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What is IUPAC nomenclature?

This is a method of naming the organic compounds as recommended by the international Union of Pure and Applied Chemistry (IUPAC).

Each part of the IUPAC name gives you some useful information about the compound.

Before starting the IUPAC rules, lets see an example of organic compound and it’s IUPAC name.

IUPAC nomenclature of organic compounds

To understand the name you need to take the name to pieces. In the given example “5-methylhex-3-en-2-ol” there are 4 pieces- ‘methyl’, ‘hex’, ‘en’ and ‘ol’.

‘methyl’ tells that –CH3 is present as substituent.

‘hex’ tells that there are 6 carbon atoms on parent carbon chain.

‘en’ tells that there is at least one carbon to carbon double bond.

‘ol’ tells that there is –OH group(alcohol) as functional group.

Thus, general format for IUPAC name of all compounds is:

IUPAC name of all compounds contain word root and primary suffix but prefix and secondary suffix may not be present because all organic compounds must contain carbon chain and bond but substituent and functional group may not be present. For example.

Now see the four parts ( prefix, word root, bond and functional group) separately.

 1. Word root : It indicates the parent carbon chain, which is the the longest continuous chain of carbon atoms including functional group and multiple bonds( if present).

No. of Carbon atoms in the parent chain Word root
1 Meth-
2 Eth-
3 Prop-
4 But-
5 Pent-
6 Hex-
7 Hept-
8 Oct-
9 Non-
10 Dec-
11 Undec-
12 Dodec-

 2. Primary suffix : It indicates the nature of carbon to carbon bond in the parent carbon chain.

Nature of carbon to carbon bond Primary suffix
Single -ane
Double -ene
Triple -yne

 3. Secondary suffix : It indicates the parent(main) functional group present in the compound.

S. N. Class or family General structural   formula Functional group Secondary suffix
1 Carboxylic acid

Or RCOOH

Or -COOH

-oic acid
2 Sulphonic acid (sulfonic acid) R-SO3H -SO3H -sulphonic acid
3 Acid anhydride -oic anhydride
4 Ester

Or RCOOR

– COOR

-oate
5 Acid chloride

Or RCOCl

-COCl

-oyl chloride
6 Acid amide

Or RCONH2

Or -CONH2

-amide
7 Nitrile

Or RCN

-CN -nitrile
8 Aldehyde

Or RCHO

Or -CHO

-al
9 Ketone -one
10 Alcohol R-OH -OH -ol
11 Thioalcohol R-SH -SH -thiol
12 Amine R – NH2 -NH2 -amine

 4. Prefix : It indicates the substituent ( i.e any group bonded with parent carbon chain except main functional group).

Substituent Prefix
-CH3 Methyl-
-CH2CH3 or–C2H5 Ethyl-
-OCH3 Methoxy-
-OCH2CH3 or–OC2H5 Ethoxy-
-F Fluoro-
-Cl Chloro-
-Br Bromo-
-I Iodo-
-NO2 Nitro-
-NO Nitroso-
-NH2 Amino-

{Note: -NH2 can be taken as substituent as well as functional group}

Sometimes, in case of compounds having polyfunctional groups, functional groups may be considered as prefixes. For example:

-OH Hydroxy-
-CN Cyano-
-CO- Keto- (Carbonyl or Oxo-)
-CHO Aldo- (Carbonyl or Oxo-)

General steps for IUPAC nomenclature of organic compounds:


Naming of all organic compounds can be done in three steps as,

Rules for IUPAC nomenclature

To simplify the nomenclature process, differentiate the organic compounds in four categories as,

[A] Compounds containing carbon to carbon single bonds and substituents only.

[B] Compounds containing multiple bonds ( double/ triple bonds) too.

[C] Compounds containing one functional group ( monofunctional compounds).

[D] Compounds containing more than one functional groups ( polyfunctional compounds).


IUPAC Nomenclature of organic compounds containing carbon to carbon single bonds and substituents only : [A]

There are only three steps in nomenclature of all organic compounds.

Step-I : Selection of parent chain : The longest continuous carbon chain is selected as the parent chain. Eg.

