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Positional Isomerism – Isomerism, its Classification, Structural Isomerism, Positional Isomerism, Practice Problems and FAQ

Positional Isomerism – Isomerism, its Classification, Structural Isomerism, Positional Isomerism, Practice Problems and FAQ

Consider the following scenario.

There is a teacher who is taking classes in a school. She gets disturbed by the continuous talking of the students seated on the last bench. So, she decides to swap the positions of the last bench students with the ones on the first bench.

Do you think it will make a difference?

Naturally, it will. Due to their proximity to the teacher, the loud kids on the final bench would be silent on the first bench. So, it is important to note that the same students who were making a fuss on the last bench began to stay quiet once their places (positions) were changed. The number of students in the class remained the same, but the atmosphere in the room altered as the students' places varied.

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Similar to this, when the position of a substituent, unsaturation, or functional group changes in two compounds with the same chemical formula, the compounds' characteristics likewise vary. Positional isomers are those molecules with the same chemical formula but distinct substituent positions in the parent chain.

We will learn more about isomerism and its classification on this concept page, with a focus on positional isomerism.

TABLE OF CONTENTS

  • Isomerism
  • Isomerism – Classification
  • Structural Isomerism
  • Structural Isomerism – Classification
  • Positional Isomerism – Introduction
  • Positional Isomerism – Examples
  • Practice Problems
  • Frequently Asked Questions – FAQ

Isomerism

Isomerism is the phenomenon of two or more compounds having the same molecular formula but different structural formulas, physical and chemical properties. These substances are referred to as isomers.

The Greek terms "isos" and "meros," which mean "equal" and "parts," respectively, are the source of the English word "isomer."

Isomerism – Classification

Stereoisomerism and structural isomerism are the two main categories under which isomerism can be generically categorised.

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Structural Isomerism: Structural isomers are substances with the same molecular formula but different structural formulas, meaning they differ in the order in which the atoms are bonded. This phenomenon is known as structural isomerism. Constitutional isomers are another name for structural isomers.

Example:

Stereoisomerism: Stereoisomers are isomers that differ from one another solely in the orientation of their atoms in space, and the process is known as stereoisomerism. These isomers share the identical atom and group bonding sequence.

Example:

Structural Isomerism

Chemical compounds that have the same molecular formula but different chemical structures are said to exhibit structural isomerism. Different functional groups or atoms connected to the central atom, or sometimes both, characterise structural isomers. Constitutional isomerism is another name for structural isomerism. The functional group may or may not be the same in the structural isomers, but the molecular formula stays the same.

Structural Isomerism – Classification

Structural isomerism can be classified into six types as given below.

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On this concept page, we will focus on positional isomerism in detail.

Positional Isomerism – Introduction

Positional isomers are structural isomers with a difference in the position of multiple bond/functional group/alkyl substituent on the same carbon skeleton (parent chain). The principal carbon chain remains the same in positional isomers. The size of the parent and the side chains in the isomers must be the same. The nature of the functional group, multiple bonds, and the substituent must be the same in the isomers.

Positional Isomerism – Examples

The following are a few examples of positional isomers.

1. But-1-ene and But-2-ene

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In the above example, the position of the double bond changes. In but-1-ene, the double bond is present in the first position, whereas in but-2-ene, the double bond is present in the second position. Since these two molecules have the same molecular formula (C4H8) but different positions of the double bond, they are positional isomers.

2. Propan-1-ol and Propan-2-ol

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In the example given above, the position of the hydroxyl group is different. In propan-1-ol, the hydroxyl group is present in the first position, whereas in propan-2-ol, the hydroxyl group is present in the second position. Since these two molecules have the same molecular formula (C3H8O) but different positions of the hydroxyl groups, they are positional isomers.

3. 1-Chlorobutane and 2-Chlorobutane

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In the above-given example, the position of the chlorine atom is different. In 1-chlorobutane, the chlorine atom is present in the first position, whereas in 2-chlorobutane, the chlorine atom is present in the second position. Since these two molecules have the same molecular formula (C4H9Cl) but different positions of the chlorine atoms, they are positional isomers.

