ALKENES

 

Week:  six                                                           Date:                                    

Period:                                 Duration: 1 HR 20 MIN.                                                 Average age of learners: 16YEARS

Subject:                               CHEMISTRY                                                                        Class: SS TWO

Topic:                                ALKENES

Sub topic:  Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials:

Entry behavior: The students have been taught alkanes

Behavioural objective: At the end of the lesson the students should be able to:

i.                     Explain alkenes

ii.                   Explain the laboratory preparation of alkenes

iii.                  State the physical properties and chemical properties of alkenes

iv.                 uses of alkenes

CONTENT

ALKENES

Alkenes are unsaturated hydrocarbon. They form homologous series with general molecular formula CnH2n, where n is a whole number which  is equal to or greater than 2. The first member of the series is C2H4. Alkenes contain carbon-carbon double bond, meaning the two pair of electrons  are shared. A molecule contain two double bond is called diene

Summary Table for Straight-Chain Alkenes: IUPAC Name, Structure, and Molecular Formula

no. C atoms
(n)

Systematic IUPAC Name

Other Names

Molecular Formula

Structure

2

Ethane

ethylene 
(preferred IUPAC name)

C2H4

 

H
|

 

H
|

 

 

C

=

C

 

 

|
H

 

|
H

 

3

prop-1-ene

1-propene
propene
propylene

C3H6

 

H
|

 

 

 

H
|

 

 

C

=

C

-

C

-H

 

|
H

 

|
H

 

|
H

 

4

but-1-ene

1-butene

C4H8

 

H
|

 

 

 

H
|

 

H
|

 

 

C

=

C

-

C

-

C

-H

 

|
H

 

|
H

 

|
H

 

|
H

 

but-2-ene

2-butene

C4H8

 

H
|

 

H
|

 

H
|

 

H
|

 

H-

C

-

C

=

C

-

C

-H

 

|
H

 

 

 

 

 

|
H

 

5

pent-1-ene

1-pentene

C5H10

 

H
|

 

 

 

H
|

 

H
|

 

H
|

 

 

C

=

C

-

C

-

C

-

C

-H

 

|
H

 

|
H

 

|
H

 

|
H

 

|
H

 

pent-2-ene

2-pentene

C5H10

 

H
|

 

H
|

 

H
|

 

H
|

 

H
|

 

H-

C

-

C

=

C

-

C

-

C

-H

 

|
H

 

 

 

 

 

|
H

 

|
H

 

6

hex-1-ene

1-hexene

C6H12

 

H
|

 

 

 

H
|

 

H
|

 

H
|

 

H
|

 

 

C

=

C

-

C

-

C

-

C

-

C

-H

 

|
H

 

|
H

 

|
H

 

|
H

 

|
H

 

|
H

 

hex-2-ene

2-hexene

C6H12

 

H
|

 

H
|

 

H
|

 

H
|

 

H
|

 

H
|

 

H-

C

-

C

=

C

-

C

-

C

-

C

-H

 

|
H

 

 

 

 

 

|
H

 

|
H

 

|
H

 

hex-3-ene

3-hexene

C6H12

 

H
|

 

H
|

 

H
|

 

H
|

 

H
|

 

H
|

 

H-

C

-

C

-

C

=

C

-

C

-

C

-H

 

|
H

 

|
H

 

 

 

 

 

|
H

 

|
H

 

hept-3-ene

3-heptene

C7H14

 

H
|

 

H
|

 

H
|

 

H
|

 

H
|

 

H
|

 

H
|

 

H-

C

-

C

-

C

=

C

-

C

-

C

-

C

-H

 

|
H

 

|
H

 

 

 

 

 

|
H

 

|
H

 

|
H

 

 

 

ETHENE

Ethene is the first and the most important alkenes with the formula C2H4

LABORATORY PREPARATION OF ETHENE

Ethene is prepared by heating ethanol with excess concentrated tetraoxosulphate (vi) acid at 180oC. The reaction occurs in two stages. On mixing the ethanol and the acid in a volume ratio of 1:2 respectively, the ester, ethylhydrogentetraoxosulphate(vi) is formed.

C2H5OH aq    +   H2SO4aq    à  C2H5HSO4aq  + H2Ol

On heating the ethylhydrogentetraoxosulphate (vi) in the presence of excess tetraoxosulphate (vi) acid, it decomposes to produce ethane.

C2H5HSO4aq    --> C2H4g +H2SO4aq

The overall reaction can be regarded as dehydration of ethanol by hot concentrated H2SO4 because the overall change is the removal of one molecule of water from ethanol.

C2H5OHaq          à    C2H4g

The gas evolved is passed through conc. Sodium hydroxide to remove any gaseous impurities which are mainly CO2 and SO2. The almost pure ethene is collected over water.

Ethane is prepared industrially by cracking of petroleum.

C12H26   à  C6H14  +  C4H8 +  C2H4

PHYSICAL PROPERTIES OF ETHENE

1.       Ethene is a colourless gas with a faint sweetish smell.

2.       It is slightly less dense than air

3.       It is sparingly soluble in water

4.       It is neutral to litmus

CHEMICAL PROPERTIES OF ETHENE

I.                    COMBUSTION: Ethene burns readily in air or oxygen with a smoky luminous flame because of its high carbon content, to form carbon(iv) oxide and water.

C2H4  +  3H2O    à  2CO2   + 2H2O

II.                  ADDITION REACTION: Addition reaction are characteristic of all unsaturated compound. It involves the direct addition of an attacking reagent across the double or triple bond of an unsaturated compound to yield a saturated product or at least one in which the degree of saturation is increased.

a.       CH2=CH2   +  Cl2   à  CH2ClCH2Cl (1,2- dichloroethane)

        c.   CH2=CH2    +  HI     à  CH3CH2I

        d . Addition of water

Alkene reacts with water, in the form of steam, to produce alcohol. 

Alkene + steam is passed over phosphoric acid (H3PO4) catalyst and temperature of 300oC. 

H2O molecule adds to C = C bonds to form alcohol.


3) Polymerization

The joining of several identical alkene molecules to form a big molecule. Eg: Ethene -->  poly(ethene)

 

USES OF ETHENE

I.                    In the manufacture of organic compounds, such as ethane, ethanol, ethanoic acid

II.                  It is a raw material used for making important plastic such as pvc, polythene etc

III.                It is used to produce synthetic rubber such as styrene-buta diene rubber and thiokol.

IV.                Used to hasten the ripening of fruit

V.                  1,2-dibromoethene is used as a petrol additive.

VI.                Used in the manufacture of detergents and glycol.

PRESENTATION

Step I: The teacher explain alkenes as an unsaturated hydrocarbon

Step II: The teacher leads the students to derived the molecular formula of first – ninth members of alkenes.

Step III: the teacher explain the laboratory preparation of alkenes

Step IV: The teacher states the physical and chemical properties of alkenes.

EVALUATION

The teacher evaluates the lessons by asking these questions

i.                     Explain alkenes

ii.                   Explain the laboratory preparation of alkenes

iii.                  State the physical properties and chemical properties of alkenes

iv.                 uses of alkenes

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