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3.7Organic Chemistry

Organic Mechanisms — AQA A Level Chemistry Revision Notes & Topical Questions

5 min read 10 parts Lesson 8 of 8

📚 AQA A Level Chemistry Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

Organic Mechanisms: Bond Fission, Nucleophilic Substitution and Elimination — AQA A LEVEL CHEMISTRY — COMPLETE NOTES notes: Use curly arrows to show nucleophilic substitution and elimination, and distinguish homolytic from heterolytic fission.Homolytic fission is breaking a coval…

  • nucleophile
  • electrophile
  • aqueous NaOH
  • KCN in ethanol
  • excess ammonia in ethanol
  • ethanolic NaOH

Part 1 of 4Fundamentals of reaction mechanisms

Homolytic fission is breaking a covalent bond in such a way that each atom takes an electron from the bond to form two radicals (shown by fish-hook, single-headed arrows). Heterolytic fission is breaking a bond in such a way that both electrons go to one atom, forming a positive and a negative ion (shown by a full-headed curly arrow).

Nucleophile
A nucleophile is an electron-pair donor (OH-, CN-, NH3, H2O); an electrophile is an electron-pair acceptor (H+, carbocations, δ+ atoms).
A curly arrow starts at a lone pair or bond and points to where the pair of electrons moves.

Part 2 of 4Nucleophilic substitution

In a halogenoalkane the carbon bonded to the halogen has a δ+ charge, as the halogen is more electronegative. A nucleophile uses its lone pair to attack the δ+ carbon, and the C-X bond breaks heterolytically with the halogen leaving as X-.

Nucleophilic substitution
Aqueous NaOH
With aqueous NaOH (warm, reflux): R-X + OH- → R-OH + X-, giving an alcohol.
  • Mechanismthe OH- lone pair attacks Cδ+, one curly arrow from the C-X bond to X.
KCN in ethanol
With KCN in ethanol: R-X + CN- → R-CN + X-, forming a nitrile and lengthening the carbon chain by one.
Excess ammonia in ethanol
With excess ammonia in ethanol (sealed tube): R-X + 2NH3 → R-NH2 + NH4X, forming a primary amine.
  • Excess ammonia prevents further substitution to secondary amines.

Part 3 of 4Elimination

Heating a halogenoalkane with ethanolic NaOH (not aqueous) removes HX to form an alkene.

Elimination
Base
The hydroxide acts as a base and removes a proton (H+) from a carbon adjacent to the C-X; the electrons move to form the C=C and the halide leaves.
Substitution versus elimination:
Example: CH3CH2Br + NaOH (ethanolic) → CH2=CH2 + NaBr + H2O. Substitution versus elimination: aqueous conditions and lower temperature favour substitution; ethanolic conditions and high temperature favour elimination.
Tertiary halogenoalkanes favour elimination.

Part 4 of 4Worked examples

  1. Draw/describe the mechanism for 1-bromopropane with aqueous NaOH. [3]3 marks

  2. State the conditions that favour elimination over substitution. [2]2 marks

  3. Explain why the C-Br carbon is attacked by nucleophiles. [2]2 marks

Key terms

TermMeaning
Homolytic fissionBond breaking where each atom takes one electron, forming radicals.
Heterolytic fissionBond breaking where both electrons go to one atom, forming ions.
NucleophileAn electron-pair donor.
EliminationA reaction in which a small molecule is removed from a larger one, forming a double bond.
SubstitutionOne atom or group is replaced by another.

Avoid these mistakes

1
Using aqueous NaOH for elimination.
2
Starting the curly arrow at the nucleophile's charge rather than its lone pair.
3
Starting a curly arrow at the H or at a wrong atom.
4
Mixing nucleophile and electrophile definitions.
5
Forgetting excess ammonia is needed for primary amines.

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