AS Level Chemistry · Hydrocarbons. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Alkanes are saturated hydrocarbons. They are generally unreactive because their C–C and C–H bonds are strong and non-polar, so nucleophiles and electrophiles are not attracted to them. They have two main reactions: combustion and free-radical substitution.
In free-radical substitution, ultraviolet light splits a chlorine or bromine molecule into two radicals. A radical removes a hydrogen atom from the alkane, and a chain reaction follows in which hydrogen atoms are replaced by halogen atoms one at a time.
Alkanes are made by adding hydrogen to an alkene (hydrogenation) or by cracking a longer alkane, which gives a shorter alkane and an alkene.
Students often write H• as a product of propagation. The chlorine radical removes a hydrogen atom to form HCl and an alkyl radical; no hydrogen radicals are made.
Answer: 50 cm3 of oxygen is needed and 30 cm3 of carbon dioxide is formed.
Try each one, then open the answer.
Answer: B. Propagation steps use one radical and make another, keeping the chain going. The first equation is initiation (UV light), and the last two combine radicals, so they are termination steps.
Answer: A. Hydrogenation adds H2 across the C=C bond and needs a Ni (or Pt) catalyst and heat. Steam with H3PO4 would give an alcohol.
Answer: D. Free-radical substitution is not selective. Once chloroethane forms, Cl• radicals attack its C–H bonds too, so di-, tri- and further substituted products form.
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