Why Are Alkanes Generally Unreactive?
Alkanes contain only strong, non-polar C−C and C−H bonds. There are no lone pairs, no regions of high/low electron density, and no functional groups. So they are not attacked by nucleophiles or electrophiles under normal conditions.
Exception: Under UV light (or high temperature), halogen molecules undergo homolytic fission to form free radicals, which are reactive enough to attack C−H bonds.
Bond Fission
Homolytic Fission
Bond breaks equally. One electron to each atom
Produces free radicals (species with unpaired electrons, shown with •)
Cl−Cl → Cl• + Cl•
Shown with fish-hook (single-barbed) curly arrows
Heterolytic Fission
Bond breaks unequally. Both electrons to one atom
Produces ions (cation + anion)
H−Br → H⁺ + Br⁻
Shown with full (double-barbed) curly arrows
The Free Radical Substitution Mechanism
Alkanes react with halogens under UV light via a free radical mechanism. This has three stages:
Key Features of Each Stage
- Initiation: UV light provides the energy for homolytic fission of Cl₂. This is the rate-determining step. No reaction without UV light
- Propagation: A self-sustaining chain reaction. Each step consumes one radical and produces another. There are always exactly two propagation steps
- Termination: Any two radicals combine. These are minority reactions. The chain typically propagates hundreds of times before termination
Limitations of Free Radical Substitution
⚠️ Why It Gives a Mixture of Products
- Poly-substitution: Once CH₃Cl is formed, it has C−H bonds that can react further: CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄
- Unexpected termination products: e.g. CH₃• + CH₃• → C₂H₆ (ethane, not expected from a methane + chlorine reaction)
- Low selectivity: Radicals are so reactive they attack almost any C−H bond, even in longer chains (producing multiple positional isomers)
This makes free radical substitution of limited synthetic value. Mixtures are hard to separate and purify.
Reactivity Order of Halogens
F₂ > Cl₂ > Br₂ > I₂. Fluorine is too reactive (explosive), iodine is too unreactive (endothermic). Chlorine and bromine are most commonly used.
Think About It
Why does free radical substitution typically give a mixture of products?
Further substitution can occur: CH₃Cl → CH₂Cl₂ → CHCl₃ → CCl₄. Also, termination can produce unexpected products like ethane (C₂H₆) when two methyl radicals combine.
⚠️ Common Exam Mistakes
- Using full curly arrows instead of fish-hook arrows. Radicals involve single electron movement
- Forgetting to state UV light as the condition for initiation
- Writing only one propagation step. You must write both
- Confusing homolytic fission (→ radicals) with heterolytic fission (→ ions)