go to the AUS-e-TUTE homepage
home test Join AUS-e-TUTE game contact
 

 

Introduction to the IUPAC System for Naming Carbon Compounds

Key Concepts

  • Nomenclature is a system of rules by which the names of compounds are formed.

  • For nomenclature purposes, all compounds containing carbon are said to be organic compounds.

  • When naming organic compounds, punctuation marks are used for specific purposes:1:

    (i) the first letter of the name is not a capital letter unless the name occurs at the beginning of a sentence
        Example: Acetone is the preferred IUPAC name for propan-2-one.

    (ii) commas (,) are used to separate one number from another number
        Example: the numbers 1 and 2 are written as 1,2

    (iii) hyphens (-) are used to separate a number from a letter
        Example: 2-methylpropane and propan-2-ol

    (iv) there are no spaces (gaps) between letters unless the compound is a

        (a) carboxylic acid: a space between alkanoic and acid
        Example: butanoic acid

        (b) salts: a space between the names of the cation and the anion
        Example: sodium butanoate

        (c) acid halides: a space between name derived from carboxylic acid (alkanoyl) and the halogen (halide)
        Example: butanoyl chloride

        (d) ester : a space between name derived from alcohol (alkyl) and that derived from alkanoic acid (alkanoate)
        Example: ethyl butanoate

        (e) acid anhydride: a space between alkanoic and anhydride
        Example: acetic anhydride

  • Hydrocarbon molecules, those made up of only carbon and hydrogen atoms, are the basis upon which the name of an organic molecule is constructed.

  • The name of the hydrocarbon used as the basis for the construction of the name of an organic molecule is referred to as the parent hydrocarbon2.

  • The name of the parent hydrocarbon is made up of a prefix and a suffix:

    parent hydrocarbon
    prefix suffix

  • The principal nomenclature used in organic chemistry, and which forms the basis of most IUPAC3 preferred names, is substitutive nomenclature.

  • Substitutive nomenclature is based on the concept of different atoms, or groups of atoms, substituting for the atoms on a parent hydrocarbon molecule.
    These atoms, or groups of atoms, are known as substituents.

  • The name of an organic compound containing a substituent is constructed by modifying the name of the parent hydrocarbon using prefixes and suffixes:

    organic compound name
    prefix parent hydrocarbon suffix

    Note that the prefixes and/or suffixes themselves may include infixes (numbers which tell us the location of the substituent).

  • For simple organic compounds containing only one type of functional group:

    (i) Prefixes are modified by branches (side chains) and by the presence of halogens

    (ii) Suffixes are modified by the presence of the functional groups OH, C=O, COOH, NH2

  • The construction of substitutive names using the parent hydrocarbon as a foundation requires the application of 4 general rules:

    1. numbering (of the longest carbon chain)

    2. locants (location of structural features along the chain)

    3. multiplying prefixes (di, tri, tetra etc)

    4. alphanumerical order (ascending alphabetical order by first letter)

  • Important exceptions to the use of substitutive names as preferred IUPAC names

    (i) anhydrides : functional class nomenclature is preferred
        example: acetic anhydride is the preferred name for CH3-CO-O-CO-CH3

    (ii) esters: functional class nomenclature is preferred
        example: methyl propanoate is the preferred IUPAC name for CH3-CH2-CO-O-CH3

    (iii) polymers : the name of the repeating unit can be based on either its source or its structure

  • Note the preferred IUPAC spelling for the following elements:
    (i) Name of the element with the symbol S is sulfur (not sulphur).
    (ii) Name of the element with the symbol Al is aluminium (not aluminum).
    (iii) Name of the element with the symbol Cs is caesium (not cesium).

Parent Hydrocarbon

The parent hydrocarbon can be

  • a chain of carbon atoms (called an acyclic aliphatic compound4 )

  • a ring of carbon atoms (called an alicyclic or aliphatic cyclic compound)

  • a benzene ring (called an aromatic compound5)

The prefix for the name of an aliphatic parent hydrocarbon is determined by the number of carbon atoms in the chain or ring.
The prefix for the first four hydrocarbons in the series is based on a traditional name and the prefix has been retained.
Prefixes for aliphatic parent hydrocarbons containing more than four carbon atoms are based on multiplying prefixes (that is, the multiplying prefix with the final "a" removed, eg, penta becomes pent).

