Chemistry » Organic Molecules » IUPAC Naming And Formulae

What is IUPAC Naming?

What is IUPAC naming?

In order to give compounds a name, certain rules must be followed. When naming organic compounds, the IUPAC (International Union of Pure and Applied Chemistry) nomenclature (naming scheme) is used. This is to give consistency to the names. It also enables every compound to have a unique name, which is not possible with the common names used (for example in industry). We will first look at some of the steps that need to be followed when naming a compound, and then try to apply these rules to some specific examples.

A good general rule to follow is to start at the end (the suffix) and work backwards (from right to left) in the name.

Tip:

Molecules can contain both double or triple bonds and other functional groups (e.g. an alkene and an alcohol functional group in one molecule – propenol). However, all molecules explored in this book will contain only single carbon-carbon bonds when combined with other functional groups.

  1. Recognise the functional group in the compound. This will determine the suffix of the name (see table below).

    Functional group

    suffix

    alkane

    -ane

    alkene

    -ene

    alkyne

    -yne

    alcohol

    -ol

    aldehyde

    -al

    ketone

    -one

    carboxylic acid

    -oic acid

    ester

    -oate

    Table: The suffix associated with various functional groups.

  2. Find the longest continuous carbon chain that contains the functional group (it won’t always be a straight chain) and count the number of carbon atoms in this chain. This number will determine the prefix (the beginning) of the compound’s name (see table below).

    Carbon atoms

    prefix

    1

    meth-

    2

    eth-

    3

    prop-

    4

    but-

    5

    pent-

    6

    hex-

    7

    hept-

    8

    oct-

    9

    non-

    10

    dec-

    Table: The prefix of a compound’s name is determined by the number of carbon atoms in the longest chain that contains the functional group.

  3. Number the carbons in the longest carbon chain (Important: If the molecule is not an alkane (i.e. has a functional group) you need to start numbering so that the functional group is on the carbon with the lowest possible number). Start with the carbon at the end closest to the functional group.

  4. Look for any branched groups:

    • Name them by counting the number of carbon atoms in the branched group and referring to the table above, these groups will all end in -yl.
    • Note the position of the group on the main carbon chain. If there is more than one of the same type of branched group then both numbers must be listed (e.g. 2,4 -) and one of the prefixes listed in the table below must be used. Important: If the molecule is an alkane the branched group must be on the carbon with the lowest possible number.
    • The branched groups must be listed before the name of the main chain in alphabetical order (ignoring di/tri/tetra).

    If there are no branched groups this step can be ignored.

    Number

    prefix

    2

    di-

    3

    tri-

    4

    tetra-

    Table: Prefixes for multiple substituents with the same name. These apply to multiple functional groups as well.

  5. For the alkyl halides the halogen atom is treated in much the same way as branched groups:

    • To name them take the name of the halogen atom (e.g. iodine) and replace the -ine with -o (e.g. iodo).

      Halogen

      name

      fluorine

      fluoro

      chlorine

      chloro

      bromine

      bromo

      iodine

      iodo

      Table: Naming halogen atoms in organic molecules.

    • Give the halogen atom a number to show its position on the carbon chain. If there is more than one halogen atom the numbers should be listed and a prefix should be used (e.g. 3,4-diiodo- or 1,2,2-trichloro-). See the table above for a list of the prefixes for multiple substituents with the same name.
    • The halogen atoms must be listed before the name of the main chain in alphabetical order (ignore di/tri/tetra).

    If there are no halogen atoms this step can be ignored.

  6. Combine the elements of the name into a single word in the following order:

    • branched groups/halogen atoms in alphabetical order (ignoring prefixes)
    • prefix of main chain
    • name ending according to the functional group and its position on the longest carbon chain.

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