3.7.1Physical Chemistry
Fundamentals of Reaction Mechanisms — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 90 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 5Breaking Covalent Bonds
Homolytic fission
- Homolytic fission Each atom takes one electron from the shared pair, forming two uncharged free radicals.
Heterolytic fission
- Heterolytic fission One atom, usually the more electronegative, takes both electrons, forming a negative ion and a positive ion.
Free radical
- Free radical A species with an unpaired electron, written with a dot, such as Cl• or •CH₃, and very reactive.
Half-headed arrow
- Half-headed arrow A fishhook arrow shows the movement of a single electron in radical reactions.
Full-headed arrow
- Full-headed arrow A curly arrow with a full head shows movement of an electron pair in ionic mechanisms.
Part 2 of 5Nucleophiles and Electrophiles
Nucleophiles and Electrophiles
Nucleophile
- Nucleophile An electron-pair donor, attracted to positive centres, such as OH⁻, CN⁻, NH₃ and H₂O.
Electrophile
- Electrophile An electron-pair acceptor, attracted to negative or electron-rich centres, such as H⁺ or a δ+ atom.
Why they react
- Why they react Nucleophiles attack electron-poor carbon atoms, while electrophiles attack electron-rich sites such as C=C.
Memory aid
- Memory aid A nucleophile loves nuclei (positive charge), and an electrophile loves electrons (negative charge).
Part 3 of 5Types of Organic Reaction
Types of Organic Reaction
Addition
- Addition Two molecules combine to give a single product, as when HBr adds to an alkene.
Substitution
- Substitution One atom or group is replaced by another, as when OH⁻ replaces Br in a haloalkane.
Elimination
- Elimination A small molecule such as H₂O or HBr is removed, leaving a double bond.
Hydrolysis and condensation
- Hydrolysis and condensation Hydrolysis splits a compound using water or alkali, while condensation joins molecules and loses a small molecule.
Part 4 of 5Redox and Curly Arrows
1
Oxidation in organic chemistry means gaining oxygen or losing hydrogen, and [O] represents oxygen from an oxidising agent.
2
Reduction means losing oxygen or gaining hydrogen, and [H] represents hydrogen from a reducing agent.
3
A curly arrow starts at a lone pair or a bond, never at an atom.
4
The arrow points to the atom that receives the pair of electrons, forming a new bond or a negative ion.
5
Always add partial charges (δ+ and δ−) on polar bonds, because they are often needed to gain the mark.
Part 5 of 5Exam Checklist & Tips
3.7.1 – Key Points for the Exam
- Homolytic fission Bond breaking in which each atom receives one electron from the bonding pair.
- Heterolytic fission Bond breaking in which one atom receives both electrons from the bonding pair.
- Nucleophile An electron-pair donor.
- Electrophile An electron-pair acceptor.
- Free radical A species with an unpaired electron.
Common Mistakes
- Starting a curly arrow at an atom rather than at a lone pair or a bond.
- Using a full-headed arrow for radical reactions instead of a half-headed fishhook arrow.
- Leaving out δ+ and δ− on polar bonds that the mechanism depends on.
- Saying a nucleophile accepts electrons; it donates a pair.
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