2.8.3Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
The Nucleophilic Substitution Mechanism — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 48 of 53 Unit quiz
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 5What Makes the Mechanism Possible
The mechanism begins with a bond that is already unevenly shared.
Polar C–X bond
- Polar C–X bond Halogens are more electronegative than carbon, so the bonding pair is drawn towards X and carbon becomes δ+.
Attack site
- Attack site The nucleophile is attracted to the δ+ carbon atom, not to the halogen atom itself.
Lone pair
- Lone pair Every nucleophile has a lone pair of electrons that it can donate to form a new bond to carbon.
Heterolytic fission
- Heterolytic fission The C–X bond breaks unevenly, so both bonding electrons go to the halogen, which leaves as X⁻.
Part 2 of 5Reading Curly Arrows
Curly arrows show the movement of electron pairs, never of atoms.
Reading Curly Arrows
Arrow start
- Arrow start A curly arrow starts at a lone pair or at the middle of a covalent bond, which is where electrons currently are.
Arrow end
- Arrow end The arrow points to the atom or the space between atoms where the electron pair ends up.
Full head
- Full head A full-headed arrow represents the movement of a pair of electrons, as used in all nucleophilic substitution mechanisms.
Charges
- Charges Formal charges and partial charges must balance on both sides, which provides a useful check of the mechanism drawn.
Part 3 of 5Mechanism with Hydroxide Ions
Examplebromoethane reacting with warm aqueous sodium hydroxide.
1
Draw bromoethane showing the polar bond with δ+ on the carbon attached to Br and δ– on bromine.
2
The hydroxide ion approaches the δ+ carbon; draw a curly arrow from a lone pair on the oxygen to that carbon atom.
3
Draw a second curly arrow from the middle of the C–Br bond to the bromine atom, showing the bonding pair moving onto Br.
4
The products are ethanol and a bromide ion, shown as CH₃CH₂OH and Br⁻ with a negative charge on the bromide.
5
Check that the total charge is –1 before and after the reaction, and that the lone pair on bromide is drawn.
Part 4 of 5Mechanism with Ammonia
Ammonia is a neutral nucleophile, so the mechanism passes through a charged intermediate.
Step 1
- Step 1 The lone pair on nitrogen attacks the δ+ carbon while the C–Cl bond breaks, giving [CH₃NH₃]⁺ and Cl⁻.
Step 2
- Step 2 A second ammonia molecule removes a proton from the nitrogen of the intermediate, producing methylamine, CH₃NH₂.
By-product
- By-product The proton transferred forms ammonium chloride, NH₄Cl, so two moles of ammonia are used per mole of product.
Excess ammonia
- Excess ammonia A large excess of ammonia makes it unlikely that the amine product will react with another halogenoalkane molecule.
Part 5 of 5Exam Checklist & Tips
2.8.3 – Key Points for the Exam
- Curly arrow A symbol showing the movement of an electron pair from its source to its destination.
- Lone pair A pair of non-bonding electrons on an atom that can be donated to form a covalent bond.
- Partial charge A small charge, δ+ or δ–, caused by unequal sharing of electrons in a polar bond.
- Heterolytic fission Bond breaking in which both electrons of the shared pair go to one of the atoms.
Common Mistakes
- Starting the curly arrow from the negative charge symbol instead of from the lone pair on the oxygen atom.
- Drawing the arrow from the nucleophile to the halogen atom rather than to the δ+ carbon atom.
- Forgetting the negative charge on the leaving halide ion in the products.
- Showing the C–X bond arrow pointing to carbon instead of towards the halogen.
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