Facebook Twitter Instagram
    ScienceMotive
    • Class 9
      • Matter in Our Surroundings
      • Is Matter Around Us Pure
      • Atoms and Molecules
      • Structure of the Atom
      • The Fundamental Unit of Life
    • Class 10
      • Chemistry
    • Class 11
      • Chemisrty
        • Chapter – 1 Some Basic Concepts of Chemistry
        • Chapter – 2 Structure Of Atom
        • Chapter – 3 Classification of Elements and Periodicity in Properties
        • Chapter – 4 Chemical Bonding and Molecular Structure
        • Chapter – 5 States of Matter
        • Chapter – 6 Thermodynamics
        • Chapter – 7 Equilibrium
        • Chapter – 8 Redox Reaction
        • Chapter – 10 s-Block Elements
        • Chapter – 13 Hydrocarbons
    • Class 12
      • Chemistry
        • The Solid State
        • Solutions
        • Electrochemistry
        • Chemical Kinetics
        • Surface Chemistry
        • p – Block Elements
        • d & f Block Elements
        • Coordination Compounds
        • Haloalkanes and Haloarenes
        • Alcohols, Phenols and Ethers
        • Aldehydes, Ketones and Carboxylic Acids
        • Amines
        • Biomolecules
        • Polymers
        • Chemistry in Everyday Life
    • Practice Questions
      • +1
      • +2
    • Test Series
      • Class 9 Test Series
      • Class 10 Test Series
      • Class 11 Test Series
      • Class 12 Test Series
    • World
      • Current Affairs
      • General Knowledge
    ScienceMotive
    Home » Nucleophilic Substitution and Mechanism of Haloalkanes

    Nucleophilic Substitution and Mechanism of Haloalkanes

    Dr. Vikas JasrotiaBy Dr. Vikas JasrotiaOctober 15, 2020No Comments
    Share
    Facebook WhatsApp Telegram Twitter Email

    Nucleophilic Substitution and Mechanism of Haloalkanes

    The Post Nucleophilic Substitution and Mechanism of Haloalkanes is all about various nucleophilic substitution reactions of haloalkanes along with the followed mechanisms.

    • Chemical Reactions of Haloalkanes:
    • Nucleophilic Substitution Reactions: Due to the presence of a positive charge on the carbon atom it can be easily attacked by nucleophiles to give nucleophilic substitution reactions as well as elimination and reduction reactions.

    1). Reaction with aqueous alkali: Haloalkanes react with aq. NaOH or KOH to form alcohols.
    R-X + KOH(aq) → R-OH + KX
    CH3-CH2-Br + KOH(aq) → CH3-CH2-OH + KBr

     2). Reaction with water: Haloalkanes react with water to form alcohols.
    R-X + H2O → R-OH + HX

    3). Reaction with Sodium alkoxide (R-ONa) [Williamson’s ether synthesis]
    Haloalkanes react with sodium alkoxide to give ethers (R-O-R).
    R-X + R-ONa → R-O-R + KX
    CH3-Br + CH3-CH2-ONa → CH3-O-CH2-CH3 + NaBr

    4). Reaction with Sodium iodide (NaI) [Finkelstein Reaction]: Alkyl halides
    (Chlorides or Bromides) react with sodium alkoxide to form alkyl iodides.
    R-X + NaI   →  R-I + NaX                               [X = Cl or Br]

    5). Reaction with ammonia [Hoffmann’s reaction]: Alkyl halides react with alcoholic ammonia to give a mixture of 10, 20 and 30 amines and quaternary ammonium salt.
    R-X + NH3 → R-NH2 + R2NH + R3N + R4N+X–
    If ammonia is in excess, only primary amine is formed.

     6). Reaction with KCN: Alkyl halides react with alcoholic KCN to give alkane nitriles or alkyl cyanides (R-CN).

    7). Reaction with Silver cyanide (AgCN): Alkyl halides react with AgCN to give alkyl isocyanides or carbyl amines (R-NC).
    R-X + AgCN → R-NC + AgX
     CN– is an ambident nucleophile. i.e. here both C and N contain lone pair of electrons and can bind to the carbon atom of the alkyl group either through C or through N. Another e.g. is NO2–
    Reaction with KCN gives alkyl cyanides. This is because KCN is mainly ionic and gives CN– ions in solution. So, both C and N are free to donate electron pairs. But the C – C bond is stronger than the C – N bond. So, cyanides are formed as the major product. But AgCN is mainly covalent and the only N is free to donate an electron pair. So, isocyanides are the main product.

