CONTENT

• Nomenclature
• Preparation, Properties and Uses.

 

UNSATURATED HYDROCARBONS

These are hydrocarbons in which carbon atoms join with each other by multiple bonds.  The multiple bonds can be double bonds e.gAlkenes or triple bonds e.gAlkynes.

 

NOMENCLATURE

The process of naming in alkenes is obtained by substituting “ane” in alkane with ‘ene’ e.g Ethane changes to Ethene, propane to propene

 

LABORATORY PREPARATION 

Ethene is prepared by heating ethanol with excess concentrated tetraoxosulphate(VI) acid at 170o 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

C2H5OH(aq)  + H2SO4(aq) → C2H5HSO4(aq) + H2O(l)

C2H5HSO4(aq)   →       C2H4(g)  + H2SO4.

The overall reaction is represented by the equation: 

C2H5OH(aq)H2SO4 →  C2H4(g)  + H2SO4(aq) + H2O 

 

PHYSICAL PROPERTIES

1. Ethene is a colourless gas with 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.

 

EVALUATION

1. How would you prepare a jar of ethene gas in the laboratory?
2. Mention four physical properties of Ethene.

 

CHEMCIAL PROPERTIES

1. Combustion: Ethene undergoes combustion in air or in the presence of oxygen and produce carbon (IV) oxide and steam.

C2H4(g) +  3O2(g)     →          2CO2(g)  + 2H2O(l)

 

2. Addition reaction: This is a reaction in which two molecules combine to form one molecule. 

3. Reaction with hydrogen (Hydrogenation):

H    H H        H

 

  H-C= C –   H   +  H2 H –   C  –     C – H                                                      

Ethene

H         H          Ethane

 

4. Reaction with halogens (Halogenation):

H H HH

 

H  –  C =  C- H  +  Cl2                   H – C –  C –  H

 

Ethene ClCl      1,2- dichloroethane

 

5. Reaction with hydrogen halides(Halohydrogenation):

H H

H- C = C – H + HBr     H – C C – H

H    HH Br

Ethene           1-bromoethane.

1. Reaction with acidified /Alkaline KMnO4 (Hydroxylation):  It decolourises acidified KMnO4, but turns alkaline KMnO4 to green and ethane -1,2- diol is formed.

OH     OH

H – C = C – H    +    KMnO4                       H –     C   –    C  – H

H     H                                                           H        H

 Ethane-1,2-diol (glycol)

1.     Reaction with Hydrogen peroxide in the presence of osmium trioxde to form ethan -1,2- diol.

OH  OH

H –    C  =  C – H + H2O2          H – C –  C  – H

 

H      HHH

Ethane -1,2- diol.

 

1. Reaction with concentrated H2SO4produces a fuming liquid (ethyl hydrogen       sulphate)

C2H4  + H2SO4               C2H5HSO4

When ethyl hydrogen sulphate is hydrolyzed, tetraoxosulphate (VI) acid and ethanol are produced.

C2H5SO4 C2H5OH  + H2SO4

 

1.   Ethene gas decolourizes bromine water to produce bromoethanol.

            H      H

 

H – C = C – H+HBrO             H – C  –C –  H

 

    H   H( brown)                  Br    OH   

Bromoethanol

1. Polymerization of ethane to produce polythene.

                H      HHHH H

 

 C   =  C- C  – C  – C  –  C-

 

n      H       HHHHH n

                                                                Polyethene

1.   Ethene can also undergo addition reaction with oxygen in the presence of silver catalyst at about250oC to form epoxy ethane.

                     H   H HH

 

              H – C =   C –  H   + ½ O2            H – C  –  C  – H

 

O

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.

 

GENERAL EVALUATION/REVISION

1. Write balanced equations to show the reaction of ethene with the following:

1. Bromine water
2. Chlorine water
3. Acidified KMnO4

1. State four uses of ethene.
2. Why is an empty flask inserted between the flat bottom flask and the conical flask holding the drying agent in the laboratory preparation of ethene?
3. State THREE factors that determine the spontaneity of a chemical reaction.
4. 0.92g of ethanol raised the temperature of 100g of water from 298K to 312.3K when 

burned completely. What is the heat of combustion of ethanol? 

