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Isomerism

 
2.4  Isomers and Naming Based on IUPAC Nomenclature
 
Definition of Structural Isomerism
A phenomenon where a compound has the same molecular formula but with two or more different structural formulae.
 
Definition of Isomer
Molecules that have the same molecular formula but different structural formulae.
 
Way Structural Isomerism Can Occur
  • Structural isomerism can occur in several ways:
    • Chain isomerism.
    • Position isomerism.
 
  Structural Isomerism
Type Chain Isomerism Position Isomerism
Definition The isomers have different arrangements of carbon chains; either straight chain or branched chain. The isomers have different positions of functional group on the same carbon chain.
Example

\(C_4H_{10}\)

The diagram shows the isomer of butane.

Butane

 

The diagram shows the isomer of 2-methylpropane.

2-methylpropane

\(C_4H_{8}\)

The diagram shows the isomer of but-1-ene.

But-1-ene

 

The diagram shows the isomer of but-2-ene.

But-2-ene
 

 

 
Chemical and Physical Properties of Isomer
  • The isomers show:
    • The same chemical properties because each isomer has the same functional group.
    • The physical properties, such as melting point and boiling point, are different. The more branches there are, the lower the melting point and boiling point are.
 
The Number of Isomers of Molecule
  • Generally, the number of isomers of a molecule increases with the increase of the number of carbon atoms in the molecule.
  • The isomers of alkanes are formed by chain isomers only.
  • Isomers of alkenes, alkynes and alcohol are formed from chain isomers and position isomers.
 
Steps in Drawing Isomers
For Alkanes
  1. Start by connecting the carbon atoms in a straight chain followed by a branched chain.
For Alkene and Alkyne
  1. Start with a straight chain formula and change the position of the double bond or triple bond to a different carbon position.
  2. Next, draw a structural formula with a branched chain from each straight chain that has different positions of double or triple bonds.
 
Naming of Isomers According to the IUPAC Nomenclature
  • IUPAC: International Union of Pure and Applied Chemistry.
  • There are three parts in the naming of isomers:
    1. Prefix indicated by the branch group, which is alkyl group, attached to the longest carbon chain.
    2. The root name showing the number of carbon atoms in the longest carbon chain.
    3. The suffix showing the homologous series.
 
Steps for Writing Name of Isomer
The image is a flowchart with a light blue background. It consists of four red circles with white text inside, connected by blue plus signs and an equal sign. The text in the circles reads: 1. Branch chain 2. Longest carbon chain 3. Homologous series 4. IUPAC name
 

 

 
Parts in Naming of Isomers
1.The Prefix Indicates the Branch Group, which is the Alkyl Group, with the General Formula \(C_nH_2n+1\) that is Attached to the Longest Carbon Chain

 

Name of Alklyl Group Structural Formula Molecular Formula
Methyl The diagram shows the structural formula of methyl. \(-CH_3\)
Ethyl The diagram shows the structural formula of ethyl. \(-C_2H_5\)
 

 

 

 

Propyl The diagram shows the structural formula of propyl. \(-C_3H_7\)
2. The Root Name Shows the Number of Carbon Atoms in the Longest Carbon Chain
3. The Suffix Shows the Homologous Series

 

Homolog Suffix
Alkane 'ane'
Alkene 'ene'
Alkyne 'yne'
Alcohol 'ol'
 

 

 
Steps in Naming of Isomers According to the IUPAC Nomenclature
  Isomer of Alkane
  1. Identify the longest carbon chain containing the functional group
  2. Number the carbon atoms in the longest chain  from one end so that:
  • The branch gets the lowest number for alkane.

The diagram shows isomer of alkane namely 2-methylpropane.

  • Longest carbon chain: 3 carbons
  • Root name: Prop
3. Identify branch and functional group
  • Branch: methyl
  • Functional group: Only single bond in the longest carbon chain (alkane)
4. Identify position of branch and functional group
  • Branch: attached to Carbon 2
  • Prefix
  • 2-methyl
  • Suffix
  • ane
  • Name of isomer
  • 2-methylpropane
 

 

 
Steps in Naming of Isomers According to the IUPAC Nomenclature
  Isomer of Alkene
  1. Identify the longest carbon chain containing the functional group.
  2. Number the carbon atoms in the longest chain  from one end so that:
  • the functional group gets the lowest  number for alkene.

The diagram shows isomer of alkene namely 2-methylprop-1-ene.

  • Longest carbon chain: 3 carbons
  • Root name: Prop
3. Identify branch and functional group.
  • Branch: methyl
  • Functional group: Double bond in the longest carbon chain (alkene)
4. Identify position of branch and functional group.
  • Branch: attached to Carbon 2
  • Functional group: In between Carbon 1 and Carbon 2
  • Prefix
  • 2-methyl
  • Suffix
  • -1-ene
  • Name of isomer
  • 2-methylprop-1-ene
 

 

 
Steps in Naming of Isomers According to the IUPAC Nomenclature
  Isomer of Alcohol
  1. Identify the longest carbon chain containing the functional group.
  2. Number the carbon atoms in the longest chain  from one end so that:
  • The functional group gets the lowest  number.

