Nomenclature (Oxford AQA International A Level Chemistry)

Types of Chemical Formulas

• Organic compounds can be represented by:

  • Empirical formula

  • Molecular formula

  • General formula

  • Structural formula

  • Displayed formula

  • Skeletal formula

Empirical and molecular formulae

• The empirical formula shows the simplest whole number ratio of the elements present in one molecule of the compound

  • Eg. the empirical formula of ethanoic acid is CH₂O

• The molecular formula shows the number and type of each atom in a molecule

  • Eg. the molecular formula of ethanoic acid is C₂H₄O₂

General and structural formulae

• The general formula is a formula that represents a homologous series of compounds using letters and numbers

  • For example, the general formula of alkanes is C_nH_2n+2

  • A homologous series is a group of organic compounds that have the same functional group, the same general formula and the same chemical properties

• The structural formula is a formula that shows how the atoms are bonded to each carbon atom in a molecule

Displayed and skeletal formulae

• The displayed formula is a 2D representation of an organic molecule showing all its atoms (by their symbols) and their bonds (by single, double or triple bonds)

• The skeletal formula is a simplified displayed formula with all the carbon and hydrogen (C-H) bonds removed

Overview of the formulae of organic compounds table

Homologous Series & Functional Groups

• A functional group is a specific atom or group of atoms which determine the physical and chemical properties of the molecule

  • Organic molecules are classified by the dominant functional group on the molecule

• Alkanes are the simplest hydrocarbons with no functional group

  • Hydrocarbons are compounds that are made up of carbon and hydrogen atoms ONLY

• They are made up of carbon and hydrogen atoms bonded to each other with single covalent bonds

Table of common functional groups

• Organic compounds with the same functional group, but a different number of carbon atoms, are said to belong to the same homologous series

  • Every time a carbon atom is added to the chain, two hydrogen atoms are also added

Homologous Series of Alkanes Table

• Each member of a homologous series:

  • has the same functional group

  • has the same general formula

  • has similar chemical properties

  • differs by  -CH₂ -

• Members of a homologous series have gradually changing physical properties

  • For example, boiling point, melting point and density

A graph of boiling point for the first eight alkanes

• The broad trend is that boiling point increases with increased molecular size

  • Each additional -CH₂ - increases the strength of the intermolecular forces

  • This leads to a higher boiling point

• These trends are followed in other homologous series

• Similar trends are seen in other physical properties, such as melting point and density

IUPAC Nomenclature

• IUPAC nomenclature can be used to name organic compounds and therefore make it easier to refer to them

  • It can also be referred to as systematic nomenclature

• The alkanes provide the basis of the naming system and the stem of each name indicates how many carbon atoms are in the longest chain in one molecule of the compound

Nomenclature of organic compounds table

• If there are any side-chains or functional groups present, then the position of these groups is indicated by numbering the carbon atoms in the longest chain

  • Numbering starts at the end that gives the lowest possible numbers in the name

• The hydrocarbon side-chain is shown in brackets in the structural formula

  • e.g. CH₃CH(CH₃)CH₂CH₃

• The side-chain is named by adding ‘-yl’ to the normal alkane stem

  • This type of group is called an alkyl group

 Naming branched alkanes

• If there is more than one of the same alkyl side-chain or functional groups, di- (for two), tri- (for three) or tetra- (for four) is added in front of its name

• The adjacent numbers have a comma between them

• Numbers are separated from words by a hyphen

 Naming alkanes with multiple branches

• If there is more than one type of alkyl side-chain, they are listed in alphabetic order

Naming alkanes with multiple different branches

Functional groups and their nomenclature table