When we change our clothes, we realise that our personalities change too. Similarly, replacing an "H" atom in any hydrocarbon with another atom or group of atoms affects the hydrocarbon's personality, i.e., its physical and chemical properties. This atom or group of atoms that define the properties of an organic compound is called a functional group. Each functional group has a different identity which is represented by a different prefix or suffix given by IUPAC.

A common question that comes to mind is “why is it required to name chemical molecules?”

The IUPAC system of nomenclature was formed to create a universal standard for naming substances to improve communication.

TABLE OF CONTENTS

• Functional Groups
• Rules to Assign IUPAC Names to Functional Groups
• IUPAC Naming of Carboxylic Acids
• IUPAC Naming of Acid Anhydrides
• IUPAC Naming of Esters
• IUPAC Naming of Acyl Halides
• IUPAC Naming of Amides
• IUPAC Naming of Cyanide
• IUPAC Naming of Isocyanide
• IUPAC Naming of Aldehyde
• IUPAC Naming of Thial
• IUPAC Naming of Ketone
• IUPAC naming of Alcohol
• IUPAC naming of Thiols
• IUPAC Naming of Amines
• IUPAC naming of Ethers
• Practice Problems
• Frequently Asked Questions - FAQ

Functional Groups

An atom or group of atoms that define the properties and unique chemical reactions of an organic compound is called a functional group.

For the naming of compounds containing functional groups, a secondary suffix is added after

the primary suffix in the IUPAC name of the compound to indicate the nature of the functional

group in the organic compound.

The basic skeleton of IUPAC Naming System

Secondary prefix + Primary prefix + Root word + Primary suffix + Secondary suffix

Secondary prefix - denotes the substituents attached to the parent chain

Primary prefix - denotes the nature of the parent chain (cyclo, bicyclo, spiro)

Root Word - denotes the number of carbon atoms in the parent chain

Primary Suffix - denotes the saturation of the parent chain

Secondary Suffix - denotes the functional group

Rules to Assign IUPAC Names to Functional Groups

1. The carbon atom of the functional group or the carbon bearing the functional group has to be included in the principal carbon chain even if it violates the longest chain rule.

Example:

2. Priority order: Functional Group > double/triple bond > substituent.

Example:

3. The locant of the functional group has to be written either before the name of the hydrocarbon (root word + primary suffix) or after the name of the hydrocarbon followed by the suffix of the functional group (drop the letter "e" from the primary suffix).

Example:

4. If there is more than one identical functional group present in the molecule, then we can use di, tri, tetra, etc.

Example:

IUPAC Naming of Carboxylic Acids

This functional group is of the highest priority. While naming the compound, the C of -COOH is also considered in the name of the word root.

Secondary suffix - oic acid

Secondary prefix - carboxy

Special suffix - carboxylic acid

Examples:

IUPAC Naming of Acid Anhydrides

An acid anhydride is obtained when two carboxylic acids react with each other with the removal of water.

Formation of Acid Anhydride

The functional group next in priority (highest among the derivatives of carboxylic acids) is that of

acid anhydride(-COOCOR').

Secondary suffix: - oic anhydride

Examples:

IUPAC Naming of Esters

An ester is obtained when a carboxylic acid reacts with alcohol with the removal of water.

Formation of Esters

The functional group next in priority is that of ester (-COOR'). Generally, esters are named alkyl alkanoate. The alkyl group that is bonded to O (as OR) is termed alkyl, whereas the rest of the molecule is termed alkanoate.

Secondary suffix - oate

Special suffix - carboxylate

Examples:

Secondary prefix for esters:

1. Alkoxy carbonyl: When the C-atom of the ester (carbonyl carbon) is directly attached to the parent chain.

Example:

In the given compound, the ester group (methyl group attached to the oxygen) is directly attached to the parent chain at the second position from its carbonyl carbon. The IUPAC name of the compound is

2-methoxycarbonylpentanoic acid.

2. Alkanoyloxy: When the O-atom of the ester is directly attached to the parent chain.

Example:

In the given compound, since the ester group is directly attached to the parent chain from the oxygen atom at the fourth position, the prefix ethanoyloxy is used for it. Hence, the IUPAC name of the compound is 4-ethanoyloxyhexanoic acid.

IUPAC Naming of Acyl Halide

An acyl halide is obtained when a carboxylic acid reacts with a halogen acid (HX).

Formation of Acyl Halides

The functional group next in priority is that of acid halide (-COX). Here, X is a halogen (F, Cl, Br or I).

Secondary suffix - oyl halide

Secondary prefix - halo carbonyl

Examples:

IUPAC Naming of Amides

An amide is obtained when a carboxylic acid reacts with ammonia (NH₃).

Formation of Amides

The functional group next in priority is that of amide (-CONH₂, -CONHR, -CONR₂).

