R3.3Reactivity 3 — What are the mechanisms of chemical change?
Electron sharing reactions — IB Chemistry SL Revision Notes & Topical Questions
5 min read 9 parts Lesson 3 of 12 1 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.
Lesson summary
R3.3 Electron sharing reactions — IB CHEMISTRY SL notes: Define radicals and homolytic fission, explain why alkanes are unreactive, and describe free-radical halogenation of alkanes.Definition = a chemical species with an unpaired electron.Types — atomic (Cl•, Br•, H•), polyatomi…
- Requirement
- Identifying
- Reactivity
- Typical behaviour
- Thermolysis
- Photolysis
Part 1 of 7Radicals
Radicals
Definition
Definition a chemical species with an unpaired electron.
Types
- Types atomic (Cl•, Br•, H•), polyatomic neutral (•CH3, •OH, NO•, NO2•), anionic (e.g. superoxide O2•−) and cationic.
Requirement
- Requirement the unpaired electron is the only requirement, whatever the charge.
Notation
- Notation the dot is placed on the atom carrying the unpaired electron, most clearly in displayed formulae.
Identifying
- Identifying draw the Lewis formula and look for an unpaired electron, so F and NO2 are radicals but Cl−, Mg2+ and NH3 are not.
Reactivity
- Reactivity radicals have high enthalpy and are very reactive and short-lived.
Typical behaviour
- Typical behaviour they take an electron from another species, forming a new radical, or combine with another radical to form a covalent bond.
Part 2 of 7Homolytic fission
Definition
Definition breaking a covalent bond evenly so each atom takes one electron from the shared pair, forming two radicals.
Arrows
- Arrows single-headed arrows show the movement of one electron, starting from the middle of the bond and ending at each atom.
- (fish-hook or half-curly)
Energy
- Energy bond breaking is endothermic, and the energy needed depends on bond strength.
Thermolysis
- Thermolysis weaker bonds can be broken by heating.
Photolysis
- Photolysis stronger bonds, such as halogen bonds, are broken by UV photons.
Favoured for
- Favoured for non-polar bonds such as Cl–Cl.
Part 3 of 7Why alkanes are unreactive
Strong bonds
- C–C and C–H single bonds need a lot of energy to break.
Low polarity
- Low polarity the C–H electronegativity difference is only 0.4, so alkanes are non-polar with no δ+ or δ− regions.
No attraction
- No attraction there are no electron-deficient sites to attract nucleophiles and no electron-rich sites to attract electrophiles.
Reactions
- Reactions alkanes undergo only combustion and radical substitution.
Bond energies
- Cl–Cl (242 kJ mol−1) is weaker than C–H (414 kJ mol−1), so UV breaks the halogen bond.
Part 4 of 7Free-radical substitution of alkanes
Free-radical substitution of alkanes
Overall
Overall alkane + halogen → halogenoalkane + hydrogen halide, e.g. CH4 + Cl2 → CH3Cl + HCl.
Type
- Type a substitution in which a hydrogen atom is replaced by Cl or Br.
Condition
- Condition ultraviolet light (sunlight) is essential.
Evidence
- Evidence bromine colour disappears with an alkane only when exposed to sunlight.
Chain reaction
- Chain reaction propagation regenerates the halogen radical, so one initiation can form thousands of product molecules.
Part 5 of 7The three stages
1
Initiation: UV light breaks the halogen bond homolytically, X2 → 2X•.
2
Propagation 1: a halogen radical removes an H atom, CH4 + Cl• → •CH3 + HCl.
3
Propagation 2: the alkyl radical attacks a halogen molecule, •CH3 + Cl2 → CH3Cl + Cl•.
4
The regenerated Cl• repeats the cycle.
5
Termination: two radicals combine to end the chain.
1Termination examples
- Cl• + Cl• → Cl2, •CH2CH3 + Cl• → CH3CH2Cl and 2 •CH2CH3 → CH3CH2CH2CH3.
2Ethane example
- CH3CH3 + Cl• → •CH2CH3 + HCl, then •CH2CH3 + Cl2 → CH3CH2Cl + Cl•.
3Common error
- Common error a step producing a free H•, such as CH3CH3 + Cl• → CH3CH2Cl + H•, does not occur.
Part 6 of 7Mixture of products
Further substitution
- Further substitution the halogenoalkane product can itself be attacked, giving di-, tri- and tetrahalogenated products, up to C2Cl6 from ethane.
Isomers
- Isomers with longer alkanes, different hydrogen atoms can be replaced, giving structural isomers.
Control
- Control excess alkane favours monosubstitution and excess halogen favours multiple substitution, e.g. CCl4.
Traces of longer alkanes
- Traces of longer alkanes termination between alkyl radicals explains small amounts of products such as butane.
Preparation
- Preparation the reaction is unsuitable for making a specific halogenoalkane.
Bromination
- Bromination works the same way, and bromine water is slowly decolourised by alkanes in UV light.
Part 7 of 7More from the specification
- Mechanism
- The sequence of steps by which reactants form products.
Key terms
| Term | Meaning |
|---|---|
| Radical | A species with an unpaired electron. |
| Homolytic fission | Breaking a covalent bond so each atom takes one electron. |
| Initiation | The step in which radicals are first formed. |
| Propagation | Steps in which radicals react and are regenerated, continuing the chain. |
| Termination | A step in which two radicals combine and the chain ends. |
| Free-radical substitution | Replacement of a hydrogen atom in an alkane by a halogen through a radical chain. |
Avoid these mistakes
1
Showing a hydrogen radical in propagation — the halogen radical forms HX instead.
2
Using full curly arrows for radical steps — use single-headed arrows for one electron.
3
Saying alkanes are polar or reactive because of C–H polarity — the bonds are strong and almost non-polar.
4
Leaving out UV light — without it the radical chain does not start.
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