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*Revision Materials and Past Papers* 1 Atomic Structure 2 Amounts of Substance 3 Bonding 4 Energetics 5 Kinetics 6 Chemical Equilibria & Kc 7 Redox Equations 8 Thermodynamics 9 Rate Equations 10 Kp (Equilibrium Constant) 11 Electrode Potentials & Cells 12 Acids and Bases 13 Periodicity 14 Group 2: Alkaline Earth Metals 15 Group 7: The Halogens 16 Period 3 Elements & Oxides 17 Transition Metals 18 Reactions of Ions in Aqueous Solution 19 Intro to Organic Chemistry 20 Alkanes 21 Halogenoalkanes 22 Alkenes 23 Alcohols 24 Organic Analysis 25 Optical Isomerism 26 Aldehydes & Ketones 27 Carboxylic Acids & Derivatives 28 Aromatic Chemistry 29 Amines 30 Polymers 31 Amino Acids, Proteins & DNA 32 Organic Synthesis 33 NMR Spectroscopy 34 Chromatography RP1–RP12 Required Practicals

3.3 Halogenoalkanes

3.3.1 Nucleophilic Substitution 3.3.2 Elimination 3.3.3 Ozone Depletion

Elimination Reactions

Specification Reference Organic chemistry, Halogenoalkanes 3.3.3.2

Quick Notes

  • Halogenoalkanes can undergo both substitution and elimination reactions with KOH. AQA A-Level Chemistry overview comparing substitution to give alcohols versus elimination to give alkenes from halogenoalkanes with KOH
    • Substitution: KOH acts as a nucleophile (electron pair donor), replacing the halogen with OH−.
    • Elimination: KOH acts as a base (H+ ion acceptor), removing a hydrogen atom to form an alkene.
  • Reaction conditions determine the dominant reaction:
    • Aqueous KOH (warm, reflux) → Substitution
    • Ethanolic KOH (hot, reflux) → Elimination

Full Notes

Role of the Reagent (KOH) in Different Reactions

Hydroxide (OH−) ions can act as both a nucleophile (electron pair donor) or a base (H+ ion acceptor) when reacting with halogenoalkanes, depending on the reaction conditions.

When acting as a nucleophile, OH− ions attack the δ+ carbon, replacing the halogen and forming an alcohol (see nucleophilic substitution).

AQA A-Level Chemistry schematic showing OH− acting as a nucleophile to form an alcohol from a halogenoalkane

When acting as a base, OH− ions remove a proton (H+) from a carbon adjacent to the C–X bond, forming an alkene. This is an example of an elimination reaction and mechanism.

AQA A-Level Chemistry E2 elimination showing OH− removing H+ adjacent to C–X to form an alkene as X− leaves

Steps:

  1. Curly arrow from OH− to hydrogen adjacent to C–X (OH− accepts H+).
  2. C–H bond breaks, forming a C=C double bond.
  3. Curly arrow shows the C–X bond breaking, X− leaves.

Controlling Substitution and Elimination

Both reactions can happen simultaneously, forming both an alcohol and an alkene in the same reaction.

However, reaction conditions can be used to control the major product:

Summary

Reagent & Conditions Role of OH− Major Product Notes / Mechanism
Aqueous KOH, warm, reflux Nucleophile (electron pair donor) Alcohol Nucleophilic substitution of C–X
Ethanolic KOH, hot, reflux Base (H+ acceptor) Alkene Elimination: remove H+ and X−