    8). Reaction with Silver nitrite (AgNO2): Alkyl halides react with AgNO2 to give nitroalkane (R-NO2)
    R-X + AgNO2    → R-NO2 + AgX

    9). Reduction: Alkyl halides when reduced with lithium aluminium hydride (LiAlH4) to give alkane.
    R-X + [H]  → R-H + HX

    • Mechanism of Nucleophilic Substitution Reactions: There are two types of mechanisms:
    • Bimolecular Nucleophilic Substitution (SN2)
    • Unimolecular Nucleophilic Substitution (SN1)

     Bimolecular Nucleophilic Substitution (SN2):
    Characteristics of SN2:
    i). The rate of the reaction depends on the concentration of alkyl halide and on the concentration of the nucleophile.
    CH3 -CI + OH –  →  CH3 -OH + CI
    Thus Rate = K [CH3-CI][OH–]
    In this reaction, two species CH3-CI and OH– are involved in the rate-determining step hence reaction is bimolecular reaction.

    ii). It takes place in one step. They are Concerted Reactions i.e. bond breaking and bond making takes place in one step. No intermediate is formed.

    iii).  Product formation takes place by formation of transition state                                                                                                                                                                                                                                            

    It takes place in one step. They are Concerted Reactions. bond breaking and bond making takes place in one step. No intermediate is formed.

    Stereochemistry of SN2: The attack of nucleophile occurs from a direction opposite to the one from where the halogen atom leaves. Thus, the inversion of the configuration takes place.

    Bulky groups cause steric hindrance in the reaction and thus reactivity of alkyl halides is,

    Methyl halide > 1° > 2° > 3°

    Unimolecular Nucleophilic Substitution (SN1):
     Step 1. This step is slow and reversible so it is the rate-determining step of the reaction.

    Step 2. In the second step, the carbocation being a reactive chemical species is immediately attacked by the nucleophile i.e., OH¯ ion to give the substitution product, i.e., tertbutyl alcohol. This step is fast and hence does not affect the rate of the reaction.

    Stereochemistry of SN1 reactions:
    In SN1 reactions, if the alkyl halide is optically active, then the product is a racemic mixture, i.e., SN1 reactions are accompanied by racemization. This is due to the reason that carbocations are the intermediates in SN1 reactions. Since carbocations being sp2 hybridized are planar (achiral) species, therefore, the attack of the nucleophile on it can occur from both the faces.

    Advertisement
    Nucleophilic substitution Reactions of Haloalkanes Sn1 and Sn2 Mechanisms
    Share. Facebook Twitter Pinterest LinkedIn Tumblr Email
    Dr. Vikas Jasrotia
    • Website

    Related Posts

    CBSE Sample Paper Session 2022-23 (Chemistry) PDF

    September 16, 2022

    Haloalkanes and Haloarenes Class 12 – Questions & Answers

    August 2, 2022

    CBSE Important Questions Electrochemistry

    March 16, 2022

    Leave A Reply Cancel Reply

    READ ALSO

    Periodic Classification of Elements Questions PDF (Answers)

    March 1, 2023

    Periodic Classification of Elements Questions

    February 28, 2023

    Carbon and Its Compounds Class 10 Solutions of Practice Questions

    February 22, 2023

    Carbon and Its Compounds Class 10 Practice Questions

    February 21, 2023
    Class 10 Test Series

    Periodic Classification of Elements Questions PDF (Answers)

    By Dr. Vikas JasrotiaMarch 1, 2023

    Periodic Classification of Elements Questions PDF (Answers) Periodic Classification of Elements Questions PDF Ans 1.…

    Class 10 Test Series

    Periodic Classification of Elements Questions

    By Dr. Vikas JasrotiaFebruary 28, 2023

    Periodic Classification of Elements Questions Periodic Classification of Elements Questions Que 1. What is the…

    Class 10 Test Series

    Carbon and Its Compounds Class 10 Solutions of Practice Questions

    By Dr. Vikas JasrotiaFebruary 22, 2023

    Carbon and Its Compounds Class 10 Solutions of Practice Questions Carbon and Its Compounds Class…

    Advertisement
    Advertisement
    Facebook Twitter Instagram Pinterest YouTube
    • Disclaimer
    • Contact Us
    • Privacy Policy 
    • Terms and Conditions
    © 2023 All Rights Reserved ScienceMotive.

    Type above and press Enter to search. Press Esc to cancel.