Unsaturated Hydrocarbon (Alkynes)

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In today’s Chemistry class, We will be learning about Unsaturated Hydrocarbons (Alkynes). We hope you enjoy the class!

 

 

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’.

 

 

ETHYNE

Ethyne is the first member of the alkynes series.

It has a molecular formula, C₂H₂, and 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 a non-luminous very hot flame of about 3000^oC.

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

3. Polymerization: In the presence of complex organic –nickel as catalyst ethyne polymerizes to produce benzene.

3 C₂H₂        C₆H₆

4. Substitution Reaction
   1. • Ethyne reacts with ammoniacal solution of copper (1) chloride to form reddish brown solution of copper (I) dicarbide

C₂H₂  +  2CuCl          Cu₂C₂  +  2HCl

1. 1. • With ammoniacal silver trioxonitrate (v), ethyne forms white silver dicarbide

C₂H₂ + 2AgNO₃             Ag₂C₂  + 2HNO₃

These reactions to form dicarbide are used to distinguish ethyne from ethene.

 

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 fibre.

 

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 the molecular formula C₆H₆.

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 three double 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:

 

PREPARATION: Benzene can be prepared from:

1. Coal tar: The destructive distillation of coal produced coal tar which contains benzene.
2. Petroleum: The dehydrogenated of alkane using vanadium (V) oxide (V₂O₅) as a catalyst at 500^oC and 20 atm gives benzene.

C₆H₁₄     V₂O₂C₆H₆  +  4H₂

The process is known as catalytic reforming.

3. Polymerization of ethyne

3 ( H – C = C – H )      =     C₆H₆

 

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 a boiling point of 80^oC.
3. It is insoluble in water.
4. It burns with a sooty flame.

 

CHEMICAL PROPERTIES

Benzene can undergo both addition reactions and substitution reactions.

1. Addition Reaction
2. Hydrogenation:  Benzene reduces to cyclo-hexane if hydrogen gas is passed through finely divided nickel at 150^oC.
3. Halogenation:  In the presence of ultra-violet light, benzene reacts with halogen to produce a cyclic compound.
4. Substitution Reaction: Benzene undergoes substitution reaction due to the presence of single bonds in its structure.
5. Halogenation
6. Nitration:  This occurs in the mixture of HNO₃ and H₂SO₄ together with benzene
7. Sulphonation: Benzene reacts with concentrated H₂SO₄ to form benzene sulphonic acid.
8. 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 compounds.
2. It is used as a 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.
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 methylbenzene 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 the 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 a catalyst to form

a. polythene b. benzene c. polythene d. methylbenzene

4. Which of these compounds exhibits resonance?

a. Ethanol b. Ethane c. Benzene d. Butyne

1. Which of these is an aromatic hydrocarbon?

a. Benzene b. Cyclohexane c. Ethene d. Methylamine

 

SECTION B

1. With the aid of a labelled diagram, describe the laboratory preparation of ethyne.
2. Give a chemical test to distinguish between ethane and ethyne.
3. What is resonance? Draw two resonance structure of benzene.
4. Write a balanced equation to the following reactions of benzene:

(i) Reaction with ethene(ii) Reaction with chlorine.

 

 

 

We have come to the end of this class. We do hope you enjoyed the class?

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

In our next class, we will be learning about Alkanols. We are very much eager to meet you there.

 

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