SS2 Chemistry Lesson Note on Isomerism: Structural and Geometric

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TOPIC: HYDROCARBONS

CONTENTS:

1. ISOMERISM-STRUCTURAL AND GEOMETRIC
2. ALKENE-IUPAC NOMENCLATURE, METHOD OF PREPARATION, PROPERTIES AND USES
3. ALKYNE- IUPAC NOMENCLATURE, METHOD OF PREPARATION, PROPERTIES AND USES
4. AROMATIC HYDROCARBON: BENZENE-PROPERTIES AND USES

ISOMERISM

Definition: Isomerism is the existence of two or more compounds (known as isomers) with the same molecular formulae but different molecular structures

ISOMERISM IN ALKANES

1. Structural isomerism in alkanes is obtained by branchind the straight chain isomer.

(a)   Draw the isomers of the compound with molecular formula

(i) C₄H₁₀ 

CH₃CH₂CH₂CH₃ (butane)      CH₃CH(CH₃)CH₃ 2-methylpropane

(ii) C₅H₁₂   CH₃CH₂CH₂CH₂CH₃ Pentane     CH₃CH(CH₃)CH₂CH₂ 2-methylbutane    CH₃CH(CH₃)₂CH₃ 2,2-dimethypropane

GEOMETRIC ISOMERISM

This is the existence of of organic compounds with the same molecular formula but different spatial arrangement of compound atoms. Geometric isomers have similar chemical properties but different physical properties. We have the cis-isomers and trans-isomers. Geometric isomerism is common in alkenes.

ALKENES

The alkenes are the homologous series with the general molecular formula CnH₂n; n starts from 2. They are unsaturated hydrocarbons due to the double bond in their structures.

ISOMERISM IN ALKENES

1. Isomers of C₄H₈

2. Isomers of C₅H₁₀

Note: The carbon atoms forming the straight chain is numbered from the carbon closest to the double bond.

Laboratory preparation of Ethene

Ethene is prepared in the laboratory by the dehydration of ethanol by concentrated tetraoxosulphate(vi). Equations for the reaction

Stage 1: C₂H₅OH_(L) + H₂SO_4(aq)         →     C₂H₅HSO_4(aq)  +   H₂O_(L)

Stage 2: C₂H₅HSO_4(aq)      →    C₂H_4(g)  +  H₂SO_4(aq)

PROPERTIES OF ETHENE

Physical properties

1. Ethane is a colourless gas with a 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.

Chemical properties

1. Ethene undergoes polymerization reaction e.g

nCH₂ = CH₂      →    (CH₂ – CH₂)n polythene

2. Combustion reaction: Ethene or any alkene burn in air (or oxygen) to form CO₂ and H₂O e.g

C₂H_4(g)  +  3O₂      →   2CO_2(g)  +  2H₂O_(g)

3. Addition reaction: An addition reaction involves the direct addition of an attacking reagent across the double bend or triple bond of an unsaturated compound to form a saturated compound or at least one in which the degree of saturation is increased. Example of addition reaction between

 

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