3.3: An Introduction to Nomenclature

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The increasingly large number of organic compounds identified with each passing day, together with the fact that many of these compounds are isomers of other compounds, requires that a systematic nomenclature system be developed. Just as each distinct compound has a unique molecular structure which can be designated by a structural formula, each compound must be given a characteristic and unique name.

Introduction

As organic chemistry grew and developed, many compounds were given trivial names, which are now commonly used and recognized. Some examples are:

Name	methane	butane	acetone	toluene	acetylene	ethyl alcohol
Formula	CH₄	C₄H₁₀	CH₃COCH₃	CH₃C₆H₅	C₂H₂	C₂H₅OH

Such common names often have their origin in the history of the science and the natural sources of specific compounds, but the relationship of these names to each other is arbitrary, and no rational or systematic principles underlie their assignments.

The IUPAC Systematic Approach to Nomenclature

A rational nomenclature system should do at least two things. First, it should indicate how the carbon atoms of a given compound are bonded together in a characteristic lattice of chains and rings. Second, it should identify and locate any functional groups present in the compound. Since hydrogen is such a common component of organic compounds, its amount and locations can be assumed from the tetravalency of carbon, and need not be specified in most cases.

The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature. Knowing these rules and given a structural formula, one should be able to write a unique name for every distinct compound. Likewise, given a IUPAC name, one should be able to write a structural formula. In general, an IUPAC name will have three essential features:

A root or base indicating a major chain or ring of carbon atoms found in the molecular structure.
A suffix or other element(s) designating functional groups that may be present in the compound.
Names of substituent groups, other than hydrogen, that complete the molecular structure.

As an introduction to the IUPAC nomenclature system, we shall first consider compounds that have no specific functional groups. Such compounds are composed only of carbon and hydrogen atoms bonded together by sigma bonds (all carbons are sp³ hybridized).

An excellent presentation of organic nomenclature is provided on a Nomenclature Page. created by Dave Woodcock. A full presentation of the IUPAC Rules is also available.

Contributor

William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry
Layne Morsch (University of Illinois Springfield)