IUPAC Nomenclature: Trivial Nomenclature System, Cyclic Hydrocarbons, Aromatic Compounds, Bicyclo Compounds, Spiro Compounds, Practice Problems & FAQs

Do you have an Adhar card? It is simply an ID for all the citizens of India. Each Aadhaar card contains a unique number that can be used to identify the said person and basic information about him/her like permanent address, mobile number, etc. In a similar fashion, we have the IUPAC nomenclature for a large number of organic compounds. It contains a set of rules through which we can assign a unique IUPAC name to any compound, which makes the classification and analysis easy.

Table of content 

• Trivial Nomenclature System
• Drawbacks of Trivial Nomenclature System
• Nomenclature of Cyclic Hydrocarbons
• Nomenclature of Aromatic Compounds
• Nomenclature of Bicyclo Compounds
• Nomenclature of Spiro Compounds
• Practice Problems
• Frequently asked questions

Trivial Nomenclature System

It gives name to the organic compounds in a non-systematic way. There are no rules to give naming in a trivial nomenclature system. In a trivial nomenclature system, names are given to compounds based on the sources of the compounds, uses of the compounds, and the properties of the compounds. Examples of this system are oxalic acid, toluene, acetic acid, etc. 

The names of compounds designated using trivial nomenclature are significantly shorter and simpler than the IUPAC nomenclature. As a result, this system is still relevant today.

For example,

1. The trivial name of a carboxylic acid that is found in vinegar is acetic acid and the IUPAC name of acetic acid is ethanoic acid.
2. The trivial name of a carboxylic acid that is found in tamarind is tartaric acid and the IUPAC name of tartaric acid is 2,3-dihydroxybutanedioic acid.

Drawbacks of Trivial Nomenclature System

• A single compound might have several different names. For example, Phenol's other names, such as hydroxybenzene and carbolic acid, are an example of this.
• Only a few compounds in each group are covered by the trivial nomenclature method. For example, the first two members of the carboxylic acid group are known by their common names formic acid and acetic acid. However, there are no common names for carboxylic acids having more than one atom.
• In a trivial nomenclature system, there is no set of rules for the naming of complex compounds.

Nomenclature of Cyclic Hydrocarbons

Rules for Determining of Parent Chain

Case 1: In cyclic hydrocarbons without substituents

For alicyclic compounds without substituents, the names are obtained by adding the prefix ‘cyclo’.

Examples:

Case 2: In cyclic hydrocarbons with substituents

• If the number of ‘C’ atoms in the ring ≥ the number of ‘C’ atoms in the open chain, then the ring

is considered to be the parent chain and the open chain is considered to be the substituent.

Example:

• If the number of ‘C’ atoms in the ring < the number of ‘C’ atoms in the open chain, then the ring

is considered to be the substituent and the open chain is considered to be the parent chain.

Example:

• If more than one alicyclic ring is attached to a single chain, then the rings are considered to be

the substituents and the attached alkyl group is considered to be the parent chain.

Example:

Case 3: In the presence of unsaturation

• If the ring has unsaturation and the open chain is saturated, then the ring is considered to be the

parent chain, and the open chain is considered to be a substituent irrespective of the number of

C-atoms.

Example:

• If the open chain has unsaturation and the ring is saturated, then the open chain is considered to be the parent chain, and the ring is considered to be a substituent irrespective of the number of C-atoms.

Example:

• If both the open chain and the ring have unsaturation, the one with the maximum number of double or triple bonds is considered to be the parent chain.

Example:

• If both the open chain and the ring have equal unsaturation, then the longest one is the parent chain.

Example:

• If both the open chain and the ring have equal unsaturation and an equal number of ‘C’ atoms, then the ring is considered to be the parent chain.

Example:

Rules for Assigning Locants

1. If more than one double or triple bonds are present, then they are assigned the lowest possible number.

Example:

2. If both multiple bonds and a substituent are present, then the multiple bonds gets the lower number.

Example:

3. If both double and triple bonds are present, then alphabetical order is followed in writing the name.

Example:

Nomenclature of Aromatic Compounds

Aromatic compounds are planar, closed ring compounds with alternate single and double bonds that have extra stability. They follow Hückel's rule, i.e., (4n + 2) π electrons, where n = 0, 1, 2 ...

These are of two types:

1. Benzenoid aromatic compounds
2. Non-benzenoid aromatic compounds

Rules for Nomenclature of Benzene derivatives

Rule 1: In simple compounds (monosubstituted benzene), benzene is the parent name and the substituent is simply indicated by a prefix.

Example:

Rule 2: If two substituents are present, their relative positions are indicated by either the prefixes ortho-, meta-, and para- or by the use of numbers.

Here, the ortho position refers to the (1,2) position, i.e., the adjacent position.

The meta position refers to the (1,3) position, i.e., next to the adjacent position.

The para position refers to the (1,4) position, i.e., just opposite to the substituent.

Example:

Rule 3: If more than two groups are present on the benzene ring, their positions must be indicated by numbers. The numbering is done such that the lowest possible numbers are given to the substituents.

If more than two substituents are present and the substituents are different, they are listed in alphabetical order.

Examples:

Rule 4: If a substituent (or functional group) together with the benzene ring gives a new base name, that substituent (or functional group) is assumed to be in the 1st position and a new parent name is used.

