13.1 IUPAC Nomenclature of Organic Compounds

Word Root

It represents the number of carbon atoms in the longest chain of the compound, called the parent chain. The parent chain is the longest possible carbon chain that includes the functional groups and the multiple bonds. Different word roots are given in the table below.

Chain length	Word root
1 carbon	Meth
2 carbons	Eth
3 “	Prop
4 “	But
5 “	Pent
6 “	Hex
7 “	Hept
8 “	Oct
9 “	Non
10 “	Dec
11 “	Undec
12 “	Dodec

Primary Suffix

It is used to represent the nature of carbon-carbon bonds (saturation or unsaturation) in the main chain of the compound.

Nature of carbon-carbon bond	Primary suffix
Single bond (C–C)	-ane
Double bond (C=C)	-ene
Triple bond (C≡C)	-yne
No bond (in a compound having only one carbon — Methane)	-ane

For example,

Organic compound	Word root	Primary suffix	IUPAC name
CH₃CH₂CH₂CH₃	But	ane	Butane
CH₃CH=CH₂	prop	ene	Propene
CH₃C≡C–H	prop	yne	Propyne
CH₂=CH–CH=CH₂	buta	diene	Buta-1,3-diene
CH≡C–C≡CH	buta	diyne	Butadiyne

Secondary Suffix

It is the part of the name which comes after the primary suffix. It is used to indicate the main functional group present in the organic compound. Secondary suffixes for various functional groups are given below.

Functional group	Secondary suffix	Class of compounds	General form
-OH	-ol	Alcohols	R-OH
-CHO	-al	Aldehydes	R-CHO*
>CO	-one	Ketones	RCOR
-COOH	-oic acid	Carboxylic acids	R-COOH*
-COCl	-oyl chloride	Acid chloride	RCOCl*
-CONH₂	-amide	Amides	RCONH₂*
-COOR	-oate	Esters	RCOOR
-CO-O-CO-	-oic anhydride	Acid anhydrides	RCOOCOR
-CN	-nitrile	Nitriles or cyanides	R-CN
-NC	-isonitrile	Isonitriles or isocyanides or carbyl amines	R-NC

*In these cases, -R can also be -H

e.g.,

Organic compound	Word root	Primary suffix	Secondary suffix	IUPAC name
CH₃CH₂CH₂OH	Prop	-ane	-ol	Propan-1-ol
CH₃CH(OH)CH₃	Prop	-ane	-ol	Propan-2-ol
CH₃CH₂CN	Prop	-ane	-nitrile	Propanenitrile
CH₃CHO	Eth	-ane	-al	Ethanal
CH₂=CHCHO	Prop	-ene	-al	Propenal
HCOOH	meth	-ane	-oic acid	Methanoic acid
CH₃COOH	Eth	-ane	-oic acid	Ethanoic acid
HC≡CCOOH	Prop	-yne	-oic acid	Propynoic acid
	Prop	-ane	-one	Propanone

The part of the IUPAC name that is written at the beginning, which indicates the substituent or side chain or certain functional groups of the organic compound, is a prefix. The prefixes may be further categorised into primary and secondary prefixes. The primary prefix is cyclo, used to indicate if the compound is carbocyclic and not aromatic.

The name of this compound is:

Cyclo + pent + ane = Cyclopentane

The name of this compound is:

Cyclo + hex + ane + ol

It is written as Cyclohexanol

The name of this compound is:

Cyclo hex ane one

It is written as Cyclohexanone

Following is the list of some primary prefixes. These are, by default, written as prefixes.

Substituent	Name	Prefix
-R	Alkyl	Alkyl
-CH₃	Methyl	Methyl
-CH₂CH₃	Ethyl	Ethyl
-CH₂CH₂CH₃	Propyl	Propyl etc.
-OR	Alkoxy	Alkoxy
-OCH₃	Methoxy	Methoxy
-OCH₂CH₃	Ethoxy	Ethoxy
-OCH₂CH₂CH₃	Propoxy	Propoxy etc.
-NH₂	Amine	Amino
-NO₂	Nitro	Nitro
-Cl	Chloro	Chloro
-Br	Bromo	Bromo
-I	Iodo	Iodo
-F	Fluoro	Fluoro
-NO	Nitroso	Nitroso
-N≡N	Diazo	Diazo

For example:

For alkanes (the saturated hydrocarbons), the word root is “alk,” and the primary suffix is “ane”. If the substituents are present, they are indicated prior to the word root as prefixes.

For IUPAC names of complex alkanes, the following rules should be followed:

1. Longest Chain Rule

For alkanes with branches, the word root is that of the longest chain of C-atoms, which may or may not be straight but must be continuous. This chain is called the parent chain or principal chain. All the other carbon atoms, which are not included in the parent chain, are called side chains or substituents and are written as prefixes. For example:

= 2,3-dimethylpentane

• So, first, the longest continuous carbon chain must be located. This gives the name of the parent hydrocarbon i.e., word root. For example:

Various substituents are as follows:

Substituent group or side chain	Secondary prefix
-CH₃	Methyl
-C₂H₅	Ethyl
-CH₂CH₂CH₃	Propyl

• If there are two equally long continuous chains are there, the one with the most branches should be selected as the main chain.

2. Lowest Number Rule / Lowest Sum Rule

The parent chain is numbered in such a way that the substituent gets the lowest possible number (locant) in the main chain. Locant number is the number that indicates the position of the substituent on the parent chain.

