Isomerism

 

Structural Isomerism

 
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

 

  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

\(C_4H_{10}\)

Butane

 

2-methylpropane

\(C_4H_{8}\)

But-1-ene

 

But-2-ene

 
  • 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.
  • 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

 

Name of alkyl group Structural formula Molecular formula
Methyl \(-CH_3\)
Ethyl \(-C_2H_5\)
Propyl \(-C_3H_7\)

 

Homologous series Suffix
Alkane 'ane'
Alkene 'ene'
Alkyne 'yne'
Alcohol 'ol'

 

  Isomer of alkane

 

Identify the longest carbon chain containing the functional group

Number the carbon atoms in the longest chain  from one end so that:

  • the branch gets the lowest number for alkane.

Longest carbon chain: 3 carbons

Root name: Prop

Identify branch and functional group

Branch: methyl

Functional group: Only single bond in the longest carbon chain (alkane)

Identify position of branch and functional group

Branch: attached to Carbon 2

Prefix 2-methyl
Suffix ane
Name of isomer 2-methylpropane

 

  Isomer of alkene

 

Identify the longest carbon chain containing the functional group.

Number the carbon atoms in the longest chain  from one end so that:

  • the functional group gets the lowest  number for alkene.

Longest carbon chain: 3 carbons

Root name: Prop

Identify branch and functional group

Branch: methyl

Functional group: Double bond in the longest carbon chain (alkene)

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

 

  Isomer of alcohol

 

Identify the longest carbon chain containing the functional group.

Number the carbon atoms in the longest chain  from one end so that:

  • the functional group gets the lowest  number.

Longest carbon chain: 3 carbons

Root name: Prop

Identify branch and functional group

Branch: methyl

Functional group: Hydroxyl group (alcohol)

Identify position of branch and functional group

Branch: attached to Carbon 2

Functional group: attached to Carbon 2

Prefix 2-methyl
Suffix propan-2-ol
Name of isomer 2-methylpropan-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.
 
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

  • colourless
  • good organic solvent
  • miscible in water
  • volatile
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