Number of carbon atoms:12345678910
Prefix:methethpropbutpenthexheptoctnondec
  retained names multiplying prefix

The suffix of an aliphatic parent hydrocarbon is determined by the nature of the covalent bonding between carbon atoms in the chain or the ring:

When one double or triple bond is present, an infix is required to tell us the location of the double or triple bond.
The infix is a number which is separated from the prefix and the suffix by a hyphen:

prefix-infix-suffix

If more than one double or triple bond is present in the parent hydrocarbon,
(i) an "a" may be added to the prefix of the parent hydrocarbon for euphonic reasons

buta, penta

(ii) a multiplying prefix (di, tri, tetra, etc) is also required and is added to the beginning of the suffix, in addition to an infix to describe the loction of each double or triple bond:

prefix-infix-multiplyingsuffix

If both double and triple bonds are present in the parent hydrocabon, an enyne molecule, then the en suffix is written before the yne suffix (that is, written in alphabetical order).
However, when numbering the carbon chain, the triple bond takes precedence over the double bond, so the set of locants will be chosen to give the triple bond(s) the lowest number(s) in preference to the double bond(s).

prefix-infix-multiplyingen-infix-multiplyingyne

If the aliphatic compound is a ring rather than a chain, cyclo is added to the beginning if the prefix:

cycloprefix-infix-multiplyingsuffix

Substituents

A substituent is an atom, or group of atoms, that substitutes for one or more hydrogen atoms on the parent hydrocarbon.

A substituent can be an alkyl group, a halogen atom or other functional group.

  • Alkyl groups (unbranched acyclic alkane-based hydrocarbons attached to a parent hydrocarbon)

    Alkyl groups are hydrocarbons that are themselves attached to the parent hydrocarbon molecule. The alkyl groups "branch off" the main "stem" of the parent, so these alkyl groups are often referred to as branches (or alternatively as side-chains). The presence of alkyl groups modifies the name of the parent hydrocarbon by adding a prefix to it.

    The prefix attached to the name of the parent hydrocarbon to represent a branch or side-chain ends in yl.

    Alkyl groups can thought of a straight chain alkane in which a hydrogen atom (H.)6 has been removed. This hydrogen atom can be removed from a terminal (end) carbon atom, or from a non-terminal carbon atom.

    (i) Removal of one hydrogen atom from a terminal (end) carbon atom of the alkane
    Named by removing the "ane" from the alkane and replacing it with yl to make alkyl:

    alkane
    (Lewis Structure)
      alkyl group
    (Lewis Structure)
    alkyl group
    (Valence Structure)
    H:H
    ..
    C
    ..
    H
    :H
    methane
    remove H.
    .H
    ..
    C
    ..
    H
    :H
    methyl
    -H
    |
    C
    |
    H
    -H
    methyl

    (ii) Removal of one hydrogen atom form a non-terminal carbon atom of an alkane.
    Named by dropping the final e of alkane then inserting an infix for the location of the missing hydrogen atom followed by a hyphen and the yl ending to make alkan-infix-yl:

    alkane
    (Lewis Structure)
      alkyl group
    (Lewis Structure)
    alkyl group
    (Valence Structure)
    H:H
    ..
    C
    ..
    H
    :H
    ..
    C
    ..
    H
    :H
    ..
    C
    ..
    H
    :H
    ..
    C
    ..
    H
    :H
    butane
    remove H.
    H:H
    ..
    C
    ..
    H
    :H
    ..
    C
    ..
    H
    :H
    ..
    C
    .
     
    :H
    ..
    C
    ..
    H
    :H
    butan-2-yl
    H-H
    |
    C
    |
    H
    -H
    |
    C
    |
    H
    -H
    |
    C
    |
     
    -H
    |
    C
    |
    H
    -H
    butan-2-yl

    Alkyl groups are written in ascending alphabetical order, using an infix to identify the location of each alkyl group, and a muliplying prefix if there is more than one of each type of alkyl group:

    example: infix-multiplyingethyl-infix-multiplyingmethyl-infix-multiplyingpropyl

    and all of this is added as a prefix to the name of the parent hydrocarbon.

  • Halogen atoms as substituents.

    A halogen atom (fluorine, chlorine, bromine or iodine atom) can replace any or all of the hydrogen atoms on a parent hydrocarbon molecule.
    When naming an organic molecule containing halogen substituents, the halogen is named by dropping the "ine" ending on the name of the element and adding "o" as an ending.

