Naming Alkanes & Alkenes (WJEC GCSE Chemistry)

• Simple alkanes and alkenes are the simplest compounds or isomers where the carbon atoms are arranged in a single chain 

What are alkanes?

• The general formula of the alkanes is C_nH_2n+2
• Alkanes are a group of saturated hydrocarbons
  • Carbon atoms must always have four bonds
  • If all bonds between carbon atoms are single bonds, then the compound is saturated 
• They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases
• Alkanes are generally unreactive compounds but they:
  • Undergo combustion reactions
  • Can be cracked into smaller molecules

Naming Alkanes

• The first four alkanes have the following prefixes:
  • 1 carbon atom = meth
  • 2 carbon atoms = eth
  • 3 carbon atoms = prop
  • 4 carbon atoms = but
• The prefixes of the alkanes then follow a more mathematical approach:
  • 5 carbon atoms = pent
  • 6 carbon atoms = hex
• Alkanes also contain -ane in their name
  • This indicates that they are saturated, containing only single carbon-carbon bonds
• For example, the first alkane contains one carbon atom and only single carbon-carbon bonds
  • Therefore, the first alkane is called meth + ane = methane 
• The names and structures of the first five alkanes are shown below:

Table of alkanes

Displayed formula	Name	Molecular formula
	methane	CH4
	ethane	C2H6
	propane	C3H8
	butane	C4H10
	pentane	C5H12

The first five members of the alkane homologous series

What are alkenes?

• The general formula of an alkene is C_nH_2n
• Alkenes are a group of unsaturated hydrocarbons
  • The term unsaturated means that they have at least one carbon-carbon double bond as well as carbon-carbon single bonds
  • The carbon-carbon double bond is shown as two lines between two of the carbon atoms, i.e. C=C
• They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases
• The carbon-carbon double bond in alkenes is the functional group, which allows alkenes to react in ways that alkanes cannot
• Alkenes are reactive compounds that:
  • Undergo combustion reactions
  • Can react with other chemicals in addition reactions
  • Undergo addition polymerisation to form polymers / plastics

Naming Alkenes

• It is not possible to form an alkene with only one carbon atom as the carbon-carbon double bond of an alkene requires two carbon atoms
• The first three alkenes have the following prefixes:
  • 2 carbon atoms = eth
  • 3 carbon atoms = prop
  • 4 carbon atoms = but
• The prefixes of the alkenes then follow the same mathematical approach as alkanes:
  • 5 carbon atoms = pent
  • 6 carbon atoms = hex
• Alkenes also contain -ene in their name
  • This indicates that they are unsaturated, containing at least one double carbon-carbon bond
• For example, the first alkene contains two carbon atoms and one double carbon-carbon bond
  • Therefore, the first alkene is called eth + ene = ethene 
• The names and structures of the first four alkenes are shown below:

Table of alkenes

Displayed formula	Name	Molecular formula
	ethene	C2H4
	propene	C3H6
	but-1-ene	C4H8
	pent-1-ene	C5H10

The first four members of the alkene homologous series

Higher Tier

Naming complex alkanes

• More complex alkanes have alkyl side-chains / branches attached
• These alkyl side-chains are limited to:
  • Methyl groups, -CH₃ 
  • Ethyl groups, -C₂H₅ 

• The process for naming complex alkanes is:
  • Identify the longest carbon chain and types of carbon-carbon bonds (the same as naming simple alkanes)
  • Identify the alkyl side-chain that is attached
  • Identify the carbon that the alkyl side-chain is attached to

Guide to naming complex alkanes 

	Methyl group example	Ethyl group example
Molecule
Longest carbon chain	5 = pent	5 = pent
Type of carbon-carbon bond	all single = ane	all single = ane
Side chain including carbon number	methyl on carbon-3	ethyl on carbon-3
Chemical name	3-methylpentane	3-ethylpentane

Naming complex alkenes

• More complex alkenes will, also, have alkyl side-chains / branches attached
  • The side-chains are still limited to methyl and ethyl groups 
• The process of naming complex alkenes follows a similar process to naming complex alkanes:
  • Identify the longest carbon chain including the carbon-carbon double bond
  • Identify where the carbon-carbon double bond is located
  • Identify the alkyl side-chain and which carbon it is attached to