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Lucid Guideline For I.U.P.A.C. Nomenclature Of Organic Compounds: Part-1:

Updated on October 25, 2015

List of Topics Included

(1) Meaning of the term: I. U. P. A. C.

(2) Why I. U. P. A. C. system of naming is so important?

(3) Some common rules of I. U. P. A. C. system of naming

(4) Root name (also called word root)

(5) Meaning of "suffix"

(6) Meaning of "prefix"

(7) Guidelines for nomenclature of saturated aliphatic hydrocarbons called, "alkanes"

@ References

(1) What is meant by, "I. U. P. A. C. System of naming"?

I. U. P. A. C. is an abbreviation of "International Union of Pure and Applied Chemistry".

It is an international organization which governs all issues related to the subject of chemistry.

It also issues the guidelines for assigning a systematic name to organic compound.


I. U. P. A. C. goes on modifying the existing rules time to time. Such modifications help:

(a) To assign names to the newly discovered compounds and

(b) To avoid discrepancies in naming the vast variety of organic compounds.

The present article is based on modified guidelines which were issued by I. U. P. A. C. in the year of 1993. However, an effort is made to cover several modifications made then after.

(2) Why I. U. P. A. C. System of Naming is so important?

We know that carbon possesses unique properties like catenation and isomerism. Due to this, there exist a large number of compounds of carbon. As per an estimate, the number of known compounds of carbon is more than seven million.

It is necessary to assign proper name to each and every compound of carbon. The old non-scientific method of their naming proved to be unsatisfactory. This is because the old system was not based on particular rules. Hence only a few organic compounds could be named using old system. Lots of remaining compounds were name less! This will become clear from following two examples of pentane and hexane.

(1) Consider the naming of all compounds having a common formula: C5H12.

There are three compounds (called isomers) having this formula.

Their common names are: (a) n-pentane, (b) iso-pentane and (c) neo-pentane.

(2) Now consider the naming of all compounds having a common formula: C6H14.

Here there are five compounds (called isomers) having this formula.

Out of these five isomers of hexane, only two have their characteristic common names as:

(a) n-hexane and

(b) iso-hexane.

The remaining three isomers of hexane do not have their common names. Hence in the old system of naming it was difficult to refer them.

However, in I. U. P. A. C. system of naming, they are referred as:

(c) 3-Methylpentane

(d) 2,3-dimethylbutane and

(e) 2,2-dimethylbutane.

This is not only the case. There are many more other compounds for which no common names exist.

To resolve this problem, I. U. P. A. C. introduced a systematic method for naming organic compounds. The main objectives to design this new system of naming are as below:

(A) To assign a characteristic name to each and every organic compound so that it can be differentiated from other compounds. This means each compound should have one and only one I. U. P. A. C. name and conversely each I. U. P. A. C. name should refer one and only one compound.

[Note: Though it is true that no two compounds can have same I. U. P. A. C. names, it is possible that a single compound may have more than one valid I. U. P. A. C. names. This may be due to: (a) liberal interpretation of several rules and (b) due to practical approach to appreciate the common names which become too much famous and popular.

For example, there are two valid I. U. P. A. C. names for same compound as given below:

(1) 5-sec-Butyl-4-isopropyldecane and

(2) 4-(1-Methylethyl)-5-(1-methylpropyl)decane.

(See the following figure for structures of these two compounds)].

(B) Due to numerous organic compounds, it is difficult to remember individual name of each. The new system, therefore, should be based on some systematic rules, through which one can easily assign / conclude their names from their structures. This will reduce the burden to remember their names just by cramming.

Example of a Compound having Two Valid I. U. P. A. C. Names

Some compounds possess two valid I. U. P. A. C. names. The reason is they contain such alkyl branch which contains sub-branches. Hence, in order to avoid complication, I. U. P. A. C. has allowed use of some old names in such case.
Some compounds possess two valid I. U. P. A. C. names. The reason is they contain such alkyl branch which contains sub-branches. Hence, in order to avoid complication, I. U. P. A. C. has allowed use of some old names in such case. | Source

(3) Some Common Rules of I. U. P. A. C. System of naming

(1) The first letter of each name is always written in uppercase (means in capital).

