2.7.2Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Reaction Mechanisms- Introduction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 45 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.
Section 1 of 4Curly Arrows
A mechanism shows how bonds break and form step by step, using arrows to track electrons.
Full arrow
- Full arrow A curved arrow with a full head shows the movement of a pair of electrons.
Half arrow
- Half arrow A curved arrow with a half head, a fish hook, shows the movement of a single electron.
Start of the arrow
- Start of the arrow It starts at a bond or lone pair, where the electrons come from.
End of the arrow
- End of the arrow It ends where the electrons end up, for example forming a new bond or becoming a lone pair.
Reading mechanisms
- Reading mechanisms Count electrons carefully, because every arrow must start from a source of electrons.
Radical example
- Radical example UV light splits Cl₂ homolytically into two Cl• radicals, which start the free-radical substitution of alkanes.
Section 2 of 4Bond Fission and Reactive Species
Bond Fission and Reactive Species
Homolytic fission
- Homolytic fission A covalent bond breaks evenly, each atom taking one electron to form two radicals.
Heterolytic fission
- Heterolytic fission A covalent bond breaks unevenly, with both electrons going to the more electronegative atom to form ions.
Radical
- Radical A species with an unpaired electron, for example the chlorine radical Cl•, which is very reactive.
Electrophile and nucleophile
- Electrophile and nucleophile An electrophile is an electron-pair acceptor and is usually positive,
- while a nucleophile is an electron-pair donor and is rich in electrons.
Example
- Example In hydroxide attacking bromoethane, an arrow runs from the OH⁻ lone pair to the δ⁺ carbon and the C-Br electrons move to bromine.
Section 3 of 4Predicting the Mechanism from Polarity
1
Look at the reactants and decide which bonds are non-polar or only slightly polar.
2
Non-polar bonds such as Cl-Cl and C-H often react by free-radical mechanisms, needing UV light for initiation.
3
A polar bond, such as the C-Br bond, has a δ⁺ carbon that attracts a nucleophile.
4
A C=C double bond is electron-rich, so it attracts electrophiles such as H-Br.
5
Use the mechanism's type to predict the products and to draw the arrows correctly.
6
Check that the final charges and electrons shown by your arrows match the products of the reaction.
Section 4 of 4Exam Checklist & Tips
2.7.2 – Key Points for the Exam
- Curly arrow An arrow showing the movement of electrons in a mechanism.
- Homolytic fission Breaking of a bond so that each atom keeps one of the shared electrons.
- Heterolytic fission Breaking of a bond so that both shared electrons go to one atom.
- Electrophile A species that accepts a pair of electrons.
- Nucleophile A species that donates a pair of electrons.
Common Mistakes
- Drawing curly arrows from atoms instead of from bonds or lone pairs.
- Using a full-headed arrow when a single electron moves in a radical step.
- Calling a radical an ion; radicals are uncharged with an unpaired electron.
- Saying a nucleophile is attracted to negative regions; it is attracted to δ⁺ centres.
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