Secondary suffix - amide

Secondary prefix - amino

Degree of amides:

Amides can be classified as 3⁰ (tertiary), 2⁰ (secondary), and 1⁰ (primary) based on the number

of carbon atom(s) directly attached to the N atom.

Examples:

If H attached to N in amide is replaced by alkyl groups.

If one H attached to N is replaced by an alkyl group. It is written as an N-alkyl prefix. If two H are

replaced by two alkyl groups, then N,N-dialkyl prefix is used.

Secondary Prefix for amide:

1. Alkylaminocarbonyl: For amides (-CONHR), when the carbonyl C‒atom of the amide is directly attached to the parent chain, the prefix used is alkylaminocarbonyl.

Example:

The IUPAC name of the compound is 2-N-Ethyl-N-methylaminocarbonylhexanoic acid.

2. Alkanoylamino - For amides(-CONHR), when the N‒atom of amide is directly attached to the parent chain, the prefix used is alkanoylamino.

Example:

The IUPAC name of the compound is 4-Ethanoylaminohexanoic acid.

IUPAC Naming of Cyanide

The functional group next in priority is that of cyanide (—CN).

Secondary prefix - cyano

Secondary suffix - nitrile

Special suffix - carbonitrile

Examples:

IUPAC Naming of Isocyanide

The functional group next in priority is that of isocyanide (—NC).

Secondary suffix - isonitrile

Examples:

IUPAC Naming of Aldehyde

The functional group next in priority is that of aldehyde (—CHO). Here, we count the carbon of the

aldehyde group in the parent chain.

Secondary suffix - al

Secondary prefix for aldehyde:

1. Oxo: Oxo is used as the prefix when C of CHO is included in the parent chain.
2. Formyl: Formyl is used as the prefix when C of CHO is not included in the parent chain.

Examples:

IUPAC Naming of Thial

The functional group thial is similar to aldehyde, in which the oxygen atom of aldehyde is replaced by sulphur atom.

Secondary suffix - thial

Example:

IUPAC Naming of Ketone

We count the number of carbons of the ketone group in the parent chain.

Secondary suffix - one

Secondary prefix for Ketone: Keto/oxo

Examples:

IUPAC Naming of Alcohol

The hydrocarbons containing the –OH groups are known as alcohols.

Prefix (if any) + Word root + Primary suffix (-e) + ol

While numbering the parent chain, give the lowest possible number to this functional group (−OH).

Secondary suffix - ol

Secondary prefix - hydroxy

If the primary suffix ends with a vowel, then it (vowel) is dropped while writing the IUPAC

nomenclature.

Examples:

IUPAC Naming of Thiols

The hydrocarbons containing the –SH groups are known as thiols.

IUPAC nomenclature: Add the suffix thiol after the primary suffix

Prefix (if any) + Word root + Primary suffix + thiol

Secondary suffix - thiol

Secondary prefix - mercapta

Examples:

IUPAC Naming of Amines

Amines may be regarded as the derivatives of ammonia (NH₃) formed by the replacement of one or

more hydrogen atoms by the corresponding number of alkyl groups.

All three types of amines (primary, secondary, and tertiary) act as different functional groups.

1. Primary Amines

In the IUPAC system, the primary amines are named by replacing the final –e in the parent

hydrocarbon by -amine. Therefore, the IUPAC name of the primary amine is alkanamine.

Prefix (if any) + Word root + Primary suffix (-e) + amine

Secondary prefix for amine is amino.

Example:

2. Secondary and Tertiary Amines

Secondary and tertiary amines are named as N-substituted derivatives of primary amines.

• The name of the groups attached to the nitrogen of amine should be written alphabetically and the prefix ‘N-’ should be used for it as a position.
• The largest group attached to the nitrogen atom is taken as the alkyl group of the primary amine with suffix amine.
• The other groups are written as the alkyl groups with the prefix ‘N-’.

Note:

• If the secondary suffix is starting with a vowel and the primary suffix is ending with a vowel, then the vowel in the primary suffix is dropped while writing the nomenclature.
• While numbering the parent chain, give the lowest possible number to these functional groups (-NH₂, -NHR, -NR₂ )

Examples:

IUPAC Naming of Ethers

The hydrocarbons containing the –OR groups are known as ethers.

Ethers are always used as a substituents.

General IUPAC naming of ether is alkoxy alkane where the parent chain is alkane and alkoxy is used for substituents.

Secondary prefix - alkoxy

Examples:

Practice Problems

Q. 1. Write the IUPAC name of the given compound.

Solution:

The given compound is an ester.

The general IUPAC name of an ester is Alkyl alkanoate.

The alkyl group that is bonded to O (as OR) is termed 'alkyl', whereas the rest of the molecule is termed 'alkanoate'.

Alkyl branch: The alkyl group bonded to O is cyclopentyl. So, the cyclopentyl prefix will be used.

