Explain why alkanes are described as saturated hydrocarbons and why they do not readily undergo addition reactions.

Cambridge International IGCSE Chemistry (0620) — 11.4 Alkanes · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Alkanes contain only C–H and C–C single bonds, so every carbon atom is bonded to the maximum number of hydrogen atoms – they are saturated.

Because there are no C=C double bonds, there is no site for atoms to add across; addition reactions require a multiple bond.

Thus alkanes undergo substitution reactions rather than addition reactions.

Examiner tips

Use the term "saturated" to explain maximum H atoms per C
Mention the absence of C=C bonds as the reason for no addition
Show that addition needs a double bond
State that alkanes only have single sigma bonds

Common mistakes

Claiming alkanes have double bonds
Saying addition is possible if energy is supplied
Confusing substitution with addition reactions

Mark scheme (4 marks)

Alkanes contain only carbon–hydrogen and carbon–carbon bonds / contain no double (or multiple) bonds
Saturated means all carbon atoms have the maximum number of hydrogen atoms / no more hydrogen atoms can be added
Addition reactions require a double bond / C=C bond for atoms to add across
Because alkanes have only single (sigma) bonds, they undergo substitution reactions instead of addition reactions

Key terms in this question

saturated · hydrocarbon · addition reaction