Nomenclature of Aliphatic Compounds
- Systematic nomenclature can be used to name organic compounds and therefore make it easier to refer to them
- 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
| Number of C atoms | Molecular formula of straight-chain alkane | Name of alkane | Stem used in naming |
| 1 | CH4 | methane | meth- |
| 2 | C2H6 | ethane | eth- |
| 3 | C3H8 | propane | prop- |
| 4 | C4H10 | butane | but- |
| 5 | C5H12 | pentane | pent- |
| 6 | C6H14 | hexane | hex- |
| 7 | C7H16 | heptane | hept- |
| 8 | C8H18 | octane | oct- |
| 9 | C9H20 | nonane | non- |
| 10 | C10H22 | decane | dec- |
- 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
The numbering system always gives the lowest possible number in the name. So, this example is 3-methyl hexane
- 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
The number for each carbon that has a branch is specified in the name
- If there is more than one type of alkyl side-chain, they are listed in alphabetic order
Naming alkanes with multiple different branches
The number and type of each branch is specified in the name
Functional groups & their nomenclature table
Worked example
Name the following molecules using correct systematic nomenclature:
Answers:
- The chemical name is 2,5,5-trimethylhex-2-ene
- The longest carbon chain is six carbons long
- There is an alkene / double carbon-carbon bond on carbon-2
- There are 3 methyl groups on carbons 2, 5 and 5
- The chemical name is propanal
- The longest carbon chain is 3 carbons long with only single carbon-carbon bonds
- There is an aldehyde group which is fixed as carbon-1
- The chemical name is 2-methylbutanal
- The longest carbon chain is 4 carbons long with only single carbon-carbon bonds
- There is a methyl group on carbon-2
- There is an aldehyde group which is fixed as carbon-1
- The chemical name is butanoic acid
- The longest carbon chain is 4 carbons long with only single carbon-carbon bonds
- There is a carboxylic acid group which is fixed as carbon-1
- The chemical name is 2-chlorobutane
- The longest carbon chain is 4 carbons long with only single carbon-carbon bonds
- There is a chlorine atom attached to carbon-2
Exam Tip
An aliphatic compound is straight or branched-chain and also includes cyclic organic compounds that do not contain a benzene ring.
