Chemistry (9701)
Topic 11 of 20Cambridge A Levels

Halogenoalkanes

Alkanes with halogen atoms, undergoing nucleophilic substitution and elimination reactions.

### Introduction to Halogenoalkanes


Halogenoalkanes, also known as haloalkanes or alkyl halides, are organic compounds in which one or more hydrogen atoms in an alkane have been replaced by a halogen atom (F, Cl, Br, or I). Their general formula is CnH2n+1X. The key to their reactivity is the polar C-X bond. Because halogens are more electronegative than carbon, the carbon atom becomes electron-deficient (carrying a partial positive charge, δ+) and the halogen atom becomes electron-rich (carrying a partial negative charge, δ−). This polarity makes the carbon atom susceptible to attack by electron-rich species.


Halogenoalkanes are classified based on the number of alkyl groups attached to the carbon atom bonded to the halogen:

  • Primary (1°): The C-X carbon is bonded to one other alkyl group (e.g., 1-chloropropane).
  • Secondary (2°): The C-X carbon is bonded to two other alkyl groups (e.g., 2-chloropropane).
  • Tertiary (3°): The C-X carbon is bonded to three other alkyl groups (e.g., 2-chloro-2-methylpropane).

    • ### Nucleophilic Substitution Reactions


    A nucleophile is a species (an ion or molecule) with a lone pair of electrons that it can donate to form a covalent bond. It is attracted to electron-deficient centres, like the Cδ+ in a halogenoalkane. In a nucleophilic substitution reaction, the nucleophile replaces the halogen atom.


    The general equation is: R-X + Nu⁻ → R-Nu + X⁻


    There are two main mechanisms for this process:


  • SN2 Mechanism (Substitution, Nucleophilic, Bimolecular)

    • This mechanism is typical for primary halogenoalkanes. It occurs in a single step. The nucleophile attacks the Cδ+ atom from the side opposite to the leaving group (the halogen). A transition state is formed where the nucleophile is partially bonded to the carbon, and the C-X bond is partially broken. The rate of this reaction depends on the concentration of both the halogenoalkane and the nucleophile: Rate = k[R-X][Nu⁻]. This mechanism results in an inversion of configuration at the carbon centre, similar to an umbrella turning inside out.


  • SN1 Mechanism (Substitution, Nucleophilic, Unimolecular)

    • This mechanism is characteristic of tertiary halogenoalkanes. It is a two-step process.

  • Step 1 (slow, rate-determining): The C-X bond breaks by **heterolytic fission** to form a stable **tertiary carbocation** and a halide ion (X⁻).
  • Step 2 (fast): The nucleophile attacks the planar carbocation. This attack can occur from either side with equal probability.

    • The rate depends only on the concentration of the halogenoalkane, as the first step is the slowest: Rate = k[R-X]. If the original halogenoalkane is chiral, the product is a racemic mixture (an equal mixture of two enantiomers) because the nucleophile can attack the planar carbocation from either face.


    Common Nucleophilic Substitution Reactions:

  • Hydrolysis: Formation of an alcohol using aqueous sodium or potassium hydroxide (**NaOH(aq)** or **KOH(aq)**). Conditions: **Warm**.

    • `CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻`

  • Reaction with Cyanide: Formation of a nitrile using potassium cyanide in ethanol (**KCN in ethanol**). Conditions: **Warm**. This reaction is important for **extending the carbon chain**.

    • `CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻`

  • Reaction with Ammonia: Formation of an amine using excess concentrated ammonia in ethanol (**NH₃ in ethanol**). Conditions: **Heat in a sealed tube** (to prevent ammonia gas from escaping).

    • `CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄⁺Br⁻`


    ### Elimination Reactions


    Halogenoalkanes also undergo elimination reactions to form alkenes. In this process, a hydrogen halide (H-X) is removed from adjacent carbon atoms. This reaction is favoured by using a hot, concentrated, ethanolic solution of a strong base, such as potassium hydroxide (KOH in ethanol). Here, the hydroxide ion (or more accurately, the ethoxide ion C₂H₅O⁻ formed from ethanol) acts as a base, removing a proton (H⁺), rather than as a nucleophile.


    `CH₃CH(Br)CH₃ + KOH(ethanolic) → CH₃CH=CH₂ + KBr + H₂O`


    ### Competition between Substitution and Elimination


    For many halogenoalkanes (especially secondary and tertiary), substitution and elimination are competing reactions. The outcome is determined by:

  • Structure: Primary halogenoalkanes favour SN2 substitution. Tertiary halogenoalkanes strongly favour elimination and SN1 substitution.
  • Reagent: A strong base favours elimination. A good nucleophile that is a weak base favours substitution.
  • Solvent: **Aqueous** (polar) solutions favour **substitution**. **Ethanolic** (less polar) solutions favour **elimination**.
  • Temperature: **Higher temperatures** favour **elimination** as it has a higher activation energy.