For example, correct I. U. P. A. C. name of methyl alcohol (having formula: CH3OH) is “Methanol” but not as “methanol”.

(2) No space should be left between any two words, the entire name should be written without space.

For example:

  • Di methyl ether (having formula: CH3-O-CH3) is written as, “Methoxymethane” but not as, “Methoxy methane”.
  • Likewise “1,1,1,2-Tetrachloroethane” should not be written as, “1,1,1,2-Tetra chloro ethane”.

[Note: However, there are certain exceptions in this rule which must be considered carefully. These are:

(a) The general name of carboxylic acid is, "Alkanoic acid". Here, a space is provided between the words alkanoic and acid. For example: Ethanoic acid.

(b) The general name of anhydride compound is, "Alkanoic anhydride". Here, a space is provided between the words alkanoic and anhydride. For example: Ethanoic anhydride.

(c) The general name of acid halide compound is, "Alkanoyl halide". Here, a space is provided between the words alkanoyl and halide. For example: Ethanoyl chloride.

(d) The general name of ester compound is, "Alkyl alkanoate". Here, a space is provided between the words alkyl and alkanoate. For example: Ethyl methanoate.]

(3) Longest and continuous carbon chain (either straight or bent) must be selected for assigning root name. For example, name of compound: CH3-CH(C2H5)-CH2-CH2-CH3
is, “3-Methylhexane” but not, “2-Ethylpentane”.

[Note: there are certain exceptions in this rule which are discussed later. See the figure below].

(4) Comma (,) is placed between two numbers, while hyphen (-) is placed between number and letter. For example: 2,2-Dimethyl-3-Ethyl-2,3-dimethylpentane.

(5) Numbering should be given such that substituent bears minimum number possible (minimum number rule). For example, compound: “CH3-C(CH3)2-CH2-CH(CH3)2” is named as, “2,2,4-Trimethylpentane” but not as, “2,4,4-Trimethylpentane”.

[Note: Substituent means the group which is not a part of parent hydrocarbon compound. It has substituted one or more hydrogen of the parent compound].

(6) For compound containing two or more different alkyl groups at similar position on the chain, numbering should be given from such end that alphabetically prior group bears lower number. For example, compound, “CH3-CH2-CH(CH3)-CH2-CH(C2H5)-CH2-CH3” should be named as, “3-Ethyl-5-methylheptane” but not as, “3-Methyl-5-ethylheptane”.

(7) For compound containing multiple alkyl groups at different position on the chain, numbering should be in accordance with, “lowest set of locant rule" (also called, "first point of difference rule"); but not as per classical “minimum sum rule”. For example, compound: “CH3-CH(CH3)-CH2-CH2-CH2-CH2-CH(CH3)-CH(CH3)-CH2-CH3” is named as, “2,7,8-Trimethyldecane” but not as, “3,4,9-Trimethyldecane”.

[Note: In later name, the sum of locants is, "3 + 4 + 9 = 16", which is lesser than “2 + 7 + 8 = 17” of former name. Even though the valid I. U. P. A. C. name of this compound is former and not later.

This is because in former name the first substituent is located at position number 2 which is lesser than position number 3 of later name. Thus sum of locants is not considered but position of locant is considered].

(8) For compound containing two or three different alkyl groups, alphabetically prior alkyl group should be written first, even though its position number is higher. For example, compound: “CH3-CH2-CH(CH3)-CH(C2H5)-CH2-CH2-CH3” is written as, “4-Ethyl-3-methylheptane” but not as, “3-Methyl-4-ethylheptane”.

[Note: In the condensed structural formulas of the compounds given above, the group enclosed in parenthesis should be considered to be attached with the previous carbon atom of the chain.]

