Halogenoalkanes: properties, reactions, mechanisms

Halogenoalkanes: Properties, Reactions & Mechanisms

Objective: Understand the key properties of halogenoalkanes, the major substitution and elimination reactions they undergo, and the underlying mechanisms. Use analogies to make the concepts memorable.

1. Properties of Halogenoalkanes

  • General formula: R–X where X = F, Cl, Br, I.
  • Polarity: The C–X bond is polar (δ⁺ on C, δ⁻ on X) → makes them good electrophiles.
  • Reactivity trend: I > Br > Cl > F (due to bond strength & polarizability).
  • Solubility: Often soluble in organic solvents, sparingly soluble in water.
  • Analogy: Think of the halogen as a “hand‑off” point where a new partner (nucleophile or base) can grab the carbon.

2. Major Reactions

2.1 Substitution Reactions

Two main types: SN1 (unimolecular) and SN2 (bimolecular).

  1. SN2 – “Back‑side attack” in one step. Key features:
    • Occurs with primary or secondary halogenoalkanes.
    • Base or nucleophile attacks from the side opposite the leaving group.
    • Inversion of configuration (Walden inversion).
    • Rate ∝ [Nu]·[Halogenoalkane].
  2. SN1 – “Carbocation intermediate” in two steps. Key features:
    • Favoured by tertiary halogenoalkanes.
    • First, C–X bond breaks → formation of a stable carbocation.
    • Second, nucleophile attacks the carbocation (often leading to racemization).
    • Rate ∝ [Halogenoalkane] (independent of nucleophile concentration).

2.2 Elimination Reactions

Two main types: E2 (bimolecular) and E1 (unimolecular).

  1. E2 – “Concerted” removal of H and X in one step.
    • Requires a strong base.
    • Occurs with primary or secondary halogenoalkanes.
    • Produces an alkene with anti‑syn (E2) geometry.
  2. E1 – “Carbocation intermediate” similar to SN1.
    • Favoured by tertiary halogenoalkanes.
    • First, C–X bond breaks → carbocation.
    • Second, base removes a β‑hydrogen → alkene.

3. Mechanistic Details

3.1 SN2 Mechanism (Step‑by‑Step)

  1. Base/Nucleophile approaches the electrophilic carbon from the side opposite the leaving group.
  2. Simultaneous bond formation (Nu–C) and bond breaking (C–X).
  3. Transition state: pentavalent carbon, partial bonds.
  4. Product: Inverted configuration.

3.2 SN1 Mechanism (Step‑by‑Step)

  1. Leaving group departs, forming a stable carbocation.
  2. Carbocation is attacked by the nucleophile from either side.
  3. Result: mixture of stereoisomers (racemization).

3.3 E2 Mechanism (Step‑by‑Step)

  1. Base abstracts a β‑hydrogen while the leaving group departs.
  2. Simultaneous formation of C=C double bond.
  3. Transition state: anti‑syn geometry (E2).

3.4 E1 Mechanism (Step‑by‑Step)

  1. Leaving group departs → carbocation.
  2. Base removes β‑hydrogen → alkene.
  3. Possible rearrangements of the carbocation before elimination.

4. Reaction Summary Table

Reaction Type Key Conditions Typical Substrate Outcome
SN2 Strong nucleophile, polar aprotic solvent Primary / Secondary Inverted product
SN1 Weak nucleophile, polar protic solvent Tertiary Racemized product
E2 Strong base, polar aprotic solvent Primary / Secondary Alkene (E2)
E1 Weak base, polar protic solvent Tertiary Alkene (E1)

5. Exam Tips (💡)

Remember:

  • Identify the substrate type (primary, secondary, tertiary) to decide between SN2/E2 or SN1/E1.
  • Check the solvent: polar aprotic → SN2/E2; polar protic → SN1/E1.
  • Look for the leaving group quality: I > Br > Cl > F.
  • For mechanisms, sketch the transition state: show bond breaking/ forming arrows.
  • Use the mnemonic “S N 1 & 2, E 1 & 2” to recall the reaction types.

6. Quick Quiz (🧪)

  1. Which reaction is favoured for a tertiary halogenoalkane in a polar protic solvent?
  2. What is the stereochemical outcome of an SN2 reaction?
  3. Why does a strong base favour an E2 mechanism over an SN2?

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