 

READING ASSIGNMENT

New School Chemistry for Senior Secondary School by O.Y. Ababio (6thedition) Pages 532-535

 

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1. The name of the organic compound with the structure below is

CH3     H

 

C  =  C

 

H               CH3

1. Cis- but-2-eneB.Trans –cis-but-2-eneC.Trans-1-2- but-2-eneD.1,2- dimethyl ethane.
2. In the reaction given below:

C2H5OH    Conc H2SO4       C2 H4Conc H2SO4is  acting as 

 -H2O

1. oxidizing agent B. reducing agent C. dehydrating agent    D. drying agent.
2. One of the following is not a chemical property of ethene.
3. Polymerization B. Substitution reaction C. Hydration D.  Addition reaction
4.   Function of the empty bottle during the preparation of ethane gas is 
5. to remove oxygen B. to remove CO2C. to prevent sucking back of the gas  D. None of the above
6.   Addition reaction of hydrogen and ethene is known as 
7. polymerization B. halogenation C. combustion D. hydrogenation  

 

SECTION B

1.     Write and name the geometric isomers of compound with the molecular formula C5H10
2.     Write balanced chemical equation to show how ethene reacts with the following:
3. concentrated H2SO4b. bromine water c. acidified KMnO4

 

WEEK EIGHT

TOPIC:ALKYNES 

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 CnH2n-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

H

 

H – C C – H H  – C  – C C- H

 

H

Ethyne               Prop-1-yne

 

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

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.

 

2.  Addition Reaction:Ethyne undergoes addition reaction to produce unsaturated product with double bonds and then a saturated compound with single bond.
3.   Reaction with hydrogen in the presence of nickel as catalyst.

 

H HHH

 

H- C C – H  + H2          H – C = C – H   H2(g)         H – C  – C  – H

 

EtheneH  H    Ethane

1. Reaction with halogens:

ClClClCl

 

H- CC – H + Cl2       H – C = C – H         Cl2 H – C –   C – H

 

ClCl

  1,2- dichloroethene1,1,2,2-tetrachloroethane        

1.     Reaction with hydrogen halide: Hydrogen halide reacts with ethyne toproduce halo-alkene and further halogenation produce halo-alkane. 

                      ClCl

 

H – C = C – H  +HCl       H – C = C – H  HCl        H – C –   C – H

H   ClH   H

Chloroethene         1,2- dichloro ethane.

 

1.   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

 

1. Reaction with acidified KMnO4: If ethyne is added to acidified KMnO4, it decolourises it. But with alkaline KMnO4, the solution turns to green.

 

                        O       O

H – C = C – H  +  4[O]        HO – C  –  C- OH

                                    KMnO4Ethanedioic acids

 

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

3 C2H2        C6H6

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

C2H2  +  2CuCl          Cu2C2  +  2HCl

1. 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: 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. 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:

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 catalyst at 500oC 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 boiling point of 80oC.
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:

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

 

1.   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.
3. Halogenation:

 

1. 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.

 

3. 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.
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.

 

READING ASSIGNMENT 

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

 

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1. Which of the following hydrocarbons is alkyne?
2. C2H4 b. C2H6 c. C2H2 d. C3H8
3. The final product of complete reaction between ethyne and hydrogen gas is 
4. ethane   b. methyl ethane c. ethane d. hydroethyne
5. Ethynepolymerizes in the presence of organo nickel complex as catalyst to form a. polythene b. benzene c. polythene d. methyl benzene
6. Which of these compounds exhibits resonance? a. Ethanol b. Ethane 
7. Benzene d. Butyne
8. Which of these is an aromatic hydrocarbon? a. Benzene b. Cyclohexane 
9. Ethene d. Methylamine

 

SECTION B

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

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

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

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

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