Examples:

Rule 5: When the -C6H5 (−Ph) group is named as a substituent, it is known as a phenyl group.

Note:

• When a benzene ring acts as a side chain, it is known as a phenyl ring.
• Benzyl has a methylene group ( -CH2-) having it's one end connected to benzene.
  

• Benzal has a methylene group having its one end connected to benzene.

Rule 6: If a hydrocarbon is composed of one saturated chain and one benzene ring, it is most appropriately named as a derivative of the larger structural unit. However, if named otherwise, that is also accepted.

Examples:

Nomenclature of Bicyclo Compounds

A bicyclic compounds are the compounds that has two rings joined together by at least two common atoms of each ring.

Examples:

The general format for IUPAC nomenclature of bicyclo compounds is Bicyclo[a,b,c]alkane.

Rules for IUPAC nomenclature of Bicyclo compounds.

Rule 1: Find the highest priority functional group in the molecule and suffix will be used for the principal functional group.

Example:

In the given molecule, all carbons are bonded together with a single covalent bond. So here -ane is used as a suffix.

Rule 2: Count the total number of carbon atoms in the molecule for the root word.

The total number of carbon atoms in the given molecule is eight, so the word root is oct.

Rule 3: Identify the bridgehead carbon atoms by which two rings are fused/attached.

Rule 4: Numbering starts from one bridged head carbon atom and follow the longest path to next bridged carbon atom and it continues till all carbons are numbered.

 Rule 5: Arrange in order from longest path to next longest to shortest path.

Rule 6: Write everything together starts with bicyclo

IUPAC name: Bicyclo[3.2.1]octane

Examples:

Nomenclature of Spiro Compounds

Spiro compounds are the compounds which have two rings joined together by one common atom of each ring.

The general format for IUPAC nomenclature of spiro compounds is Spiro[a,b]alkane.

Rules for IUPAC nomenclature of Spiro compounds.

Rule 1: Find the highest priority functional group in the molecule and suffix will be used for the principal functional group.

Example:

In the given molecule, all carbon atoms are bonded together with a single covalent bond. So here -ane is used as a suffix.

Rule 2: Count the total number of carbon atoms in the molecule for the root word.

The total number of carbon atoms in the given molecule is nine, so the word root is non.

Rule 3: Numbering starts from the carbon adjacent to spiro carbon and follows the shortest path to the longest path and it continues till all carbons are numbered.

 Rule 4: Arrange in order from the shortest path to the longest path.

[3.5]

Rule 6: Write everything together starting with spiro

IUPAC name: Spiro[3.5]nonane

Examples:

Practice Problems

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

Answer:

 From the structure given, we can see that the ring is the parent chain (since it contains more number of carbons than the other substituents i.e. methyl, ethyl, and propyl). If numbering is started from the carbon attached to the methyl group, two possible ways of numbering i.e., 1,3,6 and 1,2,5 are possible. If numbering is started from the carbon attached to the propyl group, then the two possible ways of numbering are 1,2,4, and 1,4,6. If numbering is started from the carbon attached to the ethyl group, then the two possible ways of numbering are 1,3,4 and 1,4,5. So, starting the numbering from the propyl group (1,2,4) is the correct one because of the sum of the lowest locant rule.

So, the IUPAC name of the compound is 4-Ethyl-2-methyl-1-propylcyclohexane.

Q 2. Write the IUPAC name of the compound.

Answer: 

Here, the ring contains more carbon atoms. So, it will be the parent chain. Following the lowest locant rule, the IUPAC name of the compound is 1,2-Dimethyl-4-methylidenecyclopentane.

Q 3. Write the IUPAC name of the compound.

Answer: 

In the given compound, the parent chain is ethyl, and two (4-chlorophenyl) substituents are on the 2nd carbon, and the three −Cl substituents are on the 1st carbon. So, the IUPAC name of the compound is,

1,1,1-Trichloro-2,2-bis(4-chlorophenyl)ethane.

Q 4. Write the IUPAC name of the compound.

Answer: 

In the given compound, the parent chain is cyclohexane. One ester functional group is connected to the cyclohexane ring from carbonyl carbon and the other by the oxygen atom. So, the first one is used as the principal functional group and the other as the prefix. Thus, the IUPAC name of the given compound

is ethyl-3-(propanoyloxy)cyclohexane-1-carboxylate.

Q 5. Write the IUPAC name of the compound.

Answer: In this case, the alcohol group is the highest priority group and there are two same functional groups. Also, there is one double bond and therefore, the numbering should start from the side where both the alcohol groups and the double bond get the lowest numbering.

Frequently asked questions

Q 1. What is the IUPAC name of C6H5COCl?
Answer: In the given compound, the principal functional group is acid chloride with a benzene ring attached to the carbonyl carbon. So, the IUPAC name of the compound is benzoyl chloride.

Q 2. What is the trivial name of 2,3-dihydroxy-1,4-Butanedioic acid?
Answer: The trivial name of 2,3-dihydroxy-1,4-Butanedioic acid is tartaric acid.

Q 3. Which system of nomenclature is suitable for naming organic compounds?
Answer: IUPAC system of nomenclature is followed for the naming of organic compounds because of having a fixed set of guidelines and rules for naming whereas the trivial nomenclature system does not have any set of rules for the naming of compounds.

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