IUPAC and Isomerism are the widely used terms when it comes to Chemistry and to be specific it is organic chemistry. Classification and IUPAC nomenclature of organic compounds, types of organic compounds are some of the important topics. The weightage of this section is around 4% in NEET examination. 

• Candidates can expect around 2-3 questions from this chapter. 
• The questions could be difficult to understand as this chapter needs a deep understanding of concepts. Check NEET Chemistry Syllabus

Read the article to know more about the definitions of various concepts of IUPAC and Isomerism with solved sample questions. 

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Quick Notes on Important Definitions

IUPAC Nomenclature of Organic Chemistry

In chemical nomenclature, the organic chemistry nomenclature IUPAC is a way of organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC).

Organic Chemistry Nomenclature

A set of laws were formed by IUPAC (the International Union of Pure and Applied Chemistry) for the methodical nomenclature of organic compounds. The IUPAC nomenclature of organic compounds primarily consists of three parts which are stem name, prefix, and suffix.

Trivial Nomenclature System

The Trivial Nomenclature method involves a non-systematic strategy to name the organic compounds. There do not endure any special set of practices for writing the trivial naming of compounds. The compounds called via trivial nomenclature have shorter and more simplistic names than the identical IUPAC nomenclature of the same compounds. An example is A type of carboxylic acid which is commonly observed in tamarind is introduced as tartaric acid as per the trivial system. The identical IUPAC nomenclature of tartaric acid would be 2,3-dihydroxy-1,4-Butanedioic acid.

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Shortcomings of the Trivial Nomenclature System

A few lapses of the trivial system are:-

• Several trivial names can exist for one particular compound. For example, Phenol has alternate names.
• The Trivial nomenclature system is confined to only a few compounds in each group. 
• There exist no appropriate set of guidelines for the classification of complex compounds in the trivial system.

IUPAC Nomenclature

According to the Guidelines set by IUPAC, the nomenclature of compounds must follow these steps:

1. The Longest Chain Rule: The parent hydrocarbon needs to be recognized and subsequently mentioned
2. The Lowest Set of Locants: The carbon atoms pertaining to the parent hydrocarbon chain must be numbered using natural numbers, starting from the end in which the lowest number is indicated to the carbon atom which bears the substituents.
3. Multiple instances of the same substituent: Prefixes that symbolize the same substituent and are repeated in organic compounds are given names such as di, tri, etc.
4. Naming of different substituents: In the organic compounds comprising of multiple substituents, the identical substituents are arranged in alphabetical order of names.
5. The naming of different substituents present at the same positions: In the situation where two differing substituent groups are there at the equivalent position of the organic compound, then the substituents are named in ascending alphabetical order.

The format of the IUPAC Name of the Compound can be written as: Locant + Prefix + Root + Locant + Suffix

Root

It shows the total number of carbon atoms present in the longest carbon chain of the compound.

Suffix

It is normally a functional group pertaining to the molecule which serves the root of the name. It is moreover divided into two types.

• A Primary Suffix, which is written after the word root as in the case of alkanes, where the suffix is ‘ane’.
• A Secondary Suffix is written after the primary suffix is written. 

Prefix

Prefixes are attached prior to the root of the compounds in IUPAC nomenclature. They are of two types:-

Primary Prefixes Symbolizes the cyclic or acyclic nature of the given compound. The prefix ‘cyclo’ is used for cyclic compounds.

Secondary Prefixes Show the presence of side chains or substituent groups.

Previous Year Question

1. The IUPAC name of neopentane is :
   1. 2–methylpropane
   2. 2, 2–dimethylbutane
   3. 2–methyl butane
   4. 2, 2–dimethylpropane

Ansewr:-IUPAC’s name is 2,2- dimethyl propane. (Option 4)

2. The IUPAC name of CH3COCH(CH3)2 is
   1. 2-methyl-3-butanone
   2. 4-methyl isopropyl ketone
   3. 3-methyl-2-butanone
   4. Isopropylmethyl ketone

Answer:-(option 3) 3-methyl-2-butanone

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IUPAC Nomenclature Methods

There are three types of Nomenclature methods:-

1. Compositional Nomenclature

• The generalized stoichiometric name is included in the compositional nomenclature of compounds. Objects are named using various prefixes so that the overall stoichiometry of the compound is very clear from the name.
• In cases of multiple components, the components are divided into two classes – electronegative components and electropositive components.

2. Substitutive Nomenclature

Substitutive Nomenclature is practiced in the IUPAC nomenclature where the parent hydride is modified by the replacement of hydrogen atoms with substituent groups.

3. Additive Nomenclature

This process was formed principally for its applications in the nomenclature of coordination compounds. It has a wide range of applications. For example, Penta-ammine-chloro-cobalt(III) chloride is applied to characterize the coordination compound given by the chemical formula [CoCl(NH3)5]Cl2.

“The prefix ‘chloro’ resembles a Chloride, whereas the prefix ‘chloride corresponds to the ligand.”

