Alkynes: Nomenclature, Preparation, Properties, Aromatic Hydrocarbons and uses

CONTENT

• Nomenclature
• Preparation, Properties and Uses
• Aromatic Hydrocarbons: Benzene Structure
• Preparation,  Properties and Uses

NOMENCLATURE

Alkynes are the homologous series of unsaturated hydrocarbon with a general molecular formula C_nH_2n-2.

Alkynes show a high degree of unsaturation than alkenes, hence, they are chemically more reactive than the corresponding alkenes or alkanes.

They are named by replacing the ‘ane’ of alkanes with ‘yne’.

Examples

ETHYNE

Ethyne is the first member of the alkynes series.  It has a molecular formula,

C₂H₂, and a structural formula of HC = CH.

LABORATORY PREPARATION

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.

Evaluation

1.       Write and name all possible structure of C₆H₈

2.      How can you prepare a few jars of ethyne in the laboratory?

PHYSICAL PROPERTIES

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

1.       Combustion:  It undergoes combustion reaction in air to form water and carbon(IV) oxide

            2C₂H_2(g) + 5O_2(g)       →     2H₂O_(l)  +   4CO_2(g)

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

2.  Addition Reaction:Ethyne undergoes addition reaction to produce unsaturated product with double bonds and then a saturated compound with single bond.

a.         Reaction with hydrogen in the presence of nickel as catalyst.

USES OF ETHYNE: Ethyne is

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

TEST FOR UNSATURATION

Unsaturated compound decolorizes bromine water.

Evaluation

1. Give a chemical test to distinguish between ethyne and ethene.
2. Write two balanced equations to show addition reaction of ethyne.

AROMATIC HYDROCARBONS

These are hydrocarbons that have the same structure as benzene.

Benzene is a typical aromatic compound with molecular formula of C₆H₆.

STRUCTURE OF BENZENE Over the years, there has been a controversy on the structure of benzene. But in 1865, August Kekule suggested a structure for benzene.

Over the years, there has been a controversy on the structure of benzene. But in 1865, August Kekule suggested a structure for benzene. Kekule proposed that benzene has a ring structure with alternate single and double carbon-carbon bonds as shown below:

These two forms of benzene structure are known as resonance forms. Resonance occurs when two forms of the same compound have the same arrangement of atoms but differ in the arrangement of electrons that form the bonds.

The Kekule structure of benzene accounted for the stability of the six carbon atoms but it was unable to explain why a highly unsaturated compound failed to undergo many of the addition reactions like decolourising bromine water, reaction with hydrogen halides etc; characteristic of alkenes. Benzene undergoes mostly substitution reactions. Thus, the structural formula with threedouble bonds describing the benzene molecule does not agree with the chemical behaviour of benzene. Therefore, the bonding in benzene cannot be described as three double bonds and three single bonds as proposed by Kekule but rather the bonding must be considered as a delocalized electron cloud spread out over the whole benzene ring. Hence, the modern structure of benzene is considered to be a plain hexagon with an inscribed ring which represents the electron cloud spread out over the whole benzene ring as shown below:

Evaluation

1. Describe three methods of preparing benzene.
2. Draw the resonance structures of benzene structure of benzene.

PHYSICAL PROPERTIES

1.         It has a pleasant smell.

2.         It has boiling point of 80^oC.

3.         It is insoluble in water.

4.         It burns with sooty flame.

CHEMICAL PROPERTIES

Benzene can undergo both addition reaction and substitution reaction.

1. Addition Reaction:

 i.   Hydrogenation:  Benzene reduces to cyclo-hexane if hydrogen gas is passed through finely divided nickel at 150^oC.

   ii.   Halogenation:  In the presence of ultra-violet light, benzene reacts with halogen to produce cyclic compound.

2.       Substitution Reaction: Benzene undergoes substitution reaction due to presence of single bonds in its structure.

i.          Halogenation:

ii.          Nitration:  This occurs in the mixture of HNO₃ and H₂SO₄ together with benzene

iii.  Sulphonation: Benzene react with concentrated H₂SO₄ to form benzene sulphonic acid.

iv.  Alkylation:-  It involves reactions of benzene with halo-alkanes in the presence of AlCl₃.

USES

1.       It is used as a solvent to dissolve organic.

2.       It is used as fuel in petrol.

3.       It is used in the manufacture of aromatic compound e.g. benzoic acid.

GENERAL Evaluation/REVISION

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

READING ASSIGNMENT

New School Chemistry for Senior Secondary School by .O.Y. Ababio (6^th edition), pages 535-539.

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1.       Which of the following hydrocarbons is alkyne?

        a. C₂H₄    b. C₂H₆ c. C₂H₂ d. C₃H₈

2.       The final product of complete reaction between ethyne and hydrogen gas is

a. ethane   b. methyl ethane c. ethane d. hydroethyne

3.       Ethynepolymerizes in the presence of organo nickel complex as catalyst to form a. polythene b. benzene c. polythene d. methyl benzene

4.       Which of these compounds exhibits resonance? a. Ethanol b. Ethane

          c. Benzene d. Butyne

5.       Which of these is an aromatic hydrocarbon? a. Benzene b. Cyclohexane

          c. Ethene d. Methylamine

SECTION B

1          a.With the aid of a labeled diagram, describe the laboratory preparation of ethyne.

            b. Give a chemical test to distinguish between ethane and ethyne.

2          a.What is resonance? Draw two resonance structure of benzene.

            b. Write balanced equation to the following reactions of benzene:

• Reaction with ethene(ii) Reaction with chlorine.