Branching and substituents


3-ethylhexane (1)
The longest carbon chain may not be all on the same line on the page.
In the alkane at the right the longest chain has six carbon atoms. If you have difficulty seeing this, move your mouse over the image.

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:

    CH3–
    CH3CH2–
    methyl
    ethyl
     
    Br–
    Cl– 
    bromo
    chloro
     




    Alkane 1 above has an ethyl substituent. 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.

    The presence of a hydrocarbon substituent causes the chain to be referred to as "branched".  Thus a two-carbon substitutent is sometimes referred to as a two-carbon branch.
     
  2. The functional group in the compound determines the numbering of the chain.
    In a haloalkane, the halogen substituent has the lowest possible number.
    For alkanes the substituent has the lowest possible number.
Dr Sheila Woodgate
© The University of Auckland 2002