3.2.5Organic Chemistry
Free Radicals — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 23 of 42
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.
Section 1 of 4What Is Needed
Free radical
- Free radical A free radical is a species with an unpaired electron, such as Cl• or •CH₃, and it is very reactive.
Homolytic fission
- Homolytic fission A covalent bond breaks so that each atom takes one electron of the shared pair, forming two radicals.
Conditions
- Conditions Ultraviolet light supplies the energy to break the Cl–Cl bond, because it is weaker than C–H.
Overall reaction
- Overall reaction CH₄ + Cl₂ → CH₃Cl + HCl, where one hydrogen is substituted by a chlorine atom.
Type
- Type The reaction is substitution because an atom replaces another, and it is a chain reaction because radicals are regenerated.
Section 2 of 4The Three Stages
1
Initiation: UV light splits chlorine molecules by homolytic fission: Cl₂ → 2Cl•.
2
Propagation step 1: a chlorine radical removes a hydrogen atom: Cl• + CH₄ → •CH₃ + HCl.
3
Propagation step 2: the methyl radical reacts with chlorine: •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating Cl•.
4
Termination: two radicals combine, removing them from the chain, for example Cl• + Cl• → Cl₂, •CH₃ + Cl• → CH₃Cl.
5
Termination can also form ethane: •CH₃ + •CH₃ → C₂H₆, a trace by-product that shows the chain has stopped.
Section 3 of 4Limitations
Limitations
Further substitution
- Further substitution If chlorine is in excess, CH₂Cl₂, CHCl₃ and CCl₄ form as well as CH₃Cl.
Mixture
- Mixture The product is a mixture which must be separated, for example by fractional distillation.
Control
- Control Using excess methane makes monosubstitution more likely, because a chlorine radical meets methane more often.
Longer alkanes
- Longer alkanes Substitution can happen at different positions in longer alkanes, so many isomers are formed.
Bromine
- Bromine Bromine reacts in the same way, but more slowly than chlorine.
Section 4 of 4Exam Checklist & Tips
3.2.5 – Key Points for the Exam
- Free radical A species with an unpaired electron.
- Homolytic fission Breaking a covalent bond so each atom receives one electron of the pair.
- Propagation Steps in a chain reaction that use up and regenerate radicals.
- Termination A step in which two radicals combine and the chain stops.
- Substitution A reaction in which one atom or group replaces another.
Common Mistakes
- Drawing the unpaired electron as a charge, when a radical carries a dot but no charge.
- Using curly arrows with full heads for homolytic fission: half-headed arrows show movement of one electron.
- Writing a propagation step that consumes two radicals or forms no radical, which makes it a termination step.
- Forgetting the UV conditions or saying the reaction occurs in the dark at room temperature.
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