IUPAC Nomenclature

This guide is an early version — the text is complete, and a few figures are still being redrawn. Spotted something unclear? Let us know.

The question this page answers: How do organic chemists name molecules unambiguously given the near infinite combinations of atoms possible?

Deeper reading: Clayden 2e: Chapter 2 Page 33–42 — see our chapter-by-chapter practice map for Clayden.

A rigorous system for unambiguous names

What are the 4 key pieces?

The International Union of Pure and Applied Chemistry (IUPAC) has come up with a rigorous system for giving each unique organic molecule an unambiguous name. This system is called IUPAC Nomenclature, and contains 4 key pieces of information:

stereochemical indicators
prefixes for other substituents
root for parent framework
suffix for principal FG

A simplified but still sufficiently detailed method for IUPAC Nomenclature follows 6 steps for each molecule.

Stereochemical indicators are discussed later in the Stereochemistry notes.

The format of an IUPAC name

Indicators–prefixes–root–suffix

IUPAC nomenclature has the following format:

stereochemical indicators–prefixes–root for parent framework–suffix for principal FG

Here are the IUPAC names for a couple common molecules:

A 6-step method

From principal FG to stereochemistry

Here is a 6-step method for IUPAC nomenclature:

Identify the principal FG (i.e. highest ranked FG) using the table on the Brief Guide to IUPAC Nomenclature Handout.
Identify and name the root for the parent framework (chain or ring) containing the FG from Step 1. If multiple possibilities exist, rings > chains, long chains > short chains, more multiple bonds > less multiple bonds.
Apply the appropriate suffix to the parent framework name from Step 2.
Assign lowest possible locant (i.e. #) to the position of the principal FG. Start from an end in chains. For ties, pick the option with lowest # at first point of difference. If none, then alphabetize prefixes and give first substituent lowest possible #.
Alphabetically list the appropriate prefixes and numbering/lettering for other substituents along the parent framework.
If applicable, add stereochemical and other indicators (e.g. R/S or E/Z)

Here are two examples of this nomenclature method in action:

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