    Symbol for halogenFClBrI
    Name of Elementfluorinechlorinebromineiodine
    Prefix Used in Nomenclaturefluorochlorobromoiodo

    These "halo" names are written in alphabetical order, preceded by an infix to identify the location of each halogen atom along the chain, as well as a multiplying prefix if more than one type of halogen is present:
    infix-multiplyingbromo-infix-multiplyingchloro-infix-multiplyingfluoro-infix-multiplyingiodo

    and all of this is then added as a prefix to the name of the parent hydrocarbon.
    Note that if the compound contains both alkyl substituents and halogen substituents, then the names of all these substituents are placed in alphabetical order
    Example: infix-multiplyingbromo-infix-multiplyingchloro-infix-multiplyingethyl-infix-multiplyingflouro-infix-multiplyingmethyl

  • Functional (or characteristic) groups as substituents

    An organic molecule may contain:
        (i) only one type of functional group
        (ii) more than one type of functional group

    (i) Organic molecules in which only one type of functional group is present.

    When only one type of functional group is present in an organic molecule, then the functional group modifies the suffix attached to the name of the parent hydrocarbon:

    parent hydrocarbonfunctional group suffix
    alkansuffix

    The functional groups you are most likely to meet during an introductory chemistry course are:

    typeformulanomenclature
    carboxylic acids COOH (terminal C atom) oic acid (suffix) (see properties of alkanoic acids)

    esters -CO-O-C (functional naming rather than substitutive naming is preferred)
    (see esters and esterification)

    aldehydes C=O (terminal C atom) al (suffix)

    ketones C=O (non-terminal C atom) one (suffix)

    alcohols OH ol (suffix) (see naming straight-chain alkanols)

    amines NH2 amine (suffix) (see properties of amines)


    Notice that these functional groups will modify the suffix of the parent hydrocarbon as shown in the table above.
    If only one type of functional group is present in the organic molecule, an infix is not required when naming carboxylic acids or aldehydes as the functional group will also be defined as attached to the first carbon atom in the chain or ring.
    If only one type of functional group is present, but only one atom of carbon is present in the parent hydrocarbon, no infix is required since the functional group can only be attached to the first carbon atom by definition. Similarly, if the molecule contains 2 carbon atoms which are otherwise identical except that one is attached to the functional group, then the infix may not be required.

    When the suffix is added directly to the name of the parent hydrocarbon, the name of the parent hydrocarbon may be changed by removal of the final "e" from a parent alkane, alkene or alkyne, if the suffix to be added begins with a vowel or the letter "y"
    Example: ethanol not ethaneol     but,     ethane-1,2-diol is preferred to ethan-1,2-diol

    (ii) The construction of the name of an organic molecule that includes more than one functional (characteristic) group relies on the principle of seniority of classes.
    The order of seniority is used to choose a parent hydrocarbon.
    The senior parent structure has the maximum number of substituents corresponding to the principal characteristic (functional) group in accord with the seniority of classes and suffixes shown in the table below:

    senioritycompound class
    1Radicals
    2Radical anions
    3Radical cations
    4Anions
    5Zwitterions
    6Cations
    7Acids
    8Anhydrides
    9Esters
    10Acid halides and pseudohalides
    11Amides
    12Hydrazides
    13Imides
    14Nitriles
    15Aldehydes and chalcogen analogues
    16Ketones, pseudoketones and heterones.
    17Hydroxy compounds and chalcogen analogues
    18Hydroperoxides (peroxols)
    19Amines
    20Imines

    Within the same class, the preferred IUPAC nomenclature gives seniority to a ring over a chain.

    The most senior class is used to construct the suffix for the parent hydrocarbon.
    Other less senior functional groups are then used to construct modifying prefixes (including infixes and multiplying prefixes) to add to the name of the parent hydrocarbon.

Construction of Substitutive Names: Numbering Rule

(i) If only one structural feature is present, the longest carbon chain is numbered so that the locant is assigned the lowest possible number.
This locant is cited as an infix, that is, placed immediately in front of the feature being named.

(ii) If more than one structural feature is present, the longest carbon chain is numbered so that locants are assigned to each structural feature in order of decreasing seniority.

For simple organic molecules you are likely to encounter in an introductory chemistry course, the lowest locants are assigned in the following order:

    1st characteristic (functional) group cited as the suffix

    2nd unsaturation (double bond "ene" or triple bond "yne")

    3rd detachable alphabetized prefixes (alkyl groups)

Example,

numbering from left to right or numbering from right to left
  H
|
  H
|
  H
|
  OH
|
  H
|
  CH3
|
  H
|
 
