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

• An alkane is an example of a hydrocarbon. In this section, we will mention alkanes, however we will go into more detail on them in a following section. 

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 belongs to the alkane family is C_nH_2n+2

• 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 formula, 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:

Organic Terminology

• Three important terms to know in this topic are homologous series, functional group and isomerism

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

Isomerism

• Isomers are compounds that have the same molecular formula but different displayed formulae

  • Eg. propene and cyclopropane

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

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

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   +   ½O₂   →   CO   +   2H₂O

• This is termed incomplete combustion