Alkynes are the homologous series of unsaturated hydrocarbon with a general molecular formula CnH2n-2.
Table of Contents
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’.
H – C C – H H – C – C C- H
Ethyne is the first member of the alkynes series. It has a molecular formula,
C2H2, and a structural formula of HC = CH.
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.
- Write and name all possible structure of C6H8
- How can you prepare a few jars of ethyne in the laboratory?
- Ethyne is a colourless gas with a characteristic sweet smell when pure.
- It is only sparingly soluble in water
- It is slightly less dense than air.
- It is unstable and may explode on compression to liquid.
- Combustion: It undergoes combustion reaction in air to form water and carbon(IV) oxide
2C2H2(g) + 5O2(g) 2H2O(l) + 4CO2(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 3000oC.
- Addition Reaction:Ethyne undergoes addition reaction to produce unsaturated product with double bonds and then a saturated compound with single bond.
- Reaction with hydrogen in the presence of nickel as catalyst.
H- C C – H + H2 H – C = C – H H2(g) H – C – C – H
EtheneH H Ethane
- Reaction with halogens:
H- C C – H + Cl2 H – C = C – H Cl2 H – C – C – H
- Reaction with hydrogen halide: Hydrogen halide reacts with ethyne toproduce halo-alkene and further halogenation produce halo-alkane.
H – C = C – H +HCl H – C = C – H HCl H – C – C – H
H ClH H
Chloroethene 1,2- dichloro ethane.
- Reaction with water: When ethyne is passed through dilute tetraoxosulphate (vi) acid in the presence of mercury (II) tetraoxosulphate (VI) as catalyst, addition of water takes place to form ethanal.
H OH H
H-C=C-H + H2O H- C = C – H H- C – C – H
Ethenol H O Ethanal
- Reaction with acidified KMnO4: If ethyne is added to acidified KMnO4, it decolourises it. But with alkaline KMnO4, the solution turns to green.
H – C = C – H + 4[O] HO – C – C- OH
- Polymerization: In the presence of complex organic –nickel as catalyst ethyne polymerizes to produce benzene.
3 C2H2 C6H6
- Substitution Reaction
- Ethyne reacts with ammoniacal solution of copper (1) chloride to form reddish brown solution of copper (I) dicarbide
C2H2 + 2CuCl Cu2C2 + 2HCl
- With ammoniacal silver trioxonitrate (v), ethyne forms white silver dicarbide
C2H2 + 2AgNO3Ag2C2 + 2HNO3
These reactions to form dicarbide are used to distinguish ethyne from ethene.
USES OF ETHYNE
- Mixed with oxygen to produce oxy ethyne flame for cutting and welding of metals.
- Used in the manufacture of PVC plastics.
- Used in miner’s lamp as fuel.
- Used in making synthetic fiber.
TEST FOR UNSATURATION
Unsaturated compound decolorizes bromine water.
- Give a chemical test to distinguish between ethyne and ethene.
- Write two balanced equations to show addition reaction of ethyne.
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. Kekule proposed that benzene has a ring structure with alternate single and double carbon-carbon bonds as shown below:
which can be conveniently represented as
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:
PREPARATION: Benzene can be prepared from:
- Coal tar: The destructive distillation of coal produced coal tar which contains benzene.
- Petroleum: The dehydrogenated of alkane using vanadium (V) oxide (V2O5) as catalyst at 500oC and 20 atm gives benzene.
C6H14 V2O2C6H6 + 4H2
The process is known as catalytic reforming.
- Polymerization of ethyne
3 ( H – C = C – H ) C6H6
- Describe three methods of preparing benzene.
- Draw the resonance structures of benzene structure of benzene.
- It has a pleasant smell.
- It has boiling point of 80oC.
- It is insoluble in water.
- It burns with sooty flame.
Benzene can undergo both addition reaction and substitution reaction.
- Addition Reaction:
- Hydrogenation: Benzene reduces to cyclo-hexane if hydrogen gas is passed through finely divided nickel at 150oC.
- Halogenation: In the presence of ultra-violet light, benzene reacts with halogen to produce cyclic compound.
- Substitution Reaction: Benzene undergoes substitution reaction due to presence of single bonds in its structure.
- 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.
- Alkylation:- It involves reactions of benzene with halo-alkanes in the presence of AlCl3.
- It is used as a solvent to dissolve organic.
- It is used as fuel in petrol.
- It is used in the manufacture of aromatic compound e.g. benzoic acid.
- How would you obtain ethanal from ethyne? Give the equation for the reaction.
- Describe how to prepare ethyne in the laboratory.
- What is resonance? Give the resonance structure of benzene.
- Explain why hydrogen fluoride exists as a liquid whereas hydrogen chloride is a gas at room temperature.
- Explain why HCl in water conducts electricity but HCl in methyl benzene does not conduct electricity.
New School Chemistry for Senior Secondary School by .O.Y. Ababio (6th edition), pages 535-539.
SECTION A: Write the correct option ONLY
- Which of the following hydrocarbons is alkyne?
- C2H4 b. C2H6 c. C2H2 d. C3H8
- The final product of complete reaction between ethyne and hydrogen gas is
- ethane b. methyl ethane c. ethane d. hydroethyne
- Ethynepolymerizes in the presence of organo nickel complex as catalyst to form a. polythene b. benzene c. polythene d. methyl benzene
- Which of these compounds exhibits resonance? a. Ethanol b. Ethane
- Benzene d. Butyne
- Which of these is an aromatic hydrocarbon? a. Benzene b. Cyclohexane
- Ethene d. Methylamine
1 a.With the aid of a labeled diagram, describe the laboratory preparation of ethyne.
- Give a chemical test to distinguish between ethane and ethyne.
2 a.What is resonance? Draw two resonance structure of benzene.
- Write balanced equation to the following reactions of benzene:
- Reaction with ethene(ii) Reaction with chlorine.