This is called the lowest number rule. For example:

This is the correct numbering, not the other way round.

It is named as

2-methyl + pent + ane + N/A = 2-methylpentane

If more than one substituent is present, numbering is done by the lowest sum rule. When two or more identical groups are attached to the parent chain, prefixes such as di-, tri- and tetra- are used.

Another example of the lowest sum rule.

3. Alphabetical Order

If two or more different types of substituents are present, they are listed alphabetically (prefixes like di, tri, are not considered for alphabetising)

For example:

If the substituents are different but locant numbers from either end of the parent chain come to be the same, then begin numbering nearest the substituent whose name has alphabetic priority.

The primary suffix “ene” is used to designate a carbon-carbon double bond. When more than one double bond is present, the suffixes like diene, triene are used. The suffix “yne” is used for a carbon-carbon triple bond. Again, when more than one triple bond is present, the suffixes like diyne, triyne are used. The compounds with both double and triple bonds in the same molecule are enynes.

1. The first longest continuous carbon chain that includes both carbons of a double or a triple bond should be selected.

For example:

2. Number the chain (parent chain) from the end nearest to the multiple bond, so that the carbon atoms in that bond have the lowest possible numbers.

But if the multiple bond is equidistant from either end of the chain, start numbering from the end nearest the first branch point.

3. Indicate the position of a multiple bond using the lower-numbered carbon atoms of that bond.

For example:

4. If more than one multiple bond is present, number using the lowest sum rule.

For example

If both double and triple bonds are present and the locant numbers from either side of the chain come to be the same, then the double-bonded carbon is numbered before the triple-bonded carbon.

Pent-1-en-4-yne

5. In the case of enynes, there are two primary suffixes, ene(s) and yne(s). In this case, the respective locants are written before their respective suffixes, and the terminal ‘e’ from the suffix ene is dropped.

Solved examples

IUPAC names of compounds containing single functional groups

3. When a chain-terminating group such as –CHO, -COOH, -COOR, -CONH₂, -COCl, -C≡N is present, the C-atoms of these functional groups are always given the number 1. For example:

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

5. When benzene is present in the parent chain, it is taken as a substituent phenyl. In case the phenyl ring is further substituted, the C-atoms of the ring are separately numbered starting from the C-atoms of the ring directly attached to the parent chain in such a way that the substituent of the ring gets the lowest possible number. Such a substituted phenyl is called a complex substituent and is enclosed in a bracket.

6. If the compound contains more than one complex substituent, then the words such as si-, tri-, tetra- are replaced by the words like –bis, -tris, -tetrakis, respectively.

Some solved examples

Polyfunctional rule (order of priority)

Name	Functional group	Prefix	Secondary Suffix
Carboxylic acid	-COOH	—	-oic acid
Sulphonic acid	-SO₃H	—	-sulphonic acid
Acid anhydride	-COOCO-	—	-oic anhydride
Ester	-COOR	Alkoxycarbonyl-	-oate
Acid chloride	-COCl	Chlorocarbonyl-	-oyl chloride
Amide	-CONH₂	Carbamoyl-	-amide
Cyanide (nitrile)	-CN	Cyano-	-nitrile
Aldehyde	-CHO	Aldo- or formyl-	-al
Ketones	-CO-	Keto- or oxo-	-one
Alcohol	-OH	Hydroxy-	-ol
Amines	-NH₂	Amino-	–
Alkene (double bond)	>C=C<	—	ene (primary suffix)
Alkyne (triple bond)	-C≡C-	—	yne (primary suffix)
Ethers	-OR	Alkoxy-	–
Nitro	-NO₂	Nitro-	–
Halo	-X	Halo-	–
Alkyl	-R	Alkyl-	–

1. Determination of Principal Functional Group

If a compound contains two or more functional groups then one of the functional groups is taken as the principal functional group and used as secondary suffix and the other one is referred to as subsidiary functional group and is used as prefixes. The principal functional group in such cases is determined on the basis of priority order as given in IUPAC rules.

All the remaining functional groups such as halo (-X), nitroso (-NO), nitro (-NO₂), alkoxy (-OR), alkyl (-R), phenyl (-C₆H₅) etc. are always used as substituent groups.

Secondary functional group	Prefix name
-X (X=F, Cl, Br, I)	Halo
-OH	Hydroxyl
-SH	Mercapto
-NH₂	Amino
-NHR	Alkyl amino
-NR₂	Dialkyl amino
-CHO	Formyl
>C=O	Keto or oxo
-COOH	Carboxy
-COOR	Alkoxycarbonyl or carbalkoxy
-COCl	Chlorocarbonyl
-CN	Cyano
-CONH₂	Carboxyamide or carbamoyl

2. Selecting the Principal Chain

After determining the principal functional group, the principal chain is selected. It is the longest, continuous carbon chain that has the principal functional group and the maximum number of secondary functional groups, and the multiple bonds (if any) are included.

3. Numbering the Carbon Atoms in the Principal Chain

The carbon atoms in the principal chain are numbered in such a way that the carbon holding the principal functional group gets the lowest possible number, followed by the double bond, triple bond, and the substituent.

4. Alphabetical Order

The prefixes for secondary functional groups and other substituents should be written in alphabetical order before the word root. If, however, two groups of the same preference occupy the identical position from either end of the parent chain, the lower number must be given to the group whose name comes first in alphabetical order.

Some more solved examples