    • ### Reactivity of Halogenoalkanes


    The reactivity of halogenoalkanes depends on the strength of the C-X bond. Bond enthalpy decreases down Group 17: C-F > C-Cl > C-Br > C-I. The C-I bond is the weakest and therefore the easiest to break. Consequently, the reactivity increases down the group:


    R-F < R-Cl < R-Br < R-I


    Iodoalkanes are the most reactive, and fluoroalkanes are the least reactive. This factor is more significant than the polarity of the C-X bond.

    Key Points to Remember

    • 1Halogenoalkanes contain a polar C-X bond, making the carbon atom electron-deficient and a target for nucleophiles.
    • 2They undergo nucleophilic substitution via two mechanisms: SN2 (primary, one-step, bimolecular rate) and SN1 (tertiary, two-step, unimolecular rate).
    • 3Key substitution reagents include NaOH(aq) to form alcohols, KCN(ethanolic) to form nitriles (carbon chain extension), and NH₃(ethanolic) to form amines.
    • 4They undergo elimination reactions to form alkenes, favoured by a hot, concentrated, ethanolic solution of a strong base (e.g., KOH).
    • 5The outcome between substitution and elimination is influenced by the halogenoalkane's structure, reagent, solvent (aqueous vs. ethanolic), and temperature (high T favours elimination).
    • 6Reactivity increases down the halogen group (R-I > R-Br > R-Cl) due to the decreasing C-X bond enthalpy.
    • 7The SN1 mechanism proceeds via a stable carbocation intermediate and can result in a racemic mixture.
    • 8The SN2 mechanism involves a transition state and results in an inversion of stereochemical configuration.

    Pakistan Example

    CFCs and Refrigerant Smuggling in Pakistan

    Chlorofluorocarbons (CFCs), such as Freon-12 (CCl₂F₂), are a class of halogenoalkanes. Due to the very high bond enthalpy of the C-F and C-Cl bonds, they are chemically inert, non-toxic, and volatile, making them ideal refrigerants. However, their stability means they persist in the atmosphere and cause ozone depletion. Pakistan, as a signatory to the Montreal Protocol, has phased out CFC use. This has led to a black market for smuggled, cheaper CFC refrigerants to service older air conditioning units, particularly in cities like Karachi and Lahore. This scenario illustrates a real-world application where the low reactivity of certain halogenoalkanes is a desirable property for technology, but a significant problem for the environment.

    Quick Revision Infographic

    Chemistry — Quick Revision

    Halogenoalkanes

    Key Concepts

    1Halogenoalkanes contain a polar C-X bond, making the carbon atom electron-deficient and a target for nucleophiles.
    2They undergo nucleophilic substitution via two mechanisms: SN2 (primary, one-step, bimolecular rate) and SN1 (tertiary, two-step, unimolecular rate).
    3Key substitution reagents include NaOH(aq) to form alcohols, KCN(ethanolic) to form nitriles (carbon chain extension), and NH₃(ethanolic) to form amines.
    4They undergo elimination reactions to form alkenes, favoured by a hot, concentrated, ethanolic solution of a strong base (e.g., KOH).
    5The outcome between substitution and elimination is influenced by the halogenoalkane's structure, reagent, solvent (aqueous vs. ethanolic), and temperature (high T favours elimination).
    6Reactivity increases down the halogen group (R-I > R-Br > R-Cl) due to the decreasing C-X bond enthalpy.
    Pakistan Example

    CFCs and Refrigerant Smuggling in Pakistan

    Chlorofluorocarbons (CFCs), such as Freon-12 (CCl₂F₂), are a class of halogenoalkanes. Due to the very high bond enthalpy of the C-F and C-Cl bonds, they are chemically inert, non-toxic, and volatile, making them ideal refrigerants. However, their stability means they persist in the atmosphere and cause ozone depletion. Pakistan, as a signatory to the Montreal Protocol, has phased out CFC use. This has led to a black market for smuggled, cheaper CFC refrigerants to service older air conditioning units, particularly in cities like Karachi and Lahore. This scenario illustrates a real-world application where the low reactivity of certain halogenoalkanes is a desirable property for technology, but a significant problem for the environment.

    SeekhoAsaan.com — Free RevisionHalogenoalkanes Infographic

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