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IUPAC Nomenclature of Important Aliphatic Compounds

The IUPAC nomenclature of alkanes, alkenes, and alkynes are discussed in the subsections below.

1. Alkanes

The General formula of alkanes is CnH2n+2. The suffix ‘ane’ is usually applied to explain alkanes. Examples- methane for CH4 and Butane for C4H10.

2. Alkenes

The General formula of alkenes is CnH2n.

The suffix ‘ene’ is used to describe alkenes. Examples- ethene for C2H4 and Propene for C3H6

3. Alkynes

The General formula of alkynes is CnH2n-2

The suffix ‘yne’ is commonly used to describe alkynes. Example- Ethyne for C2H2.

Previous Year Questions

1. 1)The systematic name of CH3-CHBr-CH2OH is
   1. 3-hydroxy-2-bromopropane
   2. 2-bromopropanol-1
   3. 2-Bromo-3-propanol
   4. 3-hydroxy isopropyl bromide

Answer:-(Option 2) 2-bromopropanol-1

2. The IUPAC name of acetyl salicylic acid is
   1. m-benzoic acid
   2. 2-acetoxy benzoic acid
   3. p-benzoic acid
   4. p-acetyl benzoic acid

Answer:-(Option 2) 2-acetoxy benzoic acid

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What is Isomerism?

Isomerism is the phenomenon in which more than one compounds have the same chemical formula but different chemical structures.

Types

It is primarily of two types of isomerism namely Structural Isomerism and Stereoisomerism. The classification of different types of isomers is illustrated below.

Structural Isomerism

Structural isomerism is usually mentioned as constitutional isomerism. The functional groups and the atoms in the molecules of these isomers are linked in different ways. The different types of structural isomerism are:-

Chain Isomerism

• It is also identified as skeletal isomerism.
• The segments of these isomers exhibit adversely branched arrangements.
• Commonly, chain isomers contrast in the branching of carbon
• Example of chain isomerism =C5H12.

Position Isomerism

• The position of the functional groups is distinctive in position isomers.
• Typically, this isomerism includes the addition of the functional groups to various carbon atoms in the carbon chain.
• Example = C3H7Cl.
   

Functional Isomerism

• It is also called functional group isomerism.
• As the name implies, it indicates the compounds that have identical chemical formulas but different functional groups attached to them.
• Example = C3H6O.
   

Metamerism

• Metamerism occurs due to the occupation of several alkyl chains on each side of the functional group.
• It is a unique type of isomerism and is usually confined to molecules that include a divalent atom enclosed by alkyl groups.
• Example = C4H10O can be represented as ethoxyethane (C2H5OC2H5) and methoxy-propane (CH3OC3H7).

Tautomerism

• A tautomer of a compound relates to the isomer of the compound which only varies in the location of protons and electrons.
• The Tautomers of a compound live together in balance and quickly interchange.
• It happens through an intramolecular proton transfer.
•  Example = Keto-enol tautomerism.

Ring-Chain Isomerism

• In ring-chain isomerism, one of the isomers has an open-chain formation whereas the other has a ring structure.
• They usually hold a variety of a number of pi bonds.
• Example = In C3H6, Propene and cyclopropane are the resulting isomers.

Stereoisomerism

This type of isomerism occurs in compounds possessing identical chemical formulas but a difference in their orientations. The compounds which show stereoisomerism is often recognized as stereoisomers. It is further categorized into two subtypes:-

Geometric Isomerism

• It is commonly known as cis-trans isomerism.
• These isomers have diverse arrangements of atoms in a three-dimensional range.

Optical Isomerism

• Compounds that come under optical isomerism have a characteristic of similar bonds but an unconventional spatial arrangement of atoms constituting non-superimposable mirror images.
• They are known as enantiomers.
• Enantiomers dissent from each other in terms of their optical behavior.
• Dextro enantiomers rotate the plane of polarized light to the right whereas laevo enantiomers rotate it to the left.

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Previous Year Question On Isomerism

1. Monocarboxylic acids are functional isomers of :
   1. Esters
   2. Amines
   3. Ethers
   4. Alcohols

Solution:Option (1)

2. Dipole moment is shown by:-
   1. trans-2, 3-dichloro- 2-butene
   2. 1, 2-dichlorobenzene
   3. 1, 4-dichlorobenzene
   4. trans-1, 2-dinitroethene

Solution:option (2).

3. Which of the following does not show geometrical isomerism?
   1. 1, 2-dichloro -1 – pentene
   2. 1, 3 – dichloro – 2 – pentene
   3. 1, 1 – dichloro – 1 – pentene
   4. 1, 4 – dichloro – 2 – pentene

Solution:option (3) .

4. A similarity between optical and geometrical isomerism is that
   1. each forms an equal number of isomers for a given compound
   2. If in a compound one is present then so is the other
   3. both are included in stereoisomerism
   4. they have no similarity

Solution: option (3)

5. Which of the following will have meso-isomer also?
   1. 2- chlorobutane
   2. 2- hydroxypropanoic acid
   3. 2,3- dichloropentane
   4. 2-3- dichlorobutane

Solution: option (4)

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