Chemical Properties: Substitution Reactions
Focus on free radical halogenation (chlorination and bromination) of alkanes, including its mechanism, regioselectivity, and reactivity.
What is Chemical Properties: Substitution Reactions?
A chemical reaction in which one atom or group of atoms is replaced by another atom or group of atoms.
Key formula / rule: Free Radical Formation (Initiation)
Key points
- Understand why alkanes primarily undergo substitution reactions.
- Explain the free-radical mechanism of halogenation of alkanes.
- Identify the role of UV light or heat in the reaction.
- Predict the major product(s) of halogenation based on regioselectivity.
Common exam trap
Confusing substitution with addition reactions.
Definitions
- Term
Substitution Reaction
- Meaning
A chemical reaction in which one atom or group of atoms is replaced by another atom or group of atoms.
- Term
Free Radical
- Meaning
An atom, molecule, or ion that has an unpaired valence electron, making it highly reactive.
- Term
Homolytic Cleavage
- Meaning
A type of bond breaking where each fragment of the original molecule retains one of the bonding electrons, forming radicals.
- Term
Regioselectivity
- Meaning
The preference for a reaction to occur at one specific position or direction over other possible positions.
- Term
Alkyl Radical
- Meaning
A radical formed by the removal of a hydrogen atom from an alkane.
Learning objectives
Understand why alkanes primarily undergo substitution reactions.
Explain the free-radical mechanism of halogenation of alkanes.
Identify the role of UV light or heat in the reaction.
Predict the major product(s) of halogenation based on regioselectivity.
Differentiate between chlorination and bromination in terms of selectivity.
Formulae
- Name
Free Radical Formation (Initiation)
- Note
Homolytic cleavage of the halogen molecule.
- Expression
X₂ + hv/Δ → 2X• (where X = Cl or Br)
- Name
Hydrogen Abstraction (Propagation)
- Note
Halogen radical abstracts a hydrogen atom from alkane.
- Expression
R-H + X• → R• + HX
- Name
Alkyl Halide Formation (Propagation)
- Note
Alkyl radical reacts with halogen molecule.
- Expression
R• + X₂ → R-X + X•
- Name
Termination Steps
- Note
Combination of radicals to form stable molecules.
- Expression
2X• → X₂ ; 2R• → R-R ; R• + X• → R-X
Prerequisites
Basic understanding of covalent bonds and σ bonds.
Knowledge of free radicals and their reactivity.
Understanding of reaction mechanisms.
Nomenclature of alkanes.
Common mistakes
Confusing substitution with addition reactions.
Not understanding the role of UV light/heat in radical formation.
Assuming monosubstitution only.
Ignoring regioselectivity and predicting the wrong major product.
Incorrectly identifying the steps of the free-radical mechanism.
Practice preview
In the free radical monochlorination of n-butane, how many monochlorinated structural isomers are possible?…
medium
Which of the following halogens is most reactive towards free radical substitution with alkanes?…
easy
What is the primary product formed when methane undergoes free radical monochlorination?…
easy