OrgoDraw

Naming and drawing haloalkanes

Fluoro, chloro, bromo and iodo prefixes, how halogen substituents are numbered alongside alkyl groups, and primary, secondary and tertiary haloalkanes.

Updated

In a haloalkane (alkyl halide) one or more hydrogens of an alkane are replaced by halogen atoms. In systematic names the halogens never appear as a suffix. They are prefixes, treated exactly like alkyl substituents: fluoro, chloro, bromo andiodo.

The rules in brief

  • The parent is the alkane chain; halogens and alkyl groups are all prefixes.
  • Number the chain to give the lowest locants to all substituents together, halogens and alkyl groups alike. The halogen gets no special priority.
  • If the locant sets tie, the substituent that comes first alphabetically gets the lower number.
  • Prefixes are cited in alphabetical order: bromo before chloro before ethyl before fluoro before iodo before methyl.

Worked example: 2-bromo-2-methylpropane

Draw propane (three carbons). Put both the bromine and a methyl group on C2. The central carbon now has four carbon or bromine neighbours and no hydrogens. This compound is also called tert-butyl bromide.

Worked example: 2-bromo-3-methylbutane

Numbering from either end of the butane chain gives the locant set {2,3}. Because the sets tie, the alphabetically earlier prefix,bromo, takes the lower number. "3-bromo-2-methylbutane" describes the same molecule but is not the correct name.

Worked example: 1,2-dichloroethane

"Dichloro" means two chlorine atoms, and the locants 1,2 put one on each carbon. Compare 1,1-dichloroethane, where both chlorines are on the same carbon. They have the same formula (C2H4Cl2) but are different compounds.

Primary, secondary and tertiary

Haloalkanes are classified by the carbon bearing the halogen: primary (1°) if that carbon is bonded to one other carbon,secondary (2°) if bonded to two, tertiary (3°) if bonded to three. 1-Bromobutane is primary, 2-bromobutane secondary and 2-bromo-2-methylpropane tertiary. The classification matters for reactivity in substitution and elimination reactions.

1-bromobutane (1°)
2-bromobutane (2°)

Other names you will meet

Older "functional class" names write the alkyl group followed by the halide: methyl iodide (iodomethane), ethyl bromide (bromoethane),tert-butyl chloride (2-chloro-2-methylpropane). Some small polyhalogenated compounds have common names, such as chloroform for trichloromethane.

trichloromethane (chloroform)

Common mistakes

  • Giving the halogen priority in numbering. Halogens and alkyl groups count equally; only a tie is broken alphabetically.
  • Putting the halogen in the parent chain. A halogen forms one bond, so it can only be at the end of a branch.
  • Missing a multiplier. "1,1,1-trichloroethane" needs three chlorines on C1.