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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

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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

TermMeaning
RadicalA species with an unpaired electron.
Homolytic fissionBreaking a covalent bond so each atom takes one electron.
InitiationThe step in which radicals are first formed.
PropagationSteps in which radicals react and are regenerated, continuing the chain.
TerminationA step in which two radicals combine and the chain ends.
Free-radical substitutionReplacement 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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