.

Do you know? Ethylene or ethene is the most important commercial organic chemical. It is used to manufacture polythene, one of the most familiar plastics. It is also converted to ethylene glycol which is used as antifreeze in automobile radiators.

9.2.1 Preparation of Alkenes:

1. By the cracking of large alkanes: i. A large alkane molecule breaks up into smaller hydrocarbons when heated at high temperatures such as 450-750°C and high pressure. This process is called thermal cracking. This process produces a mixture of alkanes and alkenes. For example, when the decane is heated at high temperatures and pressure, it breaks into octane and ethene.

• C₁₀H₂₂ → C₈H₁₈ + C₂H₄

ii. Large alkane molecules can also be broken into smaller pieces by catalytic cracking. In catalytic cracking, alkanes are heated at 550°C in the presence of a catalyst such as zeolite. Catalytic cracking gives a higher percentage of hydrocarbons containing 5 to 10 carbon atoms.

• C₁₂H₂₆ → C₅H₁₂ + C₇H₁₄

• C₇H₁₆ → C₅H₁₂ + C₂H₄

Cracking gives a mixture of smaller hydrocarbons containing a higher proportion of alkenes. These hydrocarbons are more volatile and possess lower boiling points, making them easier to use as better fuels. Moreover, their combustion can be controlled and they produce large amounts of heat per gram.

For example, the following reaction occurs when alkanes are burned.

• C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(g) + heat

• C₅H₁₂(g) + 8O₂(g) → 5CO₂(g) + 6H₂O(g) + heat

• 2. By Dehydration of Alcohols:
• Dehydration means loss of water. Alcohols dehydrate when their vapour are passed over heated alumina to give alkenes.

• Ethanol Al₂O₃, 340-450^° C→ Ethene + Water

Phosphoric acid (H₃PO₄) and phosphorus pentaoxide (P₄O₁₀) can also be used as the catalyst for the dehydration of alcohols. Concentrated sulphuric acid is also used for dehydration.

• CH₃ - CH₂OH Conc. H₂SO₄→ H₂C = CH₂ + H₂O

Note that in a dehydration reaction the -OH group is removed from one carbon and the H atom from the adjacent carbon atom, resulting in the formation of a double bond between such carbon atoms. Such a reaction is called an elimination reaction.

• 3. By dehydrohalogenation of halogenoalkanes:
• Dehydrohalogenation means the loss of hydrogen halide. Halogenoalkanes on heating with alcoholic potassium hydroxide undergo dehydrohalogenation.

• CH₃ - CH₂Cl + KOH Alcohol→ CH₂ = CH₂ + KCl + H₂O

Note that removal of H and halogen takes place from two adjacent carbon atoms. Is dehydrohalogenation an elimination reaction?

Science titbits: Alkenes occur widely in nature. Ripening fruits and vegetables give off ethene which helps in further ripening. So artificially ethene is used to hasten the normal ripening process. For example 1 kg of tomatoes can be ripened by exposure to 0.1mg of ethene for 24 hours. The red color of tomatoes is due to an alkene called Lycopene.

9.2.2 Properties of Alkenes:

a. Physical Properties: Alkenes are unsaturated hydrocarbons. The first three members i.e. ethene, propene and butene are gases while C₅ - C₁₅ members are liquids, and the higher members are solids. They are insoluble in water but soluble in organic solvents such as alcohol etc.

• b. Chemical Properties:
• The two carbon atoms forming double bond is the site where other molecules can attack giving addition reaction.

1. Reaction with halogens: Chlorine and bromine are added to the double bond C=C. One bromine atom becomes attached to one carbon and another carbon atom. Such a type of reaction is called an addition reaction. Alkenes react with bromine water in the same way.

• Ethene (colourless) + Br₂ (reddish-brown) → 1,2-Dibromoethane (colourless)

Bromine is a reddish-brown liquid and the product is colourless. When bromine water is added to an alkene, the red-brown colour disappears. The decolourization of bromine solution is frequently used as a simple test for the detection of unsaturation. Alkynes also give this reaction.

2. Reaction with KMnO₄: When an alkene is treated with a dilute alkaline aqueous solution of KMnO₄ (1%) addition of two hydroxyl groups occurs across the double bond. The pink colour of KMnO₄ the solution is discharged during the reaction. This reaction is used as a test for the presence of an unsaturated compound like alkene and is known as Baeyer's test. This test is used to distinguish saturated and unsaturated hydrocarbons. Saturated hydrocarbons do not give this reaction.

• 3CH₂ = CH₂ + 2KMnO₄ + 4H₂O → 3CH₂ - CH₂ + 2MnO₂ + 2KOH
• (with OH groups on each CH₂)
• Ethylene glycol is used as an anti-freeze.

• CH₃ - CH = CH₂ + Br₂
• CH₃ - CH = CH₂ + KMnO₄ + H₂O
• CH₃ - CH₃ + KMnO₄ + H₂O

60-Second Revision

• Alkenes = unsaturated hydrocarbons with a double bond.
• General formula: C_nH_2n.
• Simplest alkene: Ethene (CH₂ = CH₂).
• Preparation: Cracking, dehydration, dehydrohalogenation.
• Bromine water test → decolourizes (test for unsaturation).
• Baeyer's test (KMnO₄) → decolourizes (test for unsaturation).
• Addition reactions with halogens, KMnO₄, hydrogen, steam.
• Ethene is a natural plant hormone.
• Examples:
• Ethene: CH₂ = CH₂
• Propene: CH₃CH = CH₂
• Butene: CH₃CH₂CH = CH₂
• ⚠️ Warning: alkenes are unsaturated hydrocarbons — they contain at least one carbon-carbon double bond.