11.4 Alkanes
- Syllabus
- 0620–2026–2027
- Topic
- 11.4
- Level
- —
Learning objectives
11.4.1Bonding in alkanes is single covalent• State: the bonding in alkanes is single covalent and that alkanes are saturated hydrocarbons11.4.2Properties of alkanes as being• Describe properties of alkanes as being generally unreactive, except in terms of combustion and substitution by chlorine11.4.3In a substitution reaction one atom or• State: in a substitution reaction one atom or group of atoms is replaced by another atom or group of atoms11.4.4Substitution reaction of alkanes with• Describe substitution reaction of alkanes with chlorine as a photochemical reaction, with ultraviolet light providing the activation energy, Ea, and draw the structural or displayed formulae of the products, limited to monosubstitution
Recognise alkanes as saturated hydrocarbons
Alkanes are saturated hydrocarbons whose atoms are joined by single covalent bonds.
| Term | Meaning for an alkane |
|---|---|
| hydrocarbon | contains carbon and hydrogen only |
| saturated | every carbon–carbon bond is a single bond |
| covalent | atoms share pairs of electrons |
Methane, CH₄, ethane, CH₃CH₃, and propane, CH₃CH₂CH₃, are alkanes. None contains a C=C or C≡C bond.
An alkane can contain C–H and C–C bonds, but all are single covalent bonds. A molecule containing C=C is not an alkane.
Describe the limited reactivity of alkanes
Alkanes are generally unreactive, but they undergo combustion and substitution by chlorine.
| Reaction | Reactant or condition | Main change |
|---|---|---|
| complete combustion | excess oxygen | carbon dioxide and water form |
| incomplete combustion | limited oxygen | carbon monoxide and/or carbon plus water form |
| chlorine substitution | chlorine and ultraviolet light | a hydrogen atom is replaced by chlorine |
Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O.
'Generally unreactive' does not mean alkanes never react; it highlights the two reaction types required here.
Combustion changes the whole molecule by oxidation. Chlorine substitution replaces an atom; it is not an addition reaction.
Define a substitution reaction
In a substitution reaction, one atom or group of atoms is replaced by another atom or group of atoms.
| Before | Replacing species | After |
|---|---|---|
| alkane contains C–H | chlorine supplies Cl | chloroalkane contains C–Cl |
| chlorine molecule, Cl₂ | one H leaves the alkane | hydrogen chloride, HCl, also forms |
Identify the part that leaves and the part that takes its place. The carbon skeleton remains present while one bond is changed.
Substitution is not addition: in addition, atoms join across a multiple bond without another atom being replaced.
Carry out alkane chlorination by monosubstitution
Alkanes react with chlorine in a photochemical substitution reaction. Ultraviolet light supplies the activation energy, Eₐ, needed to start the reaction.
| Reactants | UV-light products after one substitution |
|---|---|
| methane + chlorine | chloromethane + hydrogen chloride |
| ethane + chlorine | chloroethane + hydrogen chloride |
| propane + chlorine | chloropropane + hydrogen chloride |
General pattern: R–H + Cl₂ → R–Cl + HCl. For methane: CH₄ + Cl₂ → CH₃Cl + HCl, with ultraviolet light above the arrow.
For propane, replacing an end hydrogen gives 1-chloropropane, CH₃CH₂CH₂Cl; replacing a middle-carbon hydrogen gives 2-chloropropane, CH₃CHClCH₃.
For a structural product, keep the alkane carbon skeleton, replace exactly one H with Cl, then include HCl as the second product and check atom balance.
This objective is limited to monosubstitution: replace one hydrogen only. Do not continue to dichloro-, trichloro- or fully chlorinated products.