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JEE Main 2018: Revision notes with important questions for Free radicals and SN1 & SN2 mechanisms

Apr 6, 2018 18:21 IST
    JEE Main chemistry exam 2018
    JEE Main chemistry exam 2018

    This is the time when all the JEE Main aspirants are searching the guess questions, important topics, important analysis about how the paper of jee main 2018 could be?

    In our previous article and video we have done an analysis regarding the most important Name Reactions asked in the previous year cbse board exam and jee main examinations. In the analysis we have found that SN1 & SN2 are the type of reaction which has been asked several times in both the examinations.

    In this article we are going to talk about the SN1 & SN2 Reactions, conditions of free radical mechanism and few important questions with solution from both the topics.  

    Let’s talk about Nucleophilic substitution reaction.

    Nucleophilic substitution reactions occur when an electron rich species, the nucleophile, reacts at an electrophilic saturated C atom attached to an electronegative group (important), the leaving group that can be displaced as shown by the general scheme:

    nucleophile is the electron rich species that will react with an electron poor species and a substitution implies that one group replaces another.

    Nucleophilic substitution reaction occur with two mechanism that is SN1 & SN2

    The detail about the reaction mechanism and how the complete reaction takes place, you have already studied and this not the time to give you the information in such detail way but the core points.

    Let’s understand the basic differences between SN1 & SN2 Reactions.

    Factor

    SN1

    SN2

    Steps

    2

    1

    Rate

    1st order

    2nd order

    Molecularity

    Unimolecular

    Bimolecular

    Stereochemistry

    Inversion and racemization

    Walden Inversion (backside attack)

    Reactivity

    3o> 2o> 1o> CH3

    CH3> 1o> 2o> 3o

    Solvent nature and its effect on Rate

    Rate increases in polar solvent

    With Nu- there is a large rate increase in polar aprotic solvents.

    Catalysis

    Lewis acid, eg. Ag+, AlCl3, ZnCl2

    None

    Effect of nucleophile

    No effect as it does not appear in the rate expression.

    Rate depends on nucleophilicity

    I-  > Br-  > Cl- ;  RS-  > RO-

    Competitive reaction

    Elimination, rearrangement

    Elimination

    Intermediate 

    Carbonium ion

    There is no intermediate but it forms transition state

    Strength and concentration of nucleophile required

    Week nucleophile, low concentration

    Strong nucleophile, high concentration





    Free radicals: A free radical can be defined as any molecular species capable of independent existence that contains an unpaired electron in an atomic orbital.

    V.V. Important: The important points regarding the free radicals are:

    1. Central carbon contains 3 bond pairs
    2. One unpaired electron means total 7 electrons in the valence shell
    3. Free radicals are paramagnetic and coloured due to presence of unpaired electron
    4. Free radicals are neutral electrophile and lewis acids
    5. Chiral free radical forms racemic mixture as product
    6. Stability of free radical is directly proportional to resonance effect and Hyperconjugation
    7. Inductive effect does not involve for the prediction of stability of free radical because inductive effect is a polarisation effect while free radicals are neutral species

    Conditions for free radical mechanism:

    1. Presence of organic peroxide
    2. Presence of non-polar solvent
    3. Presence of diffused sun light that is ultra violet light 
    4. High temperature in the range of 500 degree Celsius or above
    5. Gaseous state of reactant and reagents

    Mechanism of free radical reaction involves three steps:

    I.  Chain initiatio

    II.  Chain propagation

    III.  Chain termination

    The common examples where free radical mechanism is involved are as follows

    • Halogenation
    • Nitration
    • Sulphonation of alkane

     Note :

    • Allylic bromination or allylic chlorination with NBS (N-Bromosuccinimide)of alkene follows free radical mechanism
    • Alkane undergoes free radical substitution
    • Alkene undergoes electrophilic addition
    • Alkyl halide undergoes nucleophilic substitution and elimination
    • Carbonyl compounds undergo nucleophilic addition and nucleophilic addition- elimination reaction

    Here we have mention the important points from the topic SN1 & SN2 Reactions and Free radical mechanism. The main purpose to mention the key points are to make you aware that these points can be asked in the form of direct or indirect question in the jee main paper

    Below are the 4 solved questions from the topic SN1 & SN2 mechanism which have been asked in previous years’ JEE Main exams:

    Q 1. The increasing order of the reactivity of the following halides for the SN1 reaction is:

     (a) (I) < (III) < (II)     

    (b) (II) < (III) < (I)

    (c) (III) < (II) <  (I)

    (d) (II) < (I) <  (III)

     

                                                                                                     (JEE Main 2017)

    Correct Option: (d)

    Sol.

    Higher the stability of the carbocation, higher would be the reactivity of the SN1 reaction.

    Stability of carbocations is as follows:

    Benzyl carbocation > 2o carbocation>1o carbocation.

    So, the reactivity of the given compounds for SN1 reaction is as follows:

    II < I < III

    Hence, the correct option is (d).

    Q 2.  In SN2 reactions, the correct order of reactivity for the following compounds CH3Cl, CH3CH2Cl , (CH3)2 CHCl and (CH3)3CCl is                                                                                           

    (a) CH3Cl > (CH3)2CHCl > CH3CH2Cl > (CH3) 3CCl

    (b) CH3CI > CH3CH2 CI > (CH3)2 CHCl > (CH3)3CCl

    (c) CH3CH2Cl > CH3Cl > (CH3)2CHCl > (CH3) 3CCl

    (d) (CH3)2CHCI > CH3CH2Cl > CH3Cl > (CH3)3CCl

                                                                                                       (JEE Main 2014)

    Correct Option: (b)

    Sol.

    In SN2 reactions, the order of reactivity are followed as:

    Primary halide > Secondary halide > Tertiary halide

    So, the order of reactivity of the given compound are followed as:

    CH3CI > CH3CH2 CI > (CH3)2 CHCl > (CH3)3CCl

    Hence the correct option is (b)

     

    Q 3.

    An unknown alcohol is treated with the "Lucas reagent" to determine whether the alcohol is primary , secondary or tertiary . Which alcohol reacts fastest and by what mechanism?

    (a) Secondary alcohol by SN 1

    (b) Tertiary alcohol by SN 1

    (c) Secondary alcohol by SN 2

    (d) Tertiary alcohol by SN 2

                                                                                                                                  (JEE Main 2013)

    Correct Option: (b)

    Sol.

    The reaction of alcohol with Lucas reagent is mostly an  reaction and the rate of reaction is directly proportional to the stability of carbocation formed in the reaction. Since 3° R-OH forms 3° carbocation (most stable), hence it will react fastest.

     

    Q 4. Which of the following haloalkanes would undergo SN2 reaction faster?

    Correct Option: (b)

    Sol.

     

    In SN2 reactions, the order of reactivity is followed as:

    Primary halide > Secondary halide > Tertiary halide

    In Option (a) and option (b), the compound in option (a) will undergo SN2 reaction faster because the compound in option (a) is primary halide and the compound in option (b) is secondary halide.

    Since iodine is better leaving group because of the large size so, the compound in option (c) will undergo SN2 reaction faster.

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