Select the correct I. U. P. A. C. name


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Two Examples showing Violation of, "Longest Chain Rule"

The above examples explain the important point that root chain should be selected such that it includes a functional group or a multiple bond. No matter if it is not a longest chain.
The above examples explain the important point that root chain should be selected such that it includes a functional group or a multiple bond. No matter if it is not a longest chain. | Source

(4) Root Name (OR Word Root)

In I. U. P. A. C. system, it is assumed that saturated hydrocarbons (which are called alkanes) are the principle compounds. The remaining compounds which are not alkanes are believed to be derived from alkanes. (This assumption is true for open chain compounds).

Therefore each organic compound (which is not an alkane) essentially contains an alkane part in it. The initial part of its name is called, "Root name" or "Word root". It denotes the number of carbon atoms present in the principle chain of given organic compound.

For example if principle chain of a given compound contains seven carbon atoms, then root name of compound is, "hept". This means it is derived from an alkane called heptane.

It is due to this reason that naming of any given compound other than alkane, requires its corresponding alkane source to be identified first. This will become clear from following two examples.

(a) Consider a compound called ethanol. Since it contains two carbon atoms its root name is "eth".

(b) Likewise propanone contains three carbon atoms hence its root name is "prop".

When suffix “ane” is added to root name, it produces complete name of corresponding alkane.

As the common names of initial four alkanes have become very popular, I. U. P. A. C. has accepted them without any change.

The following table shows root names of 20 alkanes.

Names of Initial Twenty Alkanes and corresponding Word Roots

Formula of alkane (its I. U. P. A. C. name)
Word root
Primary suffix
 
CH4 (Methane)
Meth
ane
 
C2H6 (Ethae)
Eth
ane
 
C3H8 (Propane)
Prop
ane
 
C4H10 (Butane)
But
ane
 
C5H12 (Pentane)
Pent
ane
 
C6H14 (Hexane)
Hex
ane
 
C7H16 (Heptane)
Hept
ane
 
C8H18 (Octane)
Oct
ane
 
C9H20 (Nonane)
Non
ane
 
C10H22 (Decane)
Dec
ane
 
C11H24 (Unidecane)
Undec
ane
 
C12H26 (Dodecane)
Dodec
ane
 
C13H28 (Tridecane)
Tridec
ane
 
C14H30 (Tetradecane)
Tetradec
ane
 
C15H32 (Pentadecane)
Pentadec
ane
 
C16H34 (Hexadecane)
Hexadec
ane
 
C17H36 (Heptadecane)
Heptadec
ane
 
C18H38 (Octadecane)
Octadec
ane
 
C19H40 (Nonadecane)
Nonadec
ane
 
C20H42 (Ecosane)
Ecos
ane
 

(5) Suffixes

Suffixes are of two types:

(a) Primary suffixes:

Such primary suffixes are: ane, ene, yne, diene, triene, diyne etc.

They are used to denote the nature of bond between two successive carbon atoms in the given compound.

If there is a single bond between two carbon atoms, suffix “ane” is used. Likewise for double and triple bonds, suffixes “ene” and “yne” are used respectively.

For example, consider three different hydrocarbons each containing two carbon atoms. The first contains single bond, second contains double bond and third contains triple bond between two carbon atoms.

Now, due to similar number of carbon atoms in each, root name of each will be same, "eth".

As the first contains single bond, primary suffix used will be "ane", hence its I. U. P. A. C. name will be "ethane".

Likewise prefixes used for second and third will be "ene" and "yne" respectively. Hence, their I. U. P. A. C. names will be "ethene" and "ethyne" respectively.

In case compound contains more than one multiple bond, then an additional prefix prior to said primary suffix is used. In this way, suffixes “diene” and “triene” are used for naming compounds like alkadienes (containing two double bonds, as in Buta-1,3-diene) and alkatrienes (containing three double bonds as in Hepta-1,3,6-triene).

In this way suffix “diyne” is used for alkadiyne (containing two triple bonds as in Penta-1,4-diyne).

(b) Secondary suffixes:

Secondary suffixes are used to denote the nature of functional group present in the compound. For example suffix “ol” is used for alcohol, suffix “al” is used for aldehyde etc.

