Homologous Series

2.2

Homologous Series

Importance of Homologous Series

In order to study the physical properties and chemical reactions of organic compounds, they are classified into groups of compounds called the homologous series..

Characteristics of Homologous Series

The same general formula..
The same functional group.
The same chemical properties.
Consecutive members differ by one carbon atom and two hydrogen atoms (\(CH_2\) or relative molecular mass = 14).
Physical properties that gradually change from one member to the next.
The homologous series that will be learned in this topic is as shown below:
- Alkane
- Alkene
- Alkyne
- Alcohol
- Carboxylic acid
- Ester

Molecular Formula

A chemical formula that shows the type and actual number of atoms of each element in a molecule.

Structural Formula

Shows the type of bond and how the atoms in a molecule are bonded to each other.

Alkane

Alkane: a group of organic compounds that are saturated hydrocarbons.
A single carbon atom can form 4 bonds with other atoms.
The general formula of alkane: \(C_nH_{2n+2}\)
The molecular formula and structural formula of the first ten member of alkanes is as shown below:

Number of Carbon Atoms	Molecular Formula	Structural Formula
Methane
1	\(CH_4\)
Ethane
2	\(C_2H_6\)
Propane
3	\(C_3H_8\)
Butane
4	\(C_4H_{10}\)

Pentane
5	\(C_5H_{12}\)
Hexane
6	\(C_6H_{14}\)
Heptane
7	\(C_7H_{16}\)

Octane
8	\(C_8H_{18}\)
Nonane
9	\(C_9H_{20}\)
Decane
10	\(C_{10}H_{22}\)

Physical Properties of Alkanes
Name of Alkanes	Pysical State
Methane	Gas
Ethane	Gas
Propane	Gas
Butane	Gas
Pentane	Liquid
Hexane	Liquid
Heptane	Liquid
Octane	Liquid
Nonane	Liquid
Decane	Liquid

Physical Properties of Alkanes
Properties	Explaination
Melting Point
Increases as going down the group.	As going down the group, the molecular size increases. Requires more energy to overcome the attractive forces.
Boiling Point
Increases as going down the group.	As going down the group, the molecular size increases. Requires more energy to overcome the attractive forces.
Density
Increases as going down the group.	As going down the group, the molecular size increases.
Solubility in Water
Insoluble in water.	Alkanes are covalent compounds.
Conductivity of Electricity
Does not conduct electricity.	Does not contain freely moving ions.

Alkene

Alkene: a group of organic compounds that are unsaturated hydrocarbons.
The general formula of alkene: \(C_nH_{2n}\)
Homologous series: a group of organic compounds represented by a general formula, have the same functional group and every member is different from the others by a common structural unit, \(-CH_2\).
Functional group: atom or group of atoms that determines the chemical properties of members of a homologous series.
The molecular formula and structural formula of the first ten member of alkene is as stated below:

Number of Carbon Atoms	Molecular Formula	Structural Formula
Ethene
2	\(C_{2}H_{4}\)
Propene
3	\(C_{3}H_{6}\)
Butene
4	\(C_{4}H_{8}\)

Pentene
5	\(C_{5}H_{10}\)
Hexene
6	\(C_{6}H_{12}\)
Heptene
7	\(C_{7}H_{14}\)
Octene
8	\(C_{8}H_{16}\)

Nonene
9	\(C_{9}H_{18}\)
Decene
10	\(C_{10}H_{20}\)

Physical Properties of Alkenes
Name of Alkene	Physical State
Ethene	Gas
Propene	Gas
Butene	Gas
Pentene	Liquid
Hexene	Liquid
Heptene	Liquid
Octene	Liquid
Nonene	Liquid
Decene	Liquid

Properties	Explaination
Melting Point
Increases as going down the group.	As going down the group, the molecular size increases. Requires more energy to overcome the attractive forces.
Boiling Point
Increases as going down the group.	As going down the group, the molecular size increases. Requires more energy to overcome the attractive forces.
Density
Increases as going down the group.	As going down the group, the molecular size increases.
Solubility in Water
Insoluble in water.	Alkenes are covalent compounds.
Conductivity of Electricity
Does not conduct electricity.	Does not contain freely moving ions.

Alcohol

Functional group: hydroxyl group, \(-OH\).
General formula: \(C_nH_{2n+1}OH\).
The structural formula and molecular formula of alcohol is as shown below:

Naming Alcohol

Replacing the ‘e’ at the end of the alkane name with ‘ol’.
Example: methane to methanol.
For a longer alcohol chain, the position of the hydroxyl group is mentioned in the name of the alcohol.

Name of Alcohol	Structural Formula
Butan-1-ol
Butan-2-ol

Steps in Naming of Alcohol

Determine the name of the main chain based on the number of carbon atoms in the longest and continuous carbon chain.
The numbering of the main chain starts from the end closes to the hydroxyl group.
Determine the position of the hydroxyl group in the main carbon chain.
Determine the position of the branch chain.

Remember, priority is given to the hydroxyl group first, then the branch chain.

Uses of Alcohol and Its Misuse
Industry	Uses
Daily life	Solvents for paints, sticker, ink, lacquer and varnish. Used as fuel.
Production	Production of chemicals such as ethanoic acid and ester.
Manufacturing	Cosmetics, perfumes, toothpaste and skin lotions.
Pharmaceutical	To make medicines

Carboxylic Acid

Functional group: carboxyl group, \(-COOH\).
General formula: \(C_nH_{2n+1}COOH\).

Naming of Carboxylic Acid

Replacing the ‘ol’ at the end of alcohol name with ‘oic acid’.
Example: methanol to methanoic acid.

Steps in Naming Carboxylic Acid

Determine the name of the main chain based on the number of carbon atoms in the longest and continuous carbon chain.
The numbering of the longest carbon chain starts from the carboxyl group.
Determine the position of the branch chain.

Example: Structural Formula for 3,4-Dimethylhexanoic Acid

The diagram shows the structural formula of 3,4-dimethylhexanoic acid.

Uses of Carboxylic Acid

Fruits and vegetables contain carboxylic acid.
- Orange has citric acid.
Milk becomes sour because of the lactic acid formed.
Used in daily life items:
- Cosmetics.
- Liquid detergent.
Used in the food industry:
- Food flavouring.
- Food preservatives.