The diagram shows isomer of alcohol namely 2-metilpropan-2-ol.

  • Longest carbon chain: 3 carbons
  • Root name: Prop
3. Identify branch and functional group.
  • Branch: methyl
  • Functional group: Hydroxyl group (alcohol)
4. Identify position of branch and functional group
  • Branch: attached to Carbon 2
  • Functional group: attached to Carbon 2
  • Preffix
  • 2-methyl
  • Suffix
  • propan-2-ol
  • Name of isomer
  • 2-metilpropan-2-ol
 

 

 
Uses of Homologous Series in Daily Life
Uses of Alkane and Alkene
  • Alkanes have high heat of combustion.
  • Thus, the main usages of alkanes are for fuel and raw materials in the petrochemical industry.
  • Alkenes are also used as raw materials in the petrochemical industry.
  • The uses of alkane and alkene is as stated below:
Alkane or Alkene Uses
Ethane
  • Production of ethene from ethane to make detergents and plastics.
  • Liquefied natural gas (LNG) that contains ethane is used as fuel for power stations.
Butane
  • Production of fuel for lighters and portable stoves.
  • LPG cooking gas when mixed with propane.
Ethene
  • Production of alcohol, such as ethanol.
  • Production of polythene, polyvinyl chloride (PVC) and polystyrene.
But-1,3-diene
  • Production of synthetic rubber to manufacture tyres and hot water bags.

 
Uses of Alcohol

 

Fuel
  • As fuel in clean fuel, bio fuel and gasohol.
  • Properties of alcohol:
    • Highly flammable, and combustion releases a lot of heat without soot.
Solvent
  • Used in;
    • Paint, lacquer, dyes and printing ink.
    • Cosmetics such as perfume, nail varnish, cream and lotion.
  • Properties of alcohol is as below:

The image is a diagram illustrating the properties of alcohol. It consists of a central blue circle labeled ‘Properties of Alcohol’ with four surrounding blue circles connected by dashed red arrows. The four circles are labeled: 1. Colourless 2. Miscible in water 3. Volatile 4. Good organic solvent The diagram is simple and uses a light blue color scheme. The Pandai logo is at the bottom center.
Manufacturing Sector
  • Raw materials in the production of vinegar, explosives, polymer perspex and fibre.
  • Properties of alcohol:
    • Chemically reactive
 

 

 
Pharmaceutical Products
  • In the medical field
    • Antiseptics for injections, surgeries and general hygiene.
    • Solvent for medicines such as cough medicine.
  • Properties of alcohol:
    • Antiseptic
    • Good organic solvent
    • Volatile
 
Effects of Misuse of Alcohol
  • Ethanol, \(C_2H_5OH\) is widely used in alcoholic beverages.
  • Long term consumption of alcoholic beverages has adverse effects on the function of the central nervous system.
Effects of Misuse of Alcohol
Causes addiction and mental disorders, such as depression and psychosis.
Birth defects if consumed by pregnant mothers.
May cause cirrhosis of the liver, liver failure, heart failure, gastritis, ulcer, pancreatitis and oral cancer.
Inebriation, cognitive impairment and having slow physical reflxes. Drunk drivers may cause road accidents.

 
 
Uses of Carboxylic Acid
  • The most important carboxylic acid is ethanoic acid, \(CH_3COOH\) that is widely used as:
    • food preservatives in chilli sauce, ketchup and other food flavourings.
    • raw materials with other chemicals to produce dyes, paints, pesticides and plastics.
  • Methanoic acid, \(HCOOH\) is used in the rubber industry for coagulation of latex.
  • Fatty acids are long-chain carboxylic acids used to make soap.
  • Carboxylic acid is also used to manufacture polymers, namely polyester such as terylene and polyamide such as nylon.
 
Usage of Ester
  • Esters with small molecules easily evaporate and are fragrant, making them suitable for use in the manufacturing of cosmetics and perfumes.
  • Esters are also used as food flavourings.
  • Ethyl ethanoate, \(CH_3COOC_2H_5\)​ is an ester, used as a solvent for organic compounds in lotions, nail varnishes, lacquers and glue.
  • Oils and fats are esters, formed between fatty acids and glycerol that are used in the production of soap.
  • Polyester is a polymer for the production of synthetic fabrics.
Ester Flavour
Methyl butanoate, \(C_3H_7COOCH_3\) Apple
Pethyl ethanoate, \(CH_3COOC_5H_11\) Banana
Ethyl butanoate, \(C_3H_7COOC_2H_5\) Pineapple
 

 

 

 

 

 

 

The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Carbon Compounds
Topic : Isomer and Naming According to IUPAC
Form 5 Chemistry
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