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R3.3Reactivity 3 — What are the mechanisms of chemical change?

Electron sharing reactions — IB Chemistry HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 3 of 11 1 quiz

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Lesson summary

R3.3 Electron sharing reactions — IB CHEMISTRY HL notes: Describe radicals, homolytic fission and the free-radical substitution of alkanes by halogens.Radical = a chemical species with an unpaired electron.Atomic — a single atom with an unpaired electron, such as Br• and H•.Molec…

  • Requirement
  • Finding radicals
  • Highly reactive
  • Taking an electron
  • Definition
  • Thermolytic fission

Section 1 of 11Radicals

Radical
Radical a chemical species with an unpaired electron.
Atomic
  • Atomic a single atom with an unpaired electron, such as Br• and H•.
Molecular
  • Molecular a group of atoms with no overall charge but an unpaired electron, such as •CH3, •OH and NO•.
Ionic
  • Ionic an anion or cation that also has an unpaired electron, such as O2−• and C2H5OH+•.
Requirement
  • Requirement the only requirement is the unpaired electron, regardless of charge.
Notation
  • Notation a dot placed on the atom with the unpaired electron.
Finding radicals
  • Finding radicals draw the Lewis formula and look for an unpaired electron, as in F and NO2 but not Cl−, Mg2+ or NH3.

Section 2 of 11Reactivity of radicals

Highly reactive
  • Highly reactive the unpaired electron gives them high enthalpy.
Taking an electron
  • Taking an electron a radical may remove an electron from another species, which then becomes a new radical.
Combining
  • Combining two radicals may join to form a covalent bond.
Lifetime
  • Lifetime they are usually short-lived.

Section 3 of 11Homolytic fission

Definition
Definition breaking a covalent bond so that each atom takes one electron from the bond, forming two radicals.
Arrows
  • Arrows a single-headed arrow shows the movement of one electron.
    • (fish-hook or half-curly)
Role
  • Role homolytic fission of a halogen is the initiation step of a chain reaction.
Drawing
  • Drawing each arrow starts in the middle of the bond and ends at an atom.
Energy
  • Energy bond breaking is endothermic, and the energy needed depends on bond strength.
Thermolytic fission
  • Thermolytic fission heat is enough for weaker bonds.
Photolytic fission
  • UV light is needed for stronger bonds such as halogens.

Section 4 of 11Why alkanes are unreactive

Strong bonds
  • C–C and C–H single bonds are hard to break without a lot of heat.
Low polarity
  • Low polarity the electronegativity difference between C and H is only 0.4, so alkanes are nonpolar.
No polar reagents
  • No polar reagents they have no electron-deficient or electron-rich regions to attract nucleophiles or electrophiles.
Reactions
  • Reactions alkanes react only in combustion and in free-radical substitution.

Section 5 of 11Free-radical substitution

1Reaction
  • Reaction a hydrogen atom of an alkane is replaced by a halogen.
    • (chlorine or bromine)
2UV needed
  • UV needed ultraviolet light or sunlight is required;
    • bromine colour is lost only when an alkane is exposed to sunlight.
3Three stages
  • Three stages initiation, propagation and termination.

Section 6 of 11Initiation

1Process
  • Process the halogen bond is broken by UV light.
2Equation
  • Cl–Cl → 2Cl•.
3Type
  • Type homolytic fission, producing two radicals.

Section 7 of 11Propagation

1First step
  • First step a halogen radical attacks an alkane, CH4 + Cl• → •CH3 + HCl.
2Second step
  • Second step the alkyl radical attacks a halogen molecule, •CH3 + Cl2 → CH3Cl + Cl•.
3Chain
  • Chain the regenerated chlorine radical repeats the cycle.
4Ethane
  • CH3CH3 + Cl• → •CH2CH3 + HCl, then •CH2CH3 + Cl2 → CH3CH2Cl + Cl•.
5Further substitution
  • Further substitution with enough halogen all hydrogens can be replaced, up to hexachloroethane C2Cl6, so a mixture forms.
6Common error
  • Common error a hydrogen radical is never produced in propagation.

Section 8 of 11Termination

Definition
  • Definition the chain stops when two radicals react to form a single unreactive molecule.
Radical with radical
  • •CH2CH3 + Cl• → CH3CH2Cl.
Alkyl with alkyl
  • •CH2CH3 + •CH2CH3 → CH3CH2CH2CH3.
Halogen radicals
  • Cl• + Cl• → Cl2.
Mixture
  • Mixture multiple products are possible, so the reaction is not suited to preparing a specific halogenoalkane.

Section 9 of 11More from the specification

  • Mechanism
    • The sequence of steps by which reactants form products.

Section 10 of 11Key terms

TermMeaning
RadicalA species with an unpaired electron.
Homolytic fissionBond breaking in which each atom takes one electron.
InitiationThe step producing radicals from a halogen molecule.
PropagationSteps in which a radical is used and another is produced.
TerminationSteps in which two radicals combine and the chain ends.
Photolytic fissionBond breaking caused by UV light.

Section 11 of 11Exam technique

1
Showing a hydrogen radical formed in a propagation step.
2
Using a double-headed curly arrow for homolytic fission — a single-headed arrow shows one electron.
3
Saying alkanes are unreactive because they are polar or ionic — they are nonpolar with strong bonds.
4
Forgetting that UV light is needed for the initiation step.

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