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🧪 Syllabus Section 15: Halogenoalkanes (9701)

Tags: A-Level-Chemistry Organic Halogenoalkanes Mechanisms

Examiner's Golden Rule

The solvent determines the reaction! Aqueous conditions favor Substitution (forming alcohols). Ethanolic (anhydrous) conditions favor Elimination (forming alkenes). Mixing these up is the #1 reason students lose marks in Paper 2.

1. Classification of Halogenoalkanes

Depending on the carbon atom the halogen is attached to, halogenoalkanes are classified into three categories. This classification dictates the mechanism ( or ) the molecule will undergo.

  • Primary (1°): The halogen is attached to a carbon atom that is bonded to only one other alkyl group. (Undergoes ).
  • Secondary (2°): The halogen is attached to a carbon atom bonded to two other alkyl groups. (Undergoes a mixture of and ).
  • Tertiary (3°): The halogen is attached to a carbon atom bonded to three other alkyl groups. (Undergoes ).

2. Producing Halogenoalkanes (Synthesis)

From Alkanes (Free-Radical Substitution)

  • Reagents: Halogen (e.g., or ).
  • Conditions: Ultraviolet (UV) light (provides energy to break the halogen bond).
  • Mechanism Steps:
    1. Initiation: UV light causes homolytic fission of the halogen bond, producing two free radicals.
    2. Propagation: Chain reaction where radicals react with alkanes, producing new radicals.
    3. Termination: Two free radicals combine to form a stable molecule.
  • Examiner Note: This method is poor for synthesizing a specific halogenoalkane because it produces a mixture of multiply-substituted products.

From Alkenes (Electrophilic Addition)

  • Reagents: Hydrogen halide () or Halogen ().
  • Conditions: Room temperature.
  • Rule for Asymmetrical Alkenes: Apply Markovnikov’s Rule. The hydrogen (electrophile) adds to the carbon with the most hydrogens, forming the most stable carbocation intermediate. The major product forms from this most stable carbocation.

From Alcohols (Nucleophilic Substitution)

The hydroxyl (-OH) group is replaced by a halogen.

  • Using : React alcohol with (or a mixture like with concentrated or to make in situ). Heat is required.
  • Using : React alcohol with . Requires Heat. By-product is .
  • Using : React alcohol with . Occurs at Room Temperature. By-products are + .
  • Using : React alcohol with . Occurs at Room Temperature. By-products are + . (Often preferred as by-products are gases and escape).

3. Nucleophilic Substitution Reactions

Why do they react?

The C-X bond is polar due to the higher electronegativity of the halogen. The carbon carries a partial positive charge () and the halogen a partial negative charge (). A nucleophile (an electron pair donor) attacks the carbon.

Reaction with Aqueous Alkali ( / )

  • Reagents: Aqueous Sodium or Potassium Hydroxide.
  • Conditions: Warm / Heat under reflux.
  • Solvent: Aqueous (Water).
  • Product: Alcohol.
  • Mechanism: The aqueous ion acts as the nucleophile, replacing the halogen.

Reaction with Cyanide Ions ( / )

  • Reagents: Potassium or Sodium Cyanide.
  • Conditions: Heat under reflux.
  • Solvent: Ethanolic (dissolved in ethanol).
  • Product: Nitrile.
  • Examiner Note (Crucial): This reaction adds an extra carbon atom to the carbon chain. It is used by chemists to step up a carbon chain. Remember, the lone pair is on the Carbon atom of the cyanide ion ().

Reaction with Ammonia ()

  • Reagents: Ammonia gas.
  • Conditions: Heat under pressure in a sealed tube.
  • Solvent: Ethanolic (dissolved in ethanol).
  • Product: Primary Amine + .
  • Examiner Note: You MUST state that Ammonia is in excess. If ammonia is not in excess, the primary amine product (which is also a nucleophile) will attack another halogenoalkane molecule, forming unwanted secondary and tertiary amines.

Reaction with Water (Hydrolysis / Aqueous )

  • Reagents: Water (often using aqueous as the source of water and to test for the halogen).
  • Conditions: Heat under reflux / Warm.
  • Product: Alcohol + + (which precipitates with ).
  • Comparing Nucleophiles (Exam Trap): Hydrolysis with water is much slower than with aqueous . Why? The hydroxide ion () is a better nucleophile because it carries a full formal negative charge, whereas the oxygen atom in water only carries a partial negative charge ().

