Unsaturated Hydrocarbons – Alkenes

UNSATURATED HYDROCARBONS

These are hydrocarbons in which carbon atoms join with each other by multiple bonds.  The multiple bond can be double bonds e.g Alkene or triple bonds e.g Alkyne.

1. Ethene2 .Ethyne

H – C = C – H                                       H – C = C – H

H   H.

Alkenes e.gEthene

Nomenclature

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

 

PREPARATION (Lab. Preparation)

1.  Ethene is prepared by heating ethanol with excess concentrated tetraososulphate 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)  + H₂SO₄ (aq)                C₂H₅HSO₄ + H₂O

C₂H₅HSO₄               C₂H₄+ H₂SO₄.

The overall reaction is represented by the equation .

C₂H₅OH  ^H2^SO4 C₂H₄+ H₂SO₄

-H₂O

2. Ethene can be prepared through cracking of Alkane e.g C₃H₈             C₂H₄ + CH_4.
3. By dehydration

e.g                               H          H          Cl            H       H

 

H –  C –   C – H                            C   =   C  + HCL

 

H      Cl                                H       H

 

Physical Properties

1. Ethene is a colourless gas with faint sweetish odour .
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. Write four (4) physical properties of Ethene
2. How would you prepare a jar of ethane gas in the laboratory?

 

CHEMCIAL PROPERTIES

1. Combustion

Ethene undergoes combustion/oxidation in air or in the presence of oxygen and produce carbon(iv)oxide and steam .

C₂H₄  +  3O₂               2CO₂ + 2H₂O.

2. Additional reaction
3. With hydrogen known as hydrogenation

H          H                                  H        H

 

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

 

Ethene                                        H         H          Ethane

1. With halogen know as halogenation

H          H          HH

 

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

 

Ethene                         ClCl      1,2- dichloroethane

1. With Halides known as hydrogenation

e.g  H- C = C – H + HBr                                   H           H

 

H    HH    C         C – H

 

ethene                                                H        Br           1-Bromoethane.

1. With acidified /Alkaline KMnO4 known as hydroxylation. It decolourses acidified KMnO₄, but turns alkaline KMnO₄ to green and result to ethane -1,2- diol.

OH     OH

 

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

 

H     H                                                           HH

Ethane-1,2-diol (glycol)

1. 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. With conc. H₂SO₄ known as hydration to produce fuming liquid of 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          (radiation brown)                  Br     OH

Bromoethanol.

Similarly:

H     H

 

H-C  = C – H    +    HClO                   H – C –   C – H

(Brown)

Cl   OH

Chloroethanol

1. Polymerization of ethane to produce polythene.

H      HHHHH

 

C   =  CC  –  C  – C  –   C

 

H       HHHHH

polyethene

1. With ozone known as ozomolysis to from etheneozomides.

e.g                   H          H                                                      O

 

C  =      C     + O₃                                 H₂C         CH₂

Ozone

H         H                                                      O

EtheneEtheneozonide

1. Ethene can also undergo additional 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

• Used in the manufacture of plastics.
• Used in making synthetic rubber.
• Used to hasten the ripening of fruits.
• Used in the production of other organic compounds e.g halo-alkane,ethane, ethanol.

 

Evaluation

1. Describe the reaction of ethane with the following:
   1. Bromine water
   2. Chlorine water
   3. Acidified KMnO₄
   4. State four (4) uses of Ethene.

 

WEEKEND ASSIGNMENT

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

CH₃            H

 

 

C  =  C

 

 

H               CH₃

1. Cis- but-2-ene
2. Trans –cis-but-2-ene
3. Trans-1-2- but-2-ene
4. 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 ethane.

(a ) Polymerization              (b) Substitutional    (c ) Hydration     (d)  Addition.

4. Function of the empty bottle during the preparation of ethane gas is

(a) to remove oxygen    (b) to remove CO₂     ( c) to prevent sucking back of the gas            (d) None of the above.

5. Additional reaction of hydrogen and ethene is known as

(a) polymerization               (b) additional    (c) combustion   (d) hydrogenation

 

Theory

1. Describe two (2) methods of obtaining ethene industrially.

2a. Write and name the geometric isomers of compound with the molecular formular  C₅H₁₀

1. With chemical equation only, show how ethane reacts with the following:

– ozone

– oxygen

– alkaline KMnO₄

–  Conc. H₂SO₄

 

See also

ALKYNES

Saturated Hydrocarbons

SIMPLE MACHINE

AIR AND COMBUSTION

AN ALKALI