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WBJEE: Important Questions and Study Notes for Hydrocarbons

Jan 24, 2018 14:32 IST
    WBJEE 2018: Hydrocarbons
    WBJEE 2018: Hydrocarbons

    Chemistry can be categorized into three parts i.e., Physical Chemistry, Inorganic Chemistry and Organic Chemistry. Organic Chemistry is a very important part in the syllabus of WBJEE 2018. Around 40% of the questions are asked from this part only.

    Organic Chemistry is the study of structure, preparation, physical properties and chemical properties of carbon containing compound. The chapter ‘Hydrocarbons’ is the base of the Organic Chemistry.

    Hydrocarbons are further divided into three parts i.e., Alkane, Alkenes and Alkynes. The chemical formula for Alkane, Alkenes and Alkynes is CnH2n + 2, CnH2n  and CnH2n – 2 respectively.

    In this article, WBJEE aspirants will get study notes including previous years’ solved questions related to chapter Hydrocarbons which will help in your preparation for WBJEE 2018.

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    Read the following topics here:

    1. Preparation of alkanes from unsaturated hydrocarbons, alkyl halides and carboxylic acid

    2. Physical properties of alkanes

    3. Chemical properties of alkanes

    4. Preparation of alkenes from alkynes, alkyl halides, vicinal dihalides and alcohol

    5. Physical properties of alkenes

    6. Chemical properties of alkenes

    7. Preparation of alkynes from calcium carbides and vicinal dihalides

    8. Physical properties of alkynes

    9. Chemical properties of alkynes

    About the notes:

    1. These notes are prepared by Subject Experts of Chemistry after the detailed analysis of the latest syllabus and paper pattern of WBJEE.

    2. These notes contain the crux of the chapter “Hydrocarbons”.

    About the exam:

    West Bengal Joint Entrance Examination (WBJEE) is a state level common entrance test organized by West Bengal Joint Entrance Examinations Board for admission to the Undergraduate Level Engineering and Medical Courses through a common entrance test in the State of West Bengal.

    Important Concepts:

    WBJEE Hydrocarbons Concepts 1

    Physical Properties of Alkanes:

    • They are hydrophobic and dissolve in nonpolar or weakly polar solvents.
    • They are less dense than water.
    • The boiling point of alkane increases by 20-30 degree Celsius with each additional methylene group.
    • For isomeric alkanes, the boiling point decreases with branching due to an overall increase in surface area.

    WBJEE Hydrocarbons Concepts 2

    WBJEE: Important Questions and Preparation Tips –Periodic Classification & Periodicity in Properties

    WBJEE Hydrocarbons Concepts 3

    WBJEE Hydrocarbons Concepts 4

    WBJEE Hydrocarbons Concepts 4

    Physical properties of Alkynes:

    • It follows the same trend of alkenes and alkanes.
    • First three members of alkenes are gases, the next eight are liquids and the higher ones are solids.
    • They are colourless.
    • Except Ethyene all other members of alkenes are odourless.
    • They are weakly polar in nature.
    • They are immiscible with water but soluble in organic solvents like ethers, carbon tetrachloride and benzene.
    • With increase in molar mass their melting point, boiling point and density also increase.

    WBJEE Hydrocarbons Concepts 4

    Some previous years’ solved questions are given below:

    Question 1:

    From the following compounds choose the one which is not aromatic:

    WBJEE Hydrocarbons Question 1

    Solution 1:

    WBJEE Hydrocarbons Solution 1

    This compound is cyclooctatetraene which is non polar. Hence this is non aromatic compound.

    Hence, the correct option is (B).

    Question 2:

    Ozonolysis of an alkene produces only one dicarbonyl compound. The structure of the alkene is:

    WBJEE Hydrocarbons Question 2

    Solution 2:

    WBJEE Hydrocarbons Solution 2

    Hence, the correct option is (B).

    Question 3:

    The major products obtained during ozonolysis of 2, 3 – dimethyl-1-butene and subsequent reductions with Zn and H2O are

    (A) Methanoic acid and 2-methyl-2-butanone

    (B) Methanal and 3-methyl-2-butanone

    (C) Methanol and 2,2-dimethyl-3-butanone

    (D) Methanoic acid and 2-methyl-3-butanone

    Solution 3:

    The chemical reaction can be shown as below:

    WBJEE Hydrocarbons Solution 3

    Hence, the correct option is (B).

    Question 4:

    Choose the correct statements(s) among the following:

    WBJEE Hydrocarbons Question 4

    Solution 4:

    WBJEE Hydrocarbons Solution 4(a)

    So, they are not enantiomers.

    WBJEE Hydrocarbons Solution 4(b)

    Hence, the correct options are (B) and (D).

    You can also have a look on the following questions and solutions to check your preparation.

    Question 1:

    Provide an acceptable name for the alkane shown below:

    WBJEEHydrocarbons Question 1

    (A) 6–ethyl–2–methyl–5–propyldecane

    (B) 5–ethyl–6–methyl–2–propyldecane

    (C) 2–ethyl–6–methyl–2–propyldecane

    (D) 2–ethyl–6–methyl–5–propyldecane

    Solution 1:

    WBJEE Hydrocarbons Solution 1

    The name of the given compound is 6–ethyl–2–methyl–5–propyldecane.

    Hence, the correct option is (A).

    Question 2:

    Which of the following compounds will exhibit geometrical isomerism?

    (A) 1–Phenyl–2–butane

    (B) 3–Phenyl–1–butene

    (C) 2–Phenyl–1–butene

    (D) 1,1–Diphenyl–1–propene

    Solution 2:

    WBJEE Hydrocarbons Solution 2

    The compound 1–Phenyl–2–butane will exhibit geometrical isomerism.

    Hence, the correct option is (A).

    Question 3:

    Provide the systematic name of the compound shown:

    UPSEE Hydrocarbons Question 3

    (A) 4 – butyl – 1 – ethyl – 2 – methylcycloheptane

    (B) 4 – butyl – 2 – ethyl – 1 – methylcycloheptane

    (C) 1 – butyl – 4 – ethyl – 3 – methylcycloheptane

    (D) 2 – butyl – 4 – ethyl – 1 – methylcycloheptane

    Solution 3:

    WBJEE Hydrocarbons Solution 3

    Naming according to closest set of locant rule is 4 – butyl – 1 – ethyl – 2 – methylcycloheptane.

    Hence, the correct option is (A).

    Question 4:

    Which of the following compounds is insoluble even in hot concentrated H2SO4?

    (A) Ethylene (B) Benzene

    (C) Hexane (D) Aniline

    Solution 4:

    Hexane is insoluble even in hot concentrated H2SO4.

    Hence, the correct option is (C).

    Question 5:

    The number of possible alkynes with molecular formula C5H8 is

    (A) 1

    (B) 3

    (C) 5

    (D) 8

    Solution 5:

    WBJE Hydrocarbons Solution 5

    Hence, the correct option is (B).


    All the concepts related to Hydrocarbons like properties, physical properties and chemical properties are presented in a systematic manner which will help students in understanding this topic easily. Students can easily predict the difficulty level of the questions which can be asked in WBJEE 2018 with the help of previous years’ solved questions given here. Students can also track their progress after attempting the questions given here.

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