[Note: In certain cases the terminal “e” of the parent compound is dropped while adding a suffix if name of suffix begins with vowel. For example, compound, “CH3-CH2-CH2-NH2” is named as, “Propanamine” but not “Propaneamine”].

(6) Prefixes

As like suffixes, prefixes are also of two types: primary and secondary.

(a) Primary prefix: Such prefix is only one and it is, “cyclo”.

It is used just to distinguish a cyclic compound from an open chain compound.

For example, compound, “C5H10” is named as, “Cyclopentane”.

(b) Secondary prefixes: They are used to denote some functional groups like halo, nitro, alkoxy etc. Examples of such compounds are: chlorobutane, nitropropane etc.

Over and above the said functional groups, some special words like: “iso”, “tert”, “n” etc. are also used as secondary prefixes. They denote some special characteristics of the compound.

For example, in compound "isopentane", there is a methyl group on second carbon.

It must be noted carefully that some functional groups are used as both, sometimes as suffix while sometime as prefix. Their use as suffix or prefix depends upon the reactivity series (also called, priority series ) of functional groups.

The following table shows suffix as well as prefix used for same functional group.

[Note: Remember the groups like: alkyl, halo, nitro, alkoxy are always used as prefixes, while the groups like: ester, oic anhydride etc. are always used as suffixes.]

List of Functional Groups and Suffixes or Prefixes used for them

Type of compound
Functional group
Prefix
Suffix
Example of compound
Alkane
nil
nil
ane
Hexane
Alkene
>C=C<
nil
ene
Hexene
Metal carboxylate
-COOM (M=metal)
nil
oate
Sodium ethanoate
Halide
-X (X = F, Cl, Br, I)
halo (fluoro, chloro etc.)
nil
Chloroethane
Alcohol
-OH
hydroxy
ol
Butanol
Aldehyde
-CHO
formyl
al
Propanal
Ketone
>C=O
oxo
one
Propanone
Carboxylic acid
-COOH
carboxy
oic acid
Pentanoic acid
Ether
-R-O-R'
alkoxy
nil
Methoxyethane
Amine
-NH2
amino
amine
Ethanamine
Ester
-COOR (R = alkyl group)
---
oate
Ethyl ethanoate
Amide
-CONH2
carbamoyl
amide
Ethanamide
Acyl halide
-COX (X = F, Cl, Br, I)
halocarbonyl
oyl halide
Propanoyl chloride
Nitriles
-CN
cyano
nitrile
Pentanenitrile
Nitro
-NO2
nitro
Nitrite
Nitroethane
In case compound contains two functional groups; prefix is used for one which is lesser reactive, while suffix is used for another which is more reactive.(As per reactivity series of functional groups)

(7) Guidelines for Nomenclature of Saturated Aliphatic Hydrocarbons called, "Alkanes"

These types of hydrocarbons are divided into three categories:

(a) Simple alkanes

(b) Branched alkanes and

(c) Complex alkanes.

Nomenclature of simple alkanes:

These types of hydrocarbons contain a straight chain of several carbon atoms.

Examples of straight chain alkanes are: hexane, octane, dodecane etc.

These are discussed under, “root name”.

Nomenclature of branched alkanes:

These are substituted alkanes. Here, each alkyl branch is regarded as a substituent which has entered by replacing one of the hydrogen atoms of the parent alkane.

For example, compound, “CH3-CH(CH3)-CH3”, is popularly known as “isobutane”. It contains a branch of methyl group on second carbon of the chain. It is assumed to be produced when a hydrogen atom from second carbon of propane is substituted with methyl group.

Hence, its I. U. P. A. C. name is, “2-Methylpropane”.

What is meant by the term "iso"?

The word “iso” is used in common system of naming.

However, in order to avoid complication, I. U. P. A. C. has allowed the use of word “iso” only when a compound contains a complex alkyl branch.

