Naming Organic Compounds (CIE IGCSE Chemistry)

• The names of organic compounds have two main parts: 
  • the stem (sometimes called the prefix) 
  • end part (or suffix)
• The stem indicates the number of carbon atoms present in the longest continuous chain of the compound

Number of carbon atoms in the longest chain	Part of the chemical's name
1	meth
2	eth
3	prop
4	but
5	pent
6	hex

• The end part of the name tells you what functional group is in the compound

End part of the name	Functional group	Organic family
ane	none (contains only C-C bonds)	Alkane
ene	C=C bond	Alkene
anol	–OH	Alcohol
anoic acid	–COOH	Carboxylic acid
amine	–NH2	Amine
-yl -anoate	–COOC–	Ester

 

Structures of organic compounds

Extended tier only

• 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 an -OH functional group 
  • The 1 in the name indicates that the -OH group is located on the first carbon atom
  • Careful: There are many times when the numbering should start from the right - this is to keep the numbers as low as possible in chemical names 

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

Alkanes

• As before, the number of carbon atoms gives the part of the compounds name
• Alkanes contain only single carbon-carbon bonds, C–C, which means that their name ends with –ane
  • Some compounds, like alcohols and carboxylic acids, have –an– in their name 
  • This indicates that those compounds contain only single carbon-carbon bonds, C–C

 

Alkenes

• As before, the number of carbon atoms gives the part of the compounds name
• Alkenes contain at least one double carbon-carbon bond, C=C, which means that their name ends with –ene
  • The first alkene is ethene because you must have two carbons to be able to form a double carbon-carbon bond, C=C
• After propene, you must state the number of the first carbon that is part of the double carbon-carbon bond, C=C
  • e.g. but-1-ene has a double carbon-carbon bond, C=C, on the first carbon in the chain
  • But-2-ene has a double carbon-carbon bond, C=C, on the second carbon in the chain

 

Alcohols

• As before, the number of carbon atoms gives the part of the compounds name
• Alcohols contain an alcohol / hydroxyl group, –O–H, which means that their name ends with –anol
• After ethanol, you must state the number of the carbon that has the alcohol / hydroxyl group, –O–H, attached
  • e.g. propan-1-ol has the alcohol / hydroxyl group, –O–H, on the first carbon in the chain
    Propan-2-ol has the alcohol / hydroxyl group, –O–H, on the second carbon in the chain

 

Carboxylic acids

• As before, the number of carbon atoms gives the part of the compounds name
• Carboxylic acids contain a carboxylic acid group, –COOH, which means that their name ends with –anoic acid
• There is no need to number carboxylic acids because the carbon that is part of the carboxylic acid group is autmatically the first carbon of the chain

 

Esters

• Esters are one of the more challenging compounds to name
• Their name is based on the original alcohol and carboxylic acid that they were prepared from

• Ester names are confusing because the name is written backwards from the way the structure is drawn

• Methyl methanoate
  • The acid portion of the molecule contains the C–O single bond and is coloured green
    • There is one carbon, so this gives the methyl part of the name
  • The carboxylic acid portion contains the C=O double bond and is coloured purple
    • The is one carbon, so this gives the methanoate part of the name
• Methyl ethanoate
  • The acid portion of the molecule contains the C–O single bond and is coloured green
    • There is one carbon, so this gives the methyl part of the name
  • The carboxylic acid portion contains the C=O double bond and is coloured purple
    • The are two carbons, so this gives the ethanoate part of the name
• Ethyl ethanoate
  • The acid portion of the molecule contains the C–O single bond and is coloured green
    • There are two carbons, so this gives the ethyl part of the name
  • The carboxylic acid portion contains the C=O double bond and is coloured purple
    • The are two carbons, so this gives the ethanoate part of the name