Nucleophilic substitution of halogenoalkanes by hydroxide, cyanide and ammonia. Elimination of halogenoalkanes to form alkenes. The effect of bond enthalpy on rate of hydrolysis. CFCs and the depletion of the ozone layer.

What this dot point is asking

AQA wants you to give the mechanisms of nucleophilic substitution by hydroxide, cyanide and ammonia, the elimination mechanism to form an alkene, explain how carbon-halogen bond enthalpy controls hydrolysis rate, and describe how CFCs deplete the ozone layer.

Nucleophilic substitution

The carbon bonded to the halogen is $\delta+$ because the halogen is more electronegative, so it is attacked by a nucleophile (an electron-pair donor with a lone pair). The nucleophile replaces the halogen, which leaves as a halide ion (a good leaving group).

• Cyanide ($CN^-$ in ethanol): forms a nitrile, adding one carbon: $CH_3CH_2Br + CN^- \rightarrow CH_3CH_2CN + Br^-$.
• Excess ammonia: forms a primary amine: $CH_3CH_2Br + 2NH_3 \rightarrow CH_3CH_2NH_2 + NH_4Br$.

Each nucleophile is chosen for what it builds: hydroxide for an alcohol, cyanide when the carbon chain must be extended by one (the nitrile can later be hydrolysed to a carboxylic acid or reduced to an amine), and ammonia for an amine. These reactions make halogenoalkanes a central hub in synthesis, because an alcohol can be turned into a halogenoalkane and from there into many other functional groups.

Elimination

With concentrated KOH dissolved in ethanol and heat, $OH^-$ acts as a base, removing a hydrogen from the carbon next to the $C-X$ carbon, so an alkene forms:

Bond enthalpy and hydrolysis rate

The rate of hydrolysis depends on the carbon-halogen bond enthalpy, not on bond polarity. The $C-I$ bond is weakest, so iodoalkanes hydrolyse fastest; the $C-F$ bond is strongest, so fluoroalkanes are slowest. Rate of reaction is measured by adding aqueous silver nitrate and timing the precipitate (yellow $AgI$ fastest).

CFCs and ozone

CFCs (chlorofluorocarbons) were used as refrigerants and propellants. In the stratosphere, UV breaks the $C-Cl$ bond, producing chlorine radicals that catalyse ozone breakdown:

The $Cl\bullet$ is regenerated in the second step, so it acts as a catalyst and one chlorine radical can destroy thousands of ozone molecules before it is removed. This is why CFCs were banned under the Montreal Protocol and replaced by HFCs and other compounds that do not carry chlorine