Edexcel IGCSE Chemistry Your notes
4.1 Introduction
Contents
4.1.1 Organic Compounds
4.1.2 Naming Organic Compounds
4.1.3 Classifying Organic Reactions
4.1.1 Organic Compounds
Your notes
Representing Organic Compounds
Introduction
Organic Chemistry is the scientific study of the structure, properties, and reactions of organic
compounds. Organic compounds are those which contain carbon
For conventional reasons metal carbonates, carbon dioxide and carbon monoxide are not included in
organic compounds
Definition of a Hydrocarbon
A compound that contains only hydrogen and carbon atoms
Representing Organic Molecules
Organic compounds can be represented in a number of ways:
Empirical Formulae
Molecular Formulae
General Formulae
Structural Formulae
Condensed Structural Formulae
The empirical formula shows the simplest possible ratio of the atoms in a molecule
For example: Hydrogen peroxide is H2O2 but the empirical formula is HO
The molecular formula shows the actual number of atoms in a molecule
For example:
The general formula shows a ratio of atoms in a family of compounds in terms of 'n' where n is a varying
whole number
For example, the general formula of a molecule that belong to the alkane family is CnH2n+2 Your notes
The displayed formula shows the spatial arrangement of all the atoms and bonds in a molecule
This is also known as the graphical formula.
For example:
In a structural formulae enough information is shown to make the structure clear, but most of the
actual covalent bonds are omitted
Only important bonds are always shown, such as double and triple bonds
Identical groups can be bracketed together
Side groups are also shown using brackets
Straight chain alkanes are shown as follows:
Exam Tip
For defining a hydrocarbon, you must specify that they are compounds which contain hydrogen and
carbon atoms only, no other element is present.You may not be asked to name branched chain
organic compounds but you will come across them. It is useful to know that the numbers in the names
of these compounds refer to the position of the side chains with respect to the main chain.
Organic Terminology
Three important terms to know in this topic are homologous series, functional group and isomerism Your notes
Homologous Series
This is a series or family of organic compounds that have similar features and chemical properties due
to them having the same functional group
All members of a homologous series have:
The same general formula
Same functional group
Similar chemical properties
Gradation in their physical properties
The difference in the molecular formula between one member and the next is CH2
Functional Group
Functional group: A group of atoms bonded in a specific arrangement that influences the properties
of the homologous series
Some examples are shown here
Structures and Names of Common Functional Groups
Your notes
Isomerism
Isomers are compounds that have the same molecular formula but different displayed formulae
Eg. propene and cyclopropane
Your notes
Isomers of C3H6 show the same molecular formula but different structures. Isomers can show similar
physical and chemical properties or if they have different functional groups, the properties can be
different.
4.1.2 Naming Organic Compounds
Your notes
Naming Organic Compounds
Names of compounds
The names of organic compounds have two parts: the prefix or stem and the end part (or suffix)
The prefix tells you how many carbon atoms are present in the longest continuous chain in the
compound
The suffix tells you what functional group is on the compound
Examples of Naming Organic Molecules
Further rules for naming compounds
When there is more than one carbon atom where a functional group can be located it is important to
distinguish exactly which carbon the functional group is on
Each carbon is numbered and these numbers are used to describe where the functional group is
When 2 functional groups are present di- is used as a prefix to the second part of the name
Your notes
Isomers can be distinguished by following the naming rules
Exam Tip
You may not be asked to name branched chain organic compounds but you will come across them. It
is useful to know that the numbers in the names of these compounds refer to the position of the side
chains with respect to the main chain.
4.1.3 Classifying Organic Reactions
Your notes
Classifying Organic Reactions
The reactions of organic compounds can be classified into substitution, addition and combustion
Substitution
A substitution reaction takes place when one functional group is replaced by another
Example: Methane reacts with bromine under ultraviolet light
CH4 + Br2 → CH3Br + HBr
Methane + Bromine → Bromomethane + Hydrogen Bromide
Addition
An addition reaction takes place when two or more molecules combine to form a larger molecule with
no other products
Example: Bromine will react with ethene and the bromine molecule will react and add across the
double bond of the ethene
C2H4 + Br2 → C2H4Br2
Ethene + Bromine → Dibromoethane
Combustion
This is the scientific term for burning. In a combustion reaction, an organic substance reacts with
oxygen to form carbon dioxide (or carbon monoxide if incomplete combustion) and water.
Example: Alkenes burn when heated in air of oxygen
If there is an unlimited supply of air / oxygen, the products are carbon dioxide and water:
CH4 + 2O2 → CO2 + 2H2O
C3H8 + 5O2 → 3CO2 + 4H2O
This is termed complete combustion
If there is a limited supply of air / oxygen, the products are carbon monoxide and water:
CH4 + 3 2 O2 → CO + 2H2O
This is termed incomplete combustion
