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    Physical & Chemical Properties of Haloalkanes - Practice Questions & MCQ

    Edited By admin | Updated on Sep 25, 2023 25:24 PM | #NEET

    Quick Facts

    • Strong and Weak bases, SN2 Reaction, SN1 Reaction, Nucleophilic Substitution are considered the most difficult concepts.

    • 76 Questions around this concept.

    Solve by difficulty

    The major product obtained in the following reaction is :

    Which of the following reactions is an example of nucleophilic substitution reaction?

     In a face centered cubic lattice atoms A are at the corner points and atoms B at the face centered points. If atom B is missing from one of the face centered points, the formula of the ionic compound is :

    Tertiary alkyl halides are practically inert to substitution by \mathrm{SN_2} mechanism because of

    The organic chloro compound, which shows complete stereochemical inversion during a \mathrm{SN_2} reaction, is

    In an SN 1 reaction on chiral centres, there is :

    Which branched chain isomer of the hydrocarbon with molecular mass 72u gives only one isomer of mono substituted alky halide ?

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    Which one is most reactive towards SN1 reaction?

    In which of the following compounds. the C-Cl bond ionisation shall give most stable carbonium ion?

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    Which one of the following bases is not present in DNA ?

    Concepts Covered - 5

    Strong and Weak bases

    Most common bases that we have use are alc.KOH and aqueous NaOH. In alc.KOH, we have EtO-(CH3-CH2-O-) as the base while in aqueous NaOH, the base is OH-. The strength of these two bases are given below:

    CH3-CH2-O- > OH-
    Because CH3-CH2- is the releasing group and hence it unstabilize the O-, thus it reacts faster and hence it is stronger base. The reactions occurs as follows:

    For alc.KOH:
    \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{KOH}(\mathrm{alc}) \longrightarrow \mathrm{H}_{2} \mathrm{C}=\mathrm{CH}_{2}+\mathrm{KBr}+\mathrm{H}_{2} \mathrm{O}

    For aq.NaOH:

    SN2 Reaction
    • It is the substituion nucelophilic bimolecular(SN2) reaction. 
    • Rate \alpha [RX][Z-]
    • The general reaction occurs as follows:
      \mathrm{R-CH_{2}-Cl\: +\: OH^{-}\: \rightarrow \: R-CH_{2}-OH}

      Mechanism
    • Rate determinig step depends on how fast the transition state is formed, which depends on how many collisions occur between the reactants and that depends on the concentration of reactants.
    SN1 Reaction
    • SN1 reactions are nucleophilic substitution reactions, involving a nucleophile replacing a leaving group. However, SN1 reactions are unimolecular. The rate of this reaction depends only on the concentration of one reactant.
    • The mechanism occurs as follows:
    • If Z- attacks on the same side from where X- leaves, then it is called 'Retention'.
    • If Z- attacks from the opposite side from where X- leaves, then it is called 'Inversion'.
    • The rate of formation of intermediate is independent of concentration of nucleophile and depends only on the concentration of reactants.

    NOTE: Since, we have retention product and ionversion product both, therefore 'Partial racemisation' can occur.

    Elimination-Addition Mechanism(I)

    Very stong base such as sodium or potassium amide react with aryl halide, even those without electron withdrawing substituents to give products corresponding to nucleophilic substitution of halide by the base.

    Mechanism

    1. Elimination stage: Amide ion is a very strong base and brings about the dehydrohalogenation of chlorobenzene by abstracting a proton from the carbon adjacent to the one that bears the leaving group. The product of this step is an unstable intermediate called benzyne.
    2. Beginning of addition phase: Amide ion acts as a nucleophile and adds to one of the carbons of the triple bond. The product of this step is a carbanion.
    3. Completion of addition phase: The aryl anion abstracts a proton from the ammonia used as the solvent in the reaction.
    Nucleophilic Substitution

    Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons:

    • Resonance effect: In haloarenes, the electron pairs on halogen atom are in conjugation with π-electrons of the ring and the following resonating structures are possible.

      C—Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.
    • Difference in hybridisation of carbon atom in C—X bond: In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of haloarene, the carbon atom attached to halogen is sp2-hybridised. The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp3 -hybridised carbon in haloalkane with less s-chararcter. Thus, C—Cl bond length in haloalkane is 177pm while in haloarene is 169 pm. Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.
    • Instability of phenyl cation: In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance and therefore, SN1 mechanism is ruled out.
    • Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes.

    Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of 300 atmospheres.


    The presence of an electron withdrawing group (-NO2 ) at ortho- and para-positions increases the reactivity of haloarenes.

    Study it with Videos

    Strong and Weak bases
    SN2 Reaction
    SN1 Reaction

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    Books

    Reference Books

    SN1 Reaction

    Chemistry Part II Textbook for Class XI

    Page No. : 294

    Line : 24

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