4. 1,4-Benzenediol and 1,2-Benzenediol

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In the example given above, the positions of the hydroxyl groups are different. In 1,4-benzenediol, the hydroxyl groups are present in the first and the fourth positions, whereas in 1,2-benzenediol, the hydroxyl groups are present in the first and the second positions. Since these two molecules have the same molecular formula (C6H6O2) but different positions of the hydroxyl groups, they are positional isomers.

Practice Problems

1. Name the isomerism shown by the following compounds.

a. Chain Isomerism
b. Functional Group Isomerism
c. Positional Isomerism
d. None

Answer: C

Solution: Both the compounds have the same molecular formula C3H7NO2 and the same number of carbon atoms in its principal (parent) chain. Also, both the compounds have the same functional group (-NO2) attached.

The only difference is that in the first compound, -NO2 is attached to the 1st carbon of the parent chain, and in the second compound, -NO2 is attached to the 2nd carbon. Hence, they are positional isomers.

So, option C is the correct answer.

2.. Which isomerism is exhibited by 2-chloropropane and 1-chloropropane?

a. chain
b. positional
c. functional
d. metamerism

Answer: B

Solution: In the compounds given in the question, the position of the chlorine atom is different. In 1-chloropropane, the chlorine atom is present in the first position, whereas in 2-chloropropane, the chlorine atom is present in the second position. Since these two molecules have the same molecular formula (C3H7Cl) but different positions of the chlorine atoms, they are positional isomers.

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So, option B is the correct answer.

3. Which of the following pairs of compounds are positional isomers?

a. Pentan-1-ol and 2-methylbutan-1-ol
b. Pentane-1-ol and 2,2-dimethylpropan-1-ol
c. Pentan-1-ol and pentan-2-ol
d. Pentan-1-ol and 2,2-dimethylbutan-1-ol

Answer: C

Solution: Positional isomers are structural isomers with a difference in the position of multiple bond/functional group/alkyl substituent on the same carbon skeleton (parent chain). The principal carbon chain remains the same in positional isomers. The size of the parent and the side chains in the isomers must be the same. The nature of the functional group, multiple bonds, and the substituent must be the same in the isomers.

1. In the pair of compounds given in option A, the molecular formula is the same but the length of the parent chain is different. Hence, they are chain isomers and not positional isomers.

2. The pair of compounds given in option B is also chain isomers as they have the same molecular formula but the number of carbon atoms in the parent chain changes. Therefore, they are chain isomers.

3. Pentan-1-ol and pentan-2-ol are positional isomers as they differ only in the position of the hydroxyl group on the parent chain. They both have the same molecular formula but have different positions in which the hydroxyl group is attached. So, they are positional isomers.

4. The pair of compounds given in option D is not isomers as the molecular formula of these compounds is not the same.

So, option C is the correct answer.

4. How many carbon atoms must be present for alkanes, alkenes, and alkynes to show positional isomerism?

a. 8, 6, 4
b. 7, 5, 5
c. 6, 4, 2
d. 6, 4, 4

Answer: D

Solution: For alkanes to show positional isomerism, a minimum of 6 carbon atoms are required.

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For alkenes to show positional isomerism, a minimum of 4 carbon atoms are required.

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For an alkyne to show positional isomerism, a minimum of 4 carbon atoms are required.

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So, option D is the correct answer.

Frequently Asked Questions – FAQ

1. What are the differences between chain and position isomers?
Answer: Positional isomers are molecules with the same chemical formula but distinct functional groups linked to the carbon chain at various locations. Compounds with the same chemical formula but distinct carbon skeletons are known as chain isomers.

2. Which functional group does not show positional isomerism?
Answer: The "-CHO" group is always present at the end of an aliphatic aldehyde. As a result, they don't exhibit position isomerism.

3. Which isomers are a non-superimposable mirror image of each other?
Answer: A pair of stereoisomers that are non-superimposable mirror images of one another is referred to as an enantiomer. This means the molecules are made up of identical atoms that are bound together in the same way, i.e., they are connected in the same way but on the other hand, have a distinct 3D arrangement of atoms since they are mirror reflections of each other. Without breaking and reconstructing bonds, you can not superimpose one on top of the other.

4. How can the different isomers formed be separated?
Answer: Isomerism is the phenomenon in which two or more compounds have the same chemical formula but differ in one or more physical properties like melting point, boiling point, density etc. or chemical properties. Therefore the isomers can be separated by processes like fractional distillation, fractional crystallisation, chromatography technique etc.

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