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Isomerism: JEE & CBSE important question

Jan 18, 2018 17:39 IST
    Isomerism: JEE & CBSE important question
    Isomerism: JEE & CBSE important question

    Isomerism is a very important part in the syllabus of Chemistry for engineering entrance exam and CBSE class 11 exams. In this article we are going to share the important concepts related to Isomerism including some solved questions for coming entrance exams like JEE, UPSEE/UPTU, WBJEE, SRMJEEE, VITEEE etc.

    Isomerism mainly comes in CBSE class 11th chemistry syllabus and becomes the major parts in engineering entrance examinations like JEE Main, WBJEE, UPSEE, VITEEE etc.

    Following are the chapters of NCERT book where isomerism is being taught.

    NCERT Class 11 Chapter 13:  Hydrocarbons

    In the above chapter there are various topics like Classification of Hydrocarbons, Aliphatic Hydrocarbons, Alkanes - Nomenclature, isomerism and Alkenes - geometrical isomerism are being taught and have been asked the question in the class 11th chemistry paper

    In JEE and other engineering entrance examinations, every year 3 to 5 questions have been asked. So, this topic is very important for both the examinations.

    Isomerism

    Isomerism is a phenomenon in which two or more compounds have the same chemical formula but possess different properties because of different structural or spatial arrangements. Compounds which exhibit isomerism are called isomers. There are two types of isomerism:

    • Structural isomerism
    • Stereoisomerism

    Let’s see the complete overview of Isomerism and its types

    Isomerism Important Concepts

    Alkene compunds: Ozonolysis and Important questions for JEE

    Let’s discuss about the Structural isomerism

    Structural Isomerism:

    The compounds which exhibit this type of isomerism have the following important feature :

                    Molecular formula- same

                    Structural formula- different

    Structural isomerism is of following types

    1. Chain isomerism
    2. Position isomerism
    3. Functional group isomerism
    4. Metamerism


    Chain Isomerism:

    The phenomenon is termed as chain isomerism when two or more compounds have the same molecular formula but differ in the branching of carbon atoms.

    • It arises due to carbon chain or ring size
    • Compounds belongs to the same homologous series means if main compound is primary alcohol then all chain isomer should be primary alcohol
    • Number of carbon atom for Alkanes will be n≥4
    • Number of carbon atom for Alkynes and ketones will be n≥5

    Let’s have a quick overview on the number of carbon atom Vs number of chain isomers for Alkanes

    V.V.Important

    Carbon number(n)

    No. of Isomers

    4

    2

    5

    3

    6

    5

    7

    5

    8

    18

    9

    35

    10

    75

    The above table will be very much useful to attempt the direct questions in the exams

    Isomerism Important Concepts

    2. Position isomerism (Constitutional Isomers)

    The isomerism in which two or more compounds differ in the position of functional group or substituent atoms is called position isomerism

    It arises due to difference in the position of:

    • Side chain substituent
    • Double bonds and triple bonds
    • Functional groups

    In all position isomers of same molecular formula, parent chain should be same

    Q. Confirm the given structure weather they are Position isomer or not

    Isomerism Solved Question 1

    Answer: they are not position isomers 

    Q. Confirm the given structure weather they are Position isomer or not

    Isomerism Solved Question 2

    Answer: they are position isomers 

    Note:

    Aliphatic compounds having CN, CHO, COOH, COCl, COR CONH2 groups do not show position isomers

    Q. Which pair do not show position isomers

    1. Catechol- Resorcinol

    2. Benzyl alcohol- Ortho-Cresol

    3. Pyrogallol- Phloroglucinol

    4. Benzyl alcohol- Anisole

    Answer: Only option 4 is correct

    Functional group isomerism

    The isomerism in which two or more compounds have identical molecular formula but differ in the functional group present is called functional isomerism and these isomers are called functional isomers.

    It arises due to following reasons:

    • Due to difference in the nature of functional group

    Important: Ring chain isomers are a type of functional group isomers

    Examples:

    1. CnH2n  = Alkenes and cycloalkanes
    2. CnH2n-2 = alkadienes, alkynes and cycloalkenes
    3. CnH2n+2O = Alcohols and ethers
    4. CnH2nO = Aldehydes, ketones and unsaturated alcohols
    5. CnH2nO2 =  Acids, Esters, Hydroxy Aldehyde

    Important Note:

    CH3OH, HCHO, CH3CHO, C6H5CHO, HCOOH: These compounds do not show functional isomerism

    Metamerism:

    This isomerism is exhibited by compounds due to the presence of different alkyl chains on either sides of the functional group

    • It arises due to difference in alkyl or aryl group attached with polyvalent atom or group
    • All Metamers belongs to same homologous series

    Important Note:

    Compounds having following groups will exhibit the Metamerism

    Isomerism Important Concepts

    Q. for C4H8 Find out the:

    1. Number of Alkene isomers

    2. Number of all possible isomers

    Answer:

    1. Number of Alkene isomers- 3
    2. 2. Number of all possible isomers=6

    Stereoisomerism is a phenomenon in which compounds have the same molecular formula but differ in the relative positioning or orientation of atoms in space. The compounds which exhibit this type of isomerism are called as stereoisomers. These are further classified into:

    Stereoisomerism

    Stereoisomerism is a phenomenon in which compounds have the same molecular formula but differ in the relative positioning or orientation of atoms in space. It is two type.

    Geometric isomerism:

    This is being exhibited by molecules in which their special positions are locked to each other due to the presence of a ring structure or a double bond.

    Optical isomerism:

    The isomerism exhibited by two or more compounds which have the same molecular arrangement but differ in the optical activity.

    Keep visiting Engineering section of Jagranjosh.com to get detail notes and important questions on Stereoisomerism

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