IUPAC naming steps

If two or more equally long chains are present, the chain with maximum number of substituent is selected as the parent chain.

selection of carbon chain in IUPAC naming

Step-II : Numbering the parent chain :

Lowest locant rule: Carbon bearing the substituent gets the lowest possible locant. Locant is a number that locate the position of substituent.

Note : Here the first locant is same (i.e 2) so compare second locant which is 2 and 4 in (2,2,4) and (2,4,4). Hence (2,2,4) is lowest set of numbering and is correct numbering.

If substituents are present at equivalent position, follow alphabetical order.

Step III – Naming : Alphabetically i.e. if more than one substituents are present then they are written in the alphabetical order.

Note: Naming of compounds containing complex substituent (alkyl group) : 

An alkyl substituent having substituents within itself is named in bracket by putting number outside. Eg.

 

IUPAC Nomenclature of organic compounds containing multiple bonds(double/triple bond) too. [B]

Step-I : Selection of parent chain : The longest continuous carbon chain including multiple bond is selected as the parent chain. Eg.

Step-II : Numbering the parent chain :

Lowest locant rule: Carbon bearing the multiple bond gets the lowest possible locant.

If both substituent and multiple bond are present, the priority order is:

Multiple bond > Substituent

selection of carbon chain in IUPAC nomenclature

If multiple bonds are present at equivalent position, follow alphabetical order i.e double bond(-ene) gets higher priority than triple bond(-yne).

Step III – Naming : Alphabetically i.e. if more than one substituents and multiple bonds are present then they are written in the alphabetical order.

IUPAC name

 

Note: If both double and triple bonds are present, the terminal ‘e’ of first one(in name) is dropped(removed).


IUPAC Nomenclature of organic compounds containing one functional group ( monofunctional compounds): [C]

Step-I : Selection of parent chain : The longest continuous carbon chain including functional group is selected as the parent chain. Eg.

Step-II : Numbering the parent chain : Now the final priority order for numbering is :

Functional group > Multiple bond > substituent

Step III – Naming : Always follow this format:

For example,

NOTE : While adding the secondary suffix to the primary suffix, the terminal ‘e’ of the primary suffix (i.e., ane, ene or yne) is dropped if the secondary suffix begins with a vowel. However, the terminal ‘e’ is retained if the complete secondary suffix begins with a consonant.

Some examples of IUPAC names for compounds containing one functional group:

Class or family General formula Examples
Carboxylic acid

Or RCOOH

Sulphonic acid (sulfonic acid) R-SO3H
Ester

Or RCOOR

Acid chloride

Or RCOCl

Acid amide

Or RCONH2

Nitrile

Or RCN

Aldehyde

Or RCHO

Ketone
Alcohol R-OH
Amine R – NH2

IUPAC Nomenclature of organic compounds containing more than one functional groups ( polyfunctional compounds) : [D]

The priority order of functional groups is:

priority order of functional groups according to IUPAC

-COOH > -SO3H > – COO- > -COX > -CONH2 > -CN > -CHO > -CO- > -OH > -NH2

During nomenclature of polyfunctional compounds, fuctional group of higher priority is taken as principal functional group and other functional groups are considered as substituents.

The IUPAC rules applied for monofunctional compounds are also applied for polyfunctional compounds. Some additional rules are needed, which are given below:

At first, principal functional group is identified according to priority order. Eg.

IUPAC nomenclature of polyfunctional compounds

Step-I : Selection of parent chain : The longest continuous carbon chain with principal functional group is selected as the parent chain. Eg.

Step-II : Numbering the parent chain : Principal functional group gets lowest locant(number).

Step III – Naming : Substituents are written in alphabetical order as mentioned earlier. Eg.

Some other examples of polyfunctional compounds :

 

  • If a compound contains two or more functional groups, the words –di, -tri, -tetra, etc. is written before the name of secondary suffix with respective locant numbers. In such cases, the last letter ‘e’ from primary suffix is not dropped (removed). Eg.

iupac nomenclature of polyfunctional compounds


CONCLUSION :


Nomenclature of organic compounds is very easy. Remember only two things (mentioned below) during nomenclature, you will easily write correct IUPAC name of all organic compounds.

1. Always follow three steps :

2. Always write the name in general format :


References :