4. Elimination Reactions

An elimination reaction involves the loss of a small molecule (hydrogen halide) from the halogenoalkane to form a double bond.

  • Reagents: or .
  • Conditions: Hot (Heat under reflux).
  • Solvent: Ethanolic (Anhydrous).
  • Mechanism:
    1. The acts as a base (not a nucleophile), removing a hydrogen atom from a carbon adjacent to the C-X bond.
    2. The C-X bond breaks heterolytically, releasing the halide ion ().
    3. A C=C double bond forms.
  • Product: Alkene + Water + Halide salt.
  • Examiner Trap: If the halogen is on a non-terminal carbon (e.g., 2-bromobutane), elimination can happen on either adjacent carbon. This produces a mixture of isomeric alkenes (e.g., but-1-ene and but-2-ene).

5. Reaction Mechanisms: and

Mechanism (Primary Halogenoalkanes)

  • Name: Substitution, Nucleophilic, Bimolecular (Rate depends on the concentration of both the halogenoalkane and the nucleophile).
  • Steps: A one-step reaction.
  • Process: The nucleophile donates a pair of electrons to the carbon while the C-X bond simultaneously breaks (heterolytic fission).
  • Transition State: You must draw the transition state showing partial bonds (dashed lines) to both the nucleophile and the leaving group, enclosed in square brackets with a negative charge [ ]⁻.

Mechanism (Tertiary Halogenoalkanes)

  • Name: Substitution, Nucleophilic, Unimolecular (Rate depends only on the concentration of the halogenoalkane).
  • Steps: A two-step reaction.
  • Process:
    1. Slow (rate-determining) step: The C-X bond breaks heterolytically, forming a tertiary carbocation intermediate and a halide ion.
    2. Fast step: The nucleophile attacks the positive carbocation.

Carbocation Stability (Explaining vs )

  • Exam Question: “Explain why tertiary halogenoalkanes proceed via .”
  • Answer: Alkyl groups have a positive inductive effect (they push electron density away from themselves). In a tertiary carbocation, three alkyl groups push electron density towards the positively charged carbon, reducing its charge density. This makes tertiary carbocations much more stable than primary carbocations.

Drawing the Mechanisms (Where marks are lost)

  • Include all partial charges ( on C, on Halogen).
  • The curly arrow MUST start exactly from the lone pair of the nucleophile and point directly to the carbon atom.
  • The curly arrow representing bond breaking MUST start from the middle of the C-X bond and point directly to the Halogen atom.

Common Misconception

Students often assume C-F is the most reactive because it is the most polar bond. This is completely wrong. Reactivity is dictated by Bond Enthalpy (Bond Energy), not polarity.

  • The Trend: Halogenoalkanes become more reactive as you move down the group (Fluoro < Chloro < Bromo < Iodo).
  • The Explanation: As halogen atoms get larger down the group, the C-X bond gets longer. Therefore, the bond enthalpy decreases (the bond gets weaker).
    • C-F bond: Requires the most energy to break (); strongest bond. Fluoroalkanes are very unreactive.
    • C-I bond: Requires the least energy to break (); weakest bond. Iodoalkanes are the most reactive.

Aqueous Silver Nitrate () Test

Used to determine the relative rate of hydrolysis (nucleophilic substitution) of halogenoalkanes.

  • Fluoroalkanes: No precipitate (bond too strong).
  • Chloroalkanes: Forms a White precipitate () slowly.
  • Bromoalkanes: Forms a Cream precipitate ().
  • Iodoalkanes: Forms a Yellow precipitate () fastest.

7. Past Paper Specifics & MCQ Traps to Watch Out For

  • Optical Isomerism in Mechanisms (): Because the carbocation intermediate in an mechanism is planar, the nucleophile can attack equally from above or below the plane. If the resulting product has a chiral center, this will produce a racemic mixture (a 50/50 mix of enantiomers) which is optically inactive.
  • Reagents vs Conditions: Mark schemes are brutal here. If asked for “reagent and conditions”, write both distinctly. Example: Reagent = , Conditions = aqueous and heat. Writing just “aqueous NaOH” only scores the reagent mark.
  • Homolytic vs Heterolytic (MCQs): Remember that homolytic fission creates free radicals (no charge), while heterolytic fission creates ions (used in and ).
  • Reading Skeletal Structures: In Paper 1, you must rapidly identify 1°, 2°, and 3° halogenoalkanes from skeletal formulae. If the carbon bonded to the halogen is at a vertex with three other lines connecting to carbons, it is tertiary.

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