Naming Molecules (Chemical Nomenclature)

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Page author; Thomas Squire

The main purpose of chemical nomenclature is to ensure every chemical can be referred to uniquely when both spoken and written. Often some chemicals are referred to by their common name (i.e water is not called dihydrogen oxide), but it is important to be able to systematically differentiate chemicals by name in a standardized way. The most widely accepted format for nomenclature has been established by IUPAC (international union of pure and applied chemistry). Although this can become complex, there are a set of rules to follow to allow scientists around the world to name molecules in a standardised way. These rules are summarised here, with some useful tips to help apply them!

Useful to know; 

CodeNo. carbons
meth1
eth2
prop3
but4
pent5
hex6
hept7
oct8
non9
dec10
  • A prefix- is at the start of a word
  • A -suffix is at the end of a word

Rules for naming carbon chains

  • Count the longest carbon chain and name appropriately
  • Find any branched chains and count how many carbons they contain
  • Add the appropriate prefix for each branch chain
  • A carbon bonded to another branch must have the lowest possible carbon number – this identifies which end of the molecule to label as carbon 1. 
  • Alphabetical order is then taken into account when writing out the name of the molecule with multiple branches. 

Advanced rules for naming carbon chains (Not IUPAC)

Some common carbon branches are given prefixes such as iso- (double carbon branch), and but- (triple carbon branch) to shorten the name.

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Rules for naming functional groups

When compounds contain more than one functional group, the order of precedence determines which groups are named with a prefix or suffix. The highest precedence group takes the suffix, with all others taking the prefix form. However, double and triple C-C bonds only take suffix form.

Order of priority highest first:

Carboxylic acids > carboxylic acid derivative > nitriles > aldehydes > ketones > alcohols > amines

Tipwhen using a suffix, add in the following way:

  • If the suffix starts with a vowel- then remove the –e from the stem alkane name e.g. propan-1-ol, butan-1-amine, ethanoic acid, butanamide
  • If the suffix starts with a consonant or there multiple functional group meaning di, or tri needs to be used, do not remove the –e from the stem alkane name e.g. propanenitrile, ethane-1,2-diol, propanedioic acid.

Rules;

  • The position of the functional group on the carbon chain is given by a number – counting from the end of the molecule that gives the functional group the lowest number (like with carbon chain branches!).
  • You only include numbers, if they are needed to avoid ambiguity!
  • Where there are two or more of the same groups, di-, tri- or tetra are used.
  • The functional groups take precedence over branched chains in giving the lowest number (i.e 3-methylbut-1-ene is correct and NOT 2-methylbut-3-ene).
  • Words are separated by numbers with dashes
  • Numbers are separated by commas
  • If there is more than one functional group or side chain, the groups are listed in alphabetical order ignoring any di, tri or tetra (like with carbon chain branches!).

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Naming specific functional groups

To learn how to name specific functional groups, see the chemBAM page.

Quiz

To test your knowledge on naming molecules see this chemBAM quiz.

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