Naming Organic Compounds (Cambridge (CIE) O Level Chemistry): Revision Note

Naming Organic 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

Structures of organic compounds

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

• For example: 

  • Propan-1-ol is an alcohol with the -OH functional group 

  • The 1 in the name indicates that the -OH functional group is located on the first carbon atom 

In propan-1-ol the -OH group is located on the first carbon atom

Alkanes

• Alkanes are saturated hydrocarbons 

  • The term saturated means that they only have single carbon-carbon bonds, there are no double bonds

• The bonds are covalent bonds due to being between non-metal atoms 

• The general formula of the alkanes is C_nH_2n+2

  • This means that we can double the number of carbon atoms and add two to find the number of hydrogen atoms 

  • E.g. An alkane with 15 carbon atoms has (2 x 15) + 2 = 32 hydrogen atoms

  • Therefore, the formula of this alkane would be C₁₅H₃₂

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

 Alkenes

• All alkenes contain a carbon- carbon double bond, which is shown as two lines between two of the carbon atoms i.e. C=C

  • They are therefore called unsaturated hydrocarbons 

• Alkenes have the general formula C_nH_2n 

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

Alcohols

• All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecule that is responsible for their characteristic reactions

• Alcohols are a homologous series of compounds that have the general formula C_nH_2n+1OH

• They differ by one -CH₂ in the molecular formulae from one member to the next

The -OH group in alcohols

The first three alcohols

• The names and structures of the first three alcohols are shown below

• In terms of naming, the same system is used as for alkanes and alkenes, with the final ‘e’ being replaced with ‘ol’

Table to show the name, formula and displayed formula of the first three alcohols

Name	Formula	Displayed formula
Methanol	CH3OH
Ethanol	C2H5OH
Propan-1-ol	C3H7OH

Carboxylic acids

• Carboxylic acids are a homologous series of compounds that have the general formula of

C_nH_2n+1 COOH 

The general formula of a carboxylic acid

• Ethanoic acid has the formula CH₃COOH

• They differ by one -CH₂ in the molecular formulae from one member to the next

The first three carboxylic acids

Naming Esters

• Alcohols and carboxylic acids react to make esters in esterification reactions

• Esters are compounds with the functional group R-COO-R

• Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

The ester functional group

• The first part of an ester name indicates the length of the carbon chain in the alcohol, and it ends with the letters ‘- yl’

• The second part of an ester name indicates the length of the carbon chain in the carboxylic acid, and it ends with the letters ‘- oate’

  • e.g. the ester formed from ethanol and butanoic acid is called ethyl butanoate

Diagram showing the origin of each carbon chain in ester

• Some examples of common esters:

Table showing the names of Esters