3.7.4Physical Chemistry
Elimination — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 93 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5The Elimination Reaction
The Elimination Reaction
Definition
- Definition A small molecule is removed from a larger one, leaving a carbon–carbon double bond.
Reagent
- Reagent Hot ethanolic sodium hydroxide or potassium hydroxide, heated under reflux.
Equation
- Equation CH₃CH₂Br + NaOH → CH₂=CH₂ + NaBr + H₂O, with ethene as the organic product.
What is removed
- What is removed A hydrogen from the carbon next to the C–X carbon and the halogen are removed together as HX.
Part 2 of 5The Mechanism
1
The hydroxide ion acts as a base and uses a lone pair to attack a hydrogen on the carbon adjacent to the C–X carbon.
2
A curly arrow from that C–H bond moves its electrons to form the C=C double bond.
3
At the same time, a curly arrow from the C–X bond moves both electrons to the halogen atom.
4
The halide ion leaves, and the hydroxide ion becomes a water molecule.
5
All these changes occur in a single step, so there is no intermediate.
Part 3 of 5Substitution or Elimination
The same haloalkane and hydroxide can give different products depending on the solvent and temperature.
Aqueous NaOH
- Aqueous NaOH Water as the solvent favours substitution, and the product is an alcohol.
Ethanolic NaOH
- Ethanolic NaOH Ethanol as the solvent with heating favours elimination, and the product is an alkene.
Role of OH⁻
- Role of OH⁻ In substitution it acts as a nucleophile, but in elimination it acts as a base.
Hot conditions
- Hot conditions Higher temperatures favour elimination over substitution.
| Feature | Substitution | Elimination |
|---|---|---|
| Solvent | Water (aqueous) | Ethanol |
| Role of OH⁻ | Nucleophile | Base |
| Organic product | Alcohol | Alkene |
| Example product | Propan-2-ol from 2-bromopropane | Propene from 2-bromopropane |
Part 4 of 5Predicting Products
Predicting Products
Symmetrical cases
- Symmetrical cases 1-Bromopropane and 2-bromopropane both give propene, because there is only one possible alkene.
More than one alkene
- More than one alkene 2-Bromobutane can lose H from either neighbouring carbon, giving but-1-ene and but-2-ene.
Atom economy
- Atom economy The reaction produces several products, so it is less efficient than an addition reaction.
Exam tip
- Exam tip Always state both the reagent and the solvent, because aqueous and ethanolic alkali give different outcomes.
Part 5 of 5Exam Checklist & Tips
3.7.4 – Key Points for the Exam
- Elimination A reaction in which a small molecule is removed from a larger molecule, forming a double bond.
- Base A species that accepts a proton, here the hydroxide ion removing H⁺.
- Ethanolic Dissolved in ethanol rather than in water.
- Reflux Heating a reaction mixture with a condenser so that vapours condense and return to the flask.
Common Mistakes
- Using aqueous NaOH to make an alkene, which gives the alcohol by substitution.
- Drawing the hydroxide ion attacking the carbon bonded to the halogen in the elimination mechanism.
- Removing hydrogen from the same carbon as the halogen instead of from the adjacent carbon.
- Leaving out the lone pair on the hydroxide ion at the start of the arrow.
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