Alkanoate branch: Since the alkanoate branch has four carbon atoms, butanoate will be used for it.

Therefore, the IUPAC name of the compound is cyclopentylbutanoate.

Q. 2. Write the IUPAC name of the compound.

Solution:

The given compound is a carboxylic acid.

Word root: The parent chain is of five carbon atoms. So, the word root will be pent.

Prefix: The compound contains two methyl substituents at the second and third carbons. So, the prefix 2,3-dimethyl will be used.

Primary Suffix: The compound contains a carbon-carbon double bond at the second position, so 2-ene will be used as the primary suffix.

Secondary Suffix: The compound contains carboxylic acid as a functional group, so -oic acid will be used as the secondary suffix.

Therefore, the IUPAC name of the compound is 2,3-dimethylpent-2-enoic acid.

Q. 3. Write the IUPAC name of the compound.

Solution:

The given compound is an ester.

The general IUPAC name of an ester is alkyl alkanoate.

The alkyl group that is bonded to O (as OR) is termed 'alkyl', whereas the rest of the molecule is termed 'alkanoate'.

Alkyl branch: The alkyl group, which is bonded to O, is methyl. So, the methyl prefix will be used.

Alkanoate branch: Since the alkanoate branch is of four C-atoms with methyl connected to the second position, 2-methylbutanoate will be used for it.

Therefore, the IUPAC name of the compound is methyl-2-methylbutanoate.

Q. 4. Write the IUPAC name of the compound.

Solution:

The given compound is a carboxylic acid.

Word root: The parent chain is of four carbon atoms. So, the word root will be but.

Prefix: There are no substituents attached to the parent carbon chain.

Primary Suffix: The compound contains Carbon-carbon single covalent bond, so ane will be used as a primary suffix.

Secondary Suffix: The compound contains carboxylic acid as a functional group, so -oic acid will be used as a secondary suffix.

Therefore, the IUPAC name of the compound is Butan-1,4-dioic acid or 1,4-Butanedioic acid.

Q. 5. Write the IUPAC name of the compound.

Solution :

The given compound is an ether. The general IUPAC name of ether is alkoxy alkane.

Word root: The parent chain is of five carbon atoms. So, the word root will be pent.

Prefix: 1,2-dimethoxy

Primary suffix: ane,

Therefore, the IUPAC name of the compound is 1,2-dimethoxypentane.

Q. 6. Write the IUPAC name of the compound.

Solution:

The given compound is a tertiary amine.

Word root: The parent chain is of two carbon atoms. So, the word root will be eth.

Prefix: N,N-dimethyl

Primary suffix: ane

Secondary suffix: amine,

Therefore, the IUPAC name of the compound is N,N-dimethylethanamine.

Q. 7. Write the IUPAC name of the compound.

Solution :

The given compound is a ketone.

Word root: The parent chain is of five carbon atoms. So, the word root will be pent.

Prefix: 4-nitro

Primary suffix: ane

Secondary suffix: -one,

Therefore, the IUPAC name of the compound is 4-nitropentan-2-one.

Frequently Asked Questions - FAQ

Q1. Which bond should be given more priority, double bond or triple bond?
Answer: While naming an organic compound that contains double and triple bonds, the double bond is given priority over the triple bond.

Q2. What is the priority order if a molecule contains carboxylic acid, ester and amide?
Answer: When a molecule contains polyfunctional groups, IUPAC naming is done according to the priority order.

Priority order of some common functional groups: Carboxylic acid > Ester > Acyl Halide > Amide > Aldehyde > Ketone > Alcohol > Amine.

Carboxylic acid comes first, followed by esters, and finally amide.

Q3. When is the longest carbon chain selection rule violated?
Answer: When at least two functional groups or multiple bonds are present, it has to be included in the principal carbon chain even though it violates the longest chain rule.

Q4. What is the need for the selection of the longest chain as a principal carbon chain?
Answer: Every organic compound is either a hydrocarbon or a derivative of hydrocarbon. The reason behind the selection of the principal carbon chain is to select the highest possible principal carbon chain by which the compound can be prepared.

Q5. When is ‘buta’ used in place of ‘but’ as a word root?
Answer: When more than one multiple bond is present in the principal carbon chain that contains at least four carbon atoms, then we use an extra letter "a" with the word root.

Q6. Can we arrange substituents in any order in IUPAC naming?
Answer: In IUPAC naming, the substituents are always arranged in their alphabetical order with the locants.

Q7. Are two different prefixes used for the same functional group?
Answer: Yes, in polyfunctional groups containing compounds, there will be two different prefixes used for a functional group depending on the connectivity of the group. Formyl and oxo are used in aldehydes, oxo and keto are used in ketone, and alkoxy carbonyl and alkanoyloxy are used in esters.