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WBJEE: Study Notes & Important Questions for Alkyl Halides

Jan 25, 2018 10:44 IST
    WBJEE 2018: Alkyl Halides
    WBJEE 2018: Alkyl Halides

    It is very important to focus on all three subjects i.e., Physics, Chemistry and Mathematics to secure good rank in engineering entrance exam. Students can’t secure good rank in exam if they focus only on one subject out of three.

    WBJEE is a state level engineering entrance examination which is easier to crack as compared to other engineering entrance exams like JEE Main, JEE Advanced and UPSEE.

    About 70% of the questions come from theory part in Chemistry paper of WBJEE. So, it is very important to focus only on the important topics as students can’t remember everything in the examination.

    WBJEE: Important Questions and Preparation Tips for Alcohol

    In organic chemistry students should try to understand the mechanism of reactions instead of rote learning. If they understand the concepts then can easily solve any question in the examination.

    After analyzing the pattern and the syllabus of WBJEE we are providing you the revision notes including important solved questions of chapter Alkyl Halides which will help you to boost your preparation. Students always get 2-3 questions from this chapter in the examination.

    Read about the following topics here:

    1. Classification of haloalkanes and haloarenes,

    2. Preparation of alkyl halides and

    (a) From Alcohols and

    (b) From Hydrocarbons

    (i) By free radical halogenation,

    (ii) By electrophilic substitution,

    (iii) Sandmeyer's reaction,

    (iv) Addition of halogens,

    (v) Hunsdiecker reaction,

    (vi) Allylic Bromination and

    (vii) Halogen exchange

    3. Chemical properties of Alkyl Halides

    (a) Nucleophilic substitution

    (i) Substitution nucleophilic bimolecular (SN2) and

    (ii) Substitution nucleophilic unimolecular (SN1)

    (b) Elimination reactions

    (i) E1 and

    (ii) E2

    Important Concepts:

    WBJEE Alkyl Halides Concepts 1

    Chemical Reactions of Alkyl Halides

    The reaction can be broadly classified in two categories:

    (a) Nucleophilic substitution

    (b) Elimination reactions

    Nucleophilic substitution reactions: In this reaction a nucleophile, which is rich in electrons, attacks partial positive charge on the carbon atom bonded to halogen to replace the leaving group.

    WBJEE Alkyl Halides Concepts 2

     

    WBJEE Alkyl Halides Concepts 3

    WBJEE Alkyl Halides Concepts 4

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

    WBJEE Alkyl Halides Concepts 5

    Difference between E1 and E2 reaction mechanism:

    Attributes

    E1

    E2

    Rate law

     

    Depend on the concentration of substrate

    Depends on the concentration of both substrate and base

    Barrier

    Formation of carbocation 3o>2o>> 1o

    None

    Base

    Does not require strong base

    Requires strong base

    Stereochemistry

    Does not require stereochemistry

    Leaving group must be anti to hydrogen removed

    Some solved questions are given below:

    Question 1:

    Ethylidene chloride is a/an……..

    (a) vic-dihalide            

    (b) gem-dihalide         

    (c) allylic halide          

    (d) vinylic halide

    Solution 1:

    In vic-dihalides, halongen atom are present on the adjacent carbon atoms.

    In gem-dihalides, halogen atoms are present on the same carbon atom. They are known as alkylidene halides

    In allylic halides, halogen atom is bonded to sp3 hybridised carbon atom next to carbon-carbon double bond.

    In vinylic halides, halogen atom is bonded to sp2 hybridised carbon atom of a carbon-carbon double bond.

    In ethylidene chloride (H3C-CHCI2) both halogen atoms are present on same carbon atom so it is gem-dihalide.

    Hence, the correct option is (b).

    Question 2:

    A primary alkyl halide would prefer to undergo………

    (a) SN1 reaction              

    (b) SN2 reaction             

    (c) a-elimination            

    (d) racemisation

    Solution 2:

    A primary alkyl halide would prefer to undergo SN2 reaction.

    (a) SN2reactions occur only if the intermediate carbonation is stable i.e., 3° carbocation.

    (b) SN2 reactions occur if there is less steric hinderance on the a-carbon of alkyl halide. In case of primary alkyl halides carbocation is highly unstable and steric hinderance is very less. So, primary alkyl halide would prefer to undergo SN2reaction.

    (c) In a-elimination, proton and the leaving group are present on same atom.

    (d) Racemisation is the process of conversion of enantiomer into a racemic mixture.

    Hence, the correct option is (b).

    Question 3:

    Which is the correct increasing order of boiling points of the following compounds?

    1-iodobutane, 1-bromobutane, 1-chlorobutane, Butane

    (a) Butane < 1-chlorobutane < 1-bromobutane < 1-iodobutane

    (b) 1-iodobutane < 1-bromobutane < 1-chlorobutane < Butane

    (c) Butane < 1-iodobutane < 1-bromobutane < 1-chlorobutane

    (d) Butane < 1-chlorobutane < 1-iodobutane < 1-bromobutane

    Solution 3:

    Higher the surface area, higher will be the intermolecular forces of attraction and thus boiling point too. Boiling point increases with increase in molecular mass of halogen atom for the similar type of alkyl halide. Butane has no halogen atom and rests of all three compounds are halo derivatives of butane.

    Atomic mass of iodine is highest so the boiling point of 1-iodcbutane is maximum among all the given compounds.

    Hence, the correct option is (a).

    Question 4:

    Alkyl fluorides are synthesised by alkyl chloride/bromide in presence of ….…or………. .

    (a) CaF2                                                          

    (b) CoF2

    (c) Hg2F2                                                         

    (d) NaF

    Solution 4:

    Alkyl fluorides are synthesised by alkyl chloride/bromide in presence of CoF2 or Hg2F2. Only transition metal fluorides react with alkyl chloride/bromide to form alkyl fluorides. Alkali metal fluoride such as NaF and alkaline earth metal fluoride such as CaF2 do not react to form fluorides

    Hence, the correct options are (b) and (c).

    Question 5:

    The synthesis of alkyl fluorides is best accomplished by

    (a) Free radical fluorination   

    (b) Sandmeyer's reaction

    (c) Finkelstein reaction

    (d) Swarts reaction

    Solution 5:

    Option (a)

    Free radical fluorination is highly explosive reaction therefore it is not preferred for the preferred for the synthesis of alkyl fluorides.

    Option (b)

    Sandmeyer’s reaction is used to synthesize aryl halides from aryl diazonium salts.

    Option (c)

    Finkelstein reaction is used mainly for the preparation of alkyl iodides.

    Option (d)

    The synthesis of alkyl fluoride is best accompanied by Swarts Reaction. Alkyl fluoride is synthesized by heating alkyl bromide and alkyl chloride in the presence of AgF, SbF3, Hg2F2 etc.

    Hence, the correct option is (d).

    Conclusion:

    Students can refer these notes for revision instead of the books to save their precious time. It will provide you all important concepts related to chapter Alkyl Halides in a systematic manner. Students can practice the questions give here to check their preparation. Students can easily understand the mechanism of name reactions like Sandmeyer's reaction, Hunsdiecker reaction, Finkelstein reaction, Swarts reaction.

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