Naming Alkanes & Alkenes (WJEC GCSE Chemistry)

Simple Alkanes & Alkenes

• 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

Complex Alkanes & Alkenes

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