Alkanes as fuels and free-radical substitution, the reactions of alkenes including electrophilic addition and Markownikoff's rule, addition polymerisation, and the nucleophilic substitution and elimination reactions of haloalkanes.

What this dot point is asking

CCEA wants you to describe alkanes as fuels and their free-radical substitution, the electrophilic addition reactions of alkenes with Markownikoff's rule, addition polymerisation, and the nucleophilic substitution and elimination reactions of haloalkanes, including the mechanisms.

Alkanes

Alkanes are saturated hydrocarbons with the general formula $\text{C}_n\text{H}_{2n+2}$. They are used as fuels, burning in oxygen to release energy. With halogens in ultraviolet light they undergo free-radical substitution:

Taking the chlorination of methane as the model, the three stages are written as: initiation $\text{Cl}_2 \rightarrow 2\text{Cl}\cdot$ (homolytic fission, each atom keeps one electron); propagation $\text{Cl}\cdot + \text{CH}_4 \rightarrow \text{HCl} + \text{CH}_3\cdot$ then $\text{CH}_3\cdot + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}\cdot$; and termination such as $\text{CH}_3\cdot + \text{Cl}\cdot \rightarrow \text{CH}_3\text{Cl}$ or $2\text{CH}_3\cdot \rightarrow \text{C}_2\text{H}_6$. Because a chlorine radical is regenerated in propagation, a single initiation event sets off a long chain. Further substitution can replace more than one hydrogen, giving $\text{CH}_2\text{Cl}_2$, $\text{CHCl}_3$ and $\text{CCl}_4$ as well, which is why a mixture of products always forms. Burning alkanes completely gives carbon dioxide and water, for example $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$, while incomplete combustion gives toxic carbon monoxide or soot.

Alkenes

Alkenes are unsaturated, with a reactive $\text{C}=\text{C}$ double bond and general formula $\text{C}_n\text{H}_{2n}$. The high electron density of the double bond attracts electrophiles, so alkenes undergo electrophilic addition.

Alkenes also undergo addition polymerisation, where many monomers join through their double bonds to form a long-chain polymer such as poly(ethene): $n\,\text{CH}_2{=}\text{CH}_2 \rightarrow {-}(\text{CH}_2{-}\text{CH}_2){-}_n$. The double bond opens up and the monomers link end to end, with no other product. The reaction of an alkene with bromine water is also the standard test for unsaturation: the orange bromine is decolourised.

Haloalkanes

Reactivity increases as the carbon-halogen bond gets weaker down the group, so iodoalkanes react fastest and fluoroalkanes slowest. It is the bond strength that controls the rate, not the bond polarity: although the $\text{C} - \text{F}$ bond is the most polar, it is also the strongest and hardest to break, so fluoroalkanes are the least reactive. The choice of nucleophile decides the product: $\text{OH}^-$ in warm aqueous solution gives an alcohol, $\text{CN}^-$ in ethanol gives a nitrile (adding a carbon to the chain), and excess ammonia in a sealed tube gives an amine. Switching to hot ethanolic hydroxide and removing the water favours elimination instead, removing $\text{HX}$ to form an alkene.

Examples in context

Example 1. CFCs and the ozone layer. Chlorofluorocarbons were once used as refrigerants because they are unreactive and non-toxic. High in the stratosphere, ultraviolet light breaks the weak $\text{C} - \text{Cl}$ bond homolytically (an initiation step exactly like free-radical substitution), releasing chlorine radicals that catalyse the breakdown of ozone in a chain reaction. CCEA expects candidates to connect the radical chemistry of alkanes to this real environmental problem and to explain why the more inert $\text{C} - \text{F}$ bonds are not the culprits.

Example 2. Margarine from vegetable oils. Unsaturated vegetable oils are hardened into margarine by addition of hydrogen across their $\text{C}{=}\text{C}$ double bonds using a nickel catalyst, the same electrophilic-style addition reaction (here catalytic hydrogenation) that adds bromine or water to a simple alkene. Controlling how many double bonds are hydrogenated sets the melting point and spreadability of the product, a direct industrial use of alkene addition chemistry.

Try this

Q1. Name the three stages of free-radical substitution. [3 marks]

• Cue. Initiation, propagation and termination.

Q2. State the type of reaction when an alkene reacts with bromine. [1 mark]

• Cue. Electrophilic addition.