Unsaturated Hydrocarbons – SS2 Chemistry Lesson Note (Third Term)

Unsaturated hydrocarbons are hydrocarbons in which carbon atoms join with each other by multiple bonds. The multiple bonds can be double bonds, as in alkenes, or triple bonds, as in alkynes. Alkenes have names ending in -ene and alkynes have names ending in -yne. Unsaturated compounds decolourise bromine water, which is the test for unsaturation.

Behavioural objectives

By the end of the lesson, students should be able to:

• define unsaturated hydrocarbons and name alkenes and alkynes correctly;
• describe the laboratory preparation of ethene and ethyne;
• state the physical and chemical properties of ethene and ethyne;
• state the uses of ethene and ethyne;
• describe the test for unsaturation and the test that distinguishes ethyne from ethene.

Alkenes and ethene

How are alkenes named?

Alkenes are named by replacing the “ane” of the corresponding alkane with “ene”. For example, ethane changes to ethene and propane to propene.

Laboratory preparation of ethene

Ethene is prepared by heating ethanol with excess concentrated tetraoxosulphate(VI) acid at 170°C. The acid acts as a dehydrating agent by removing water from the ethanol. Thus the process is called dehydration.

The reaction occurs in two stages:

1. C₂H₅OH(aq) + H₂SO₄(aq) → C₂H₅HSO₄(aq) + H₂O(l)
2. C₂H₅HSO₄(aq) → C₂H₄(g) + H₂SO₄(aq)

The overall reaction is represented by the equation:

C₂H₅OH(aq) → C₂H₄(g) + H₂O(l) (concentrated H₂SO₄, 170°C, removal of H₂O)

Physical properties of ethene

1. Ethene is a colourless gas with a faint sweetish smell.
2. It is sparingly soluble in water.
3. It is slightly less dense than air.
4. It has no action on litmus paper.

Chemical properties of ethene

Combustion

Ethene undergoes combustion in air or in the presence of oxygen and produces carbon(IV) oxide and steam.

C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l)

Addition reactions

An addition reaction is a reaction in which two molecules combine to form one molecule. Ethene undergoes the following addition reactions:

1. Reaction with hydrogen (hydrogenation).
2. Reaction with halogens (halogenation).
3. Reaction with hydrogen halides (halohydrogenation).
4. Reaction with acidified or alkaline KMnO₄ (hydroxylation): ethene decolourises acidified KMnO₄, but turns alkaline KMnO₄ green, and ethane-1,2-diol is formed.
5. Reaction with hydrogen peroxide in the presence of osmium trioxide to form ethane-1,2-diol.
6. Reaction with concentrated H₂SO₄, which produces a fuming liquid (ethyl hydrogen sulphate):
   
   C₂H₄ + H₂SO₄ → C₂H₅HSO₄
   
   When ethyl hydrogen sulphate is hydrolysed, tetraoxosulphate(VI) acid and ethanol are produced:
   
   C₂H₅HSO₄ + H₂O → C₂H₅OH + H₂SO₄
7. Ethene gas decolourises bromine water to produce bromoethanol.
8. Ethene can also undergo an addition reaction with oxygen in the presence of a silver catalyst at about 250°C to form epoxyethane.

Polymerisation

Ethene polymerises to produce polythene.

Uses of ethene

Ethene is used:

1. In the manufacture of plastics.
2. In making synthetic rubber.
3. To hasten the ripening of fruits.
4. In the production of other organic compounds, e.g. halo-alkane, ethane and ethanol.

Alkynes and ethyne

What are alkynes?

1. Alkynes are the homologous series of unsaturated hydrocarbons with the general molecular formula C_nH_2n-2.
2. Alkynes show a higher degree of unsaturation than alkenes, hence they are chemically more reactive than the corresponding alkenes or alkanes.
3. They are named by replacing the “ane” of alkanes with “yne”.

What is ethyne?

Ethyne is the first member of the alkyne series. It has the molecular formula C₂H₂ and the structural formula HC≡CH.

Laboratory preparation of ethyne

Ethyne is usually prepared in the laboratory by the action of cold water on calcium carbide. The reaction is carried out on a heap of sand to prevent the flask from cracking as a result of the large quantity of heat evolved.

Physical properties of ethyne

1. Ethyne is a colourless gas with a characteristic sweet smell when pure.
2. It is only sparingly soluble in water.
3. It is slightly less dense than air.
4. It is unstable and may explode on compression to liquid.

Chemical properties of ethyne

Combustion

Ethyne undergoes a combustion reaction in air to form water and carbon(IV) oxide.

2C₂H₂(g) + 5O₂(g) → 2H₂O(l) + 4CO₂(g)

NB: In limited air, ethyne burns with a very smoky and luminous flame because of its high carbon content. In plenty of air and in appropriate proportion, it burns with a non-luminous, very hot flame of about 3000°C.

Addition reactions

Ethyne undergoes addition reactions to produce first an unsaturated product with a double bond and then a saturated compound with single bonds.

1. Reaction with hydrogen in the presence of nickel as a catalyst.
2. Reaction with halogens.
3. Reaction with hydrogen halides: a hydrogen halide reacts with ethyne to produce a halo-alkene, and further addition produces a halo-alkane.
4. Reaction with water: when ethyne is passed through dilute tetraoxosulphate(VI) acid in the presence of mercury(II) tetraoxosulphate(VI) as a catalyst, water adds on to form ethanal.
5. Reaction with acidified KMnO₄: ethyne decolourises acidified KMnO₄, but with alkaline KMnO₄ the solution turns green.

Polymerisation

In the presence of a complex organo-nickel catalyst, ethyne polymerises to produce benzene.

3C₂H₂ → C₆H₆

Substitution reactions

1. Ethyne reacts with an ammoniacal solution of copper(I) chloride to form reddish-brown copper(I) dicarbide:
   
   C₂H₂ + 2CuCl → Cu₂C₂ + 2HCl
2. With ammoniacal silver trioxonitrate(V), ethyne forms white silver dicarbide:
   
   C₂H₂ + 2AgNO₃ → Ag₂C₂ + 2HNO₃

These reactions that form dicarbides are used to distinguish ethyne from ethene.

Uses of ethyne

Ethyne is:

1. Mixed with oxygen to produce the oxy-ethyne flame for cutting and welding of metals.
2. Used in the manufacture of PVC plastics.
3. Used in a miner’s lamp as fuel.
4. Used in making synthetic fibre.

What is the test for unsaturation?

An unsaturated compound decolourises bromine water.

Evaluation

1. What are unsaturated hydrocarbons? Give one example each of a compound with a double bond and a compound with a triple bond.
2. Describe how ethene is prepared in the laboratory and write the overall equation.
3. State two uses of ethene and two uses of ethyne.
4. Which reactions distinguish ethyne from ethene?
5. State the test for unsaturation.

Assignment

1. How would you obtain ethanal from ethyne? Give the equation for the reaction.
2. Describe how to prepare ethyne in the laboratory.
3. What is resonance? Give the resonance structure of benzene.
4. Explain why hydrogen fluoride exists as a liquid whereas hydrogen chloride is a gas at room temperature.
5. Explain why HCl in water conducts electricity but HCl in methyl benzene does not conduct electricity.