ALKENES I

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 170^o 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)  + H₂SO_4(aq)                     C₂H₅HSO_4(aq) + H₂O_(l)

C₂H₅HSO_4(aq)               C₂H_4(g)  + H₂SO₄.

The overall reaction is represented by the equation

 

C₂H₅OH(aq)^H₂^SO₄C₂H_4(g)  + H₂SO_4(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.

C₂H_4(g) +  3O_2(g)               2CO_2(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

1. Reaction with halogens (Halogenation):

H         H         HH

 

 

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

 

Ethene                         ClCl      1,2- dichloroethane

 

1. 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 about250^oC 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
4. State four uses of ethene.
5. 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?
6. State THREE factors that determine the spontaneity of a chemical reaction.
7. 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 (6^thedition) 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 H}₂^SO₄       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

 

See also

ALKANES

INTRODUCTION TO ORGANIC CHEMISTRY

ACID/BASE REACTIONS

MASS/VOLUME RELATIONSHIP

SOLUBILITY AND SOLUTIONS