H- C1 - C2 = C3 - C4 - C5 - C6 - C7 -H
  |
H
          |
H
  |
H
  |
H
  |
H
 
 
  H
|
  H
|
  H
|
  OH
|
  H
|
  CH3
|
  H
|
 
H- C7 - C6 = C5 - C4 - C3 - C2 - C1 -H
  |
H
          |
H
  |
H
  |
H
  |
H
 
1st: characteristic group cited as suffix: OH locant is 4
2nd: unsaturation: double bond locant is 2
3rd: detachable alphabetized prefixes: methyl locant is 6
  1st: characteristic group cited as suffix: OH locant is 4
2nd: unsaturation: double bond locant is 5
3rd: detachable alphabetized prefixes: methyl locant is 2

Since both numbering methods result in the OH group having a locant of 4, we move on to compare the locants for the double bond.
Numbering from left to right results in the lowest locant for the double bond, so the preferred numbering is from left to right.
The name of the molecule is therefore 6-methylhept-2-en-4-ol

Construction of Substitutive Names: Locant Rule

Locants are arabic numbers (1, 2, 3 etc) indicating the location of a structural feature, such as a functional group, alkyl group or unsaturation (double or triple bond), on a parent hydrocarbon.
Numbers are assigned to carbon atoms in a chain in ascending order as described above in the section "Numbering".

  H
|
  Cl
|
  CH3
|
  H
|
  H
|
    Locant for the hydroxy (hydroxyl) group, is 1
Locant for the methyl group, is 3
Locant for the chlorine atom, is 4
H- C5 - C4 - C3 - C2 - C1 -OH
  |
H
  |
H
  |
H
  |
H
  |
H
 

Locants are placed immediately before that part of the name to which they relate, forming the basis of the infix of a name7.
Example:

1-ol
3-methyl
4-chloro

If more than one of the same structural feature is present, then a locant must be included for each and every feature.
For multiple instances of structural features with the same seniority, the lowest set of locants is used.
The lowest set of locants is defined as the set that, when compared term by term with other locant sets, each cited in order of increasing value, has the lowest term at the point of difference.

Example:

  H
|
  CH3
|
  CH3
|
  H
|
  H
|
 
H- C1 - C2 - C3 - C4 - C5 -H
  |
H
  |
H
  |
H
  |
H
  |
H
 
 
  H
|
  CH3
|
  CH3
|
  H
|
  H
|
 
H- C5 - C4 - C3 - C2 - C1 -H
  |
H
  |
H
  |
H
  |
H
  |
H
 
Numbering from left to right produces the locant set 2,3 for the methyl groups   Numbering from right to left produces the locant set 3,4 for the methyl groups
Compare the two sets side by side
  first term in set second term in set  
first set
(left to right)
2 3 Point of difference occurs at the beginning of the sets.
2 (first set) is less than 3 (second set)
second set
(right to left)
3 4 first set of locants is preferred

Locants should always be included in the IUPAC name unless:

  • the compound is an alkanoic acid or alkanedioc acid
    example: CH3-CH2-CH2-COOH is named as butanoic acid (not butan-1-oic acid)
    example: HOOC-CH2-CH2-COOH is named as butanedioc acid (not as butane-1,4-dioic acid)

  • the compound is an alkanal
    example: CH3-CH2-CHO is propanal (not propan-1-al)

  • the compound contains only one carbon atom
    example: CH3Cl is chloromethane (not 1-chloromethane)

  • the compound contains only two carbon atoms and only one functional group is present
    example: CH3-CH2-OH is named ethanol (not ethan-1-ol)

  • the compound results from the single substitution of a hydrogen atom on a benzene ring or cyclic alkane
    example: HBr     named as bromobenzene (not 1-bromobenzene)

  • all substitutable positions are completely substituted
    example: CF3-CF2-CF2-COOH is named heptafluorobutanoic acid (not 2,2,3,3,4,4,4-heptafluorobutanoic acid)

Construction of Substitutive Names: Multiplying Prefixes

When more than one instance of the same structural feature is present in a molecule, a multiplying prefix is used which tells us how many there are.

  multiplying Prefixes
number (1) 2 3 4 5 6 7 8 9 10
prefix (hen) di tri tetra penta hexa hepta octa nona deca

number 11 20 30 40 50 60 70 80 90 100
prefix hendeca icosa triaconta tetraconta pentaconta hexaconta heptaconta octaconta nonaconta hecta

number 101 200 300 400 500 600 700 800 900 1000
prefix henhecta dicta tricta tetracta pentacta hexacta heptacta octacta nonacta kilia

number 1001 2000 3000 4000 5000 6000 7000 8000 9000  
prefix henkilia dilia trilia tetralia pentalia hexalia heptalia octalia nonalia  

There must be as many numbers representing the locants of each feature as are indicated by the multiplying prefix.
Example: the infix 1,1- requires the multiplying prefix di because there are two numbers
Example: the infix 2,2,3,5- requires the multiplying prefix tetra because there are four numbers

Example: 2,2-dimethylbutane tells us there are two methyl groups (dimethyl) and that both methyl groups are attached to the second carbon atom (2,2-) in the chain.

Note that a multiplying prefix is not required if there is only one instance of a particular structural feature
Example: 2-chlorobutane (not 2-chlorohenbutane and not 2-chloromonobutane)

Construction of Substitutive Names: Alphanumerical Order

Simple prefixes, that is, prefixes describing atoms and unsubstituted substituents, are arranged alphabetically.

For the substituents you are likely to meet during your introductory chemistry course, the order of citation will be:

order of citation first   second   third   fourth   fith   sixth   seventh
substituent bromo   chloro   ethyl   fluoro   iodio   methyl   propyl

The addition of a multiplying prefix (di, tri, tetra etc) to the beginning of the name of the substituent does NOT change the order of citation:
example: tetrabromo would still be cited before dichloro


What would you like to do now?

1Parenthese (), brackets [] and braces {} are also used when naming oganic molecules.
While you will probably not be needing these in an introductory chemistry course, you might need them when writing a linear (condensed structural) formula :
(i) parentheses (round brackets) used when a functional group is attached to part of a chain
example: CH3-CH(OH)-CO-CH3 is the linear formula for 3-hydroxybutan-2-one.
(ii) brackets [square brackets] used when a group is repeated in a chain
example: CH3-[CH2]68-CH3 is the linear formula for heptacontane

2More correctly we should refer to the parent hydride rather than the parent hydrocarbon since other atoms can substitute for carbon within the parent structure.

3IUPAC is the abbreviation for the International Union of Pure and Applied Chemistry.
IUPAC nomenclature began in 1892 when an international assembly of Chemists met in Geneva, Switzerland, to try to come up with a rational system for naming organic molecules.
The rules for naming organic compounds are still being developed.
The most recent document for referral is "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004).
This document supercedes 'Nomenclature of Organic Chemistry, 1979 Edition' and 'A Guide to IUPAC Nomenclature of Organic Compounds, Recommendations 1993'.
The current document recognises that there is more than one way to systematically name organic compounds, and develops rules for assigning "preferred IUPAC names", but any other name, as long as it is unambiguous and follows the principles of the IUPAC recommendations is acceptable as a "general IUPAC name".
As a result, there are examples of "preferred IUPAC" names that are based on the tradition of use, rather than a system of naming. Examples of these traditional names that students may be expected to know are acetone, acetic acid and its derivatives such as ethyl acetate and acetic anhydride, formic acid and its derivatives such as ethyl formate.

4The term aliphatic comes from the Greek aleiphatos meaning "fat". Early Chemists did not know the structure of a fat, but they did know that compounds containing long hydrocarbon chains behaved in a similar way to fats. So compounds containing hydrocarbon chains came to be known as aliphatic compounds.

5Benzene is a member of a class of compounds known as aromatic compounds. It was noted that many compounds containing benzene had a distinctive odour or aroma, so historically they came to be known as aromatic compounds.

6Note that this is a hydrogen atom, that is the nucleus of the hydrogen atom as well as its valence electron, also known as monohydrogen
Removal of the hydrogen atom's valence electron would produce a positively charged hydrogen ion, H+, known as a hydron (or a proton if the original hydrogen atom was 1H)
Addition of an electron to the hydrogen atom would produce a negatively charged hydrogen ion, H-, known as hydride ("ide" indicating a negatively charged ion)

7Except in the case of the traditional contracted names when locants are added at the front of the name.

advertise on the AUS-e-TUTE website and newsletters
 
 

Search this Site

You can search this site using a key term or a concept to find tutorials, tests, exams and learning activities (games).
 

Become an AUS-e-TUTE Member

 

AUS-e-TUTE's Blog

 

Subscribe to our Free Newsletter

Email email us to
subscribe to AUS-e-TUTE's free quarterly newsletter, AUS-e-NEWS.

AUS-e-NEWS quarterly newsletter

AUS-e-NEWS is emailed out in
December, March, June, and September.

 

Ask Chris, the Chemist, a Question

The quickest way to find the definition of a term is to ask Chris, the AUS-e-TUTE Chemist.

Chris can also send you to the relevant
AUS-e-TUTE tutorial topic page.

 
 
 

Share this Page

Bookmark and Share
 
 

© AUS-e-TUTE