Branching and substituents

1


3-ethylhexane

2


1-bromo-3-methylbutane
The longest carbon chain may not be on the same line on the page.
In the 1 the longest chain has six carbon atoms.
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The name is based on the longest chain containing the functional group.
 
In 2 the longest chain containing the functional group has four carbon atoms.

Any atom or group of atoms replacing H at a carbon is called a substituent.
 
  1. Substituents on the longest chain are included in the name as prefixes.  Common substituents are:
     
    bromoBr–methylCH3–
    chloroCl–ethylCH3CH2–

    Note that saturated hydrocarbon substituents have the same name as the parent alkane with the -ane at the end of the name substituted by -yl. Compound 1 has an ethyl substituent. Compound 2 has a methyl and a bromo substituent
     
    3

    2,2-dimethylpentane

    4

    2,3-dimethylbutane
    .
  2. The functional group in the compound determines the numbering of the chain. The number is referred to as the locator in the name.

    In a haloalkane, the halogen substituent has the lowest possible number.
    See compound 2.
    For alkanes the alkyl substituent has the lowest possible number.
     
  3. The prefix di- is used if two of the same substituent are present in the compound.
    The appropriate locators separated by a comma and followed by a dash precede the name as shown for compounds 3 and 4.

 
Dr Sheila Woodgate
© The University of Auckland 2002