It is important to note that the prefix "iso" can be used under following conditions:

(a) There must exist a methyl branch in the given compound

(b) Methyl branch must be located on second carbon of the continuous carbon chain

(c) The counting of carbon atom should start from opposite end of the functional group or substitution if any.

For example, compound, “CH3-CH(CH3)-CH2-CH2-OH” is named as, “isopentyl alcohol”, but compound, “CH3-CH2-CH(CH3)-CH2-OH” can not be named as isopentyl alcohol.

Due to large variety of alkyl groups, knowledge of names of different alkyl groups is necessary. Some common alkyl groups used are listed in the following table.

Nomenclature of complex alkanes:

It is difficult to assign I. U. P. A. C. name to certain complex hydrocarbons.

This is because they contain such alkyl branch which in turn contains further branches.

Due to their complicated structure, they need a special consideration, as explained in following pictures.

I. U. P. A. C. names and structures of some Complex Alkanes

(1) Numbering is started from left end because substituted carbon comes early. (2) In order to denote two similar groups: “1-methylethyl” on third carbon, prefix “bis” is used in place of prefix “di”.
(1) Numbering is started from left end because substituted carbon comes early. (2) In order to denote two similar groups: “1-methylethyl” on third carbon, prefix “bis” is used in place of prefix “di”. | Source
(1) Numbering is started from right end because double bond comes early. (2) Numbering of carbon atoms from 1 to 6 as shown in picture, is ideal as it contains maximum substitutions of 4.
(1) Numbering is started from right end because double bond comes early. (2) Numbering of carbon atoms from 1 to 6 as shown in picture, is ideal as it contains maximum substitutions of 4. | Source
As shown in picture, correct numbering is that which contains 2 substitutions. In another type of numbering (which is wrong) there is only one substitution.
As shown in picture, correct numbering is that which contains 2 substitutions. In another type of numbering (which is wrong) there is only one substitution. | Source

Sharp Meaning of "iso"

Location of methyl group should be counted from opposite end of substitution.
Location of methyl group should be counted from opposite end of substitution. | Source

@ References:

(1) Organic chemistry by: Robert Thornton Morrison and robert Neilson Boyd, Seventh Edition, Published by, "Dorling Kindersley(India) Pvt. Ltd., licensees of Pearson Education in South Asia

(2) Oxford Dictionary Of Chemistry, published by Oxford University Press Inc., New York

(3) I. I. T. Chemistry, by Dr. O.P. Agarwal, 135th edition, Jai Prakash Nath Publications, Meerut, India

(4) Pradeep's New Course Chemistry, Class XI, Vol. II, 27th edition, Pradeep Publication, Jalandhar, India

(5) Pradeep's New Course Chemistry, Class XII, Vol. II, 27th edition, Pradeep Publication, Jalandhar, India

(6) Fundamentals Of Chemistry, Class 11, by J. D. Lee, Solomons & Fryhle, Published by: Wiley India Pvt. Ltd., 4435-35/7, Ansari Road, Daryaganj, New Delhi-110002

(7) Modern's abc of Chemistry, For Class XI, Part-II, by Dr. S. P. Jauhar, Published by: Modern Publishers, MBD House, Railway Road, Jalandhar City, India

(8) Modern's abc Of Chemistry, For Class XII, Part-II, by Dr. S. P. Jauhar, Published by: Modern Publishers, MBD House, Railway Road, Jalandhar City, India

(9) Organic Chemistry, by Bhupinder Mehta & Manju Mehta, Published by: Prentice-Hall Of India Private Limited, M-97, Connaught Circus, New Delhi, -110001, India

(10) Nootan ISC Chemistry, Class XI & XII, by Dr. H. C. Srivastava, Published by: Nageen Prakashan (Pvt.) Ltd., 310, Western Kutchery Road, Meerut-250001, U.P., India

(11) Cambridge International AS and A Level Chemistry Coursebook; by Roger Norris, Lawrie Ryan and David Acaster, Cambridge University Press; The Edinburgh Building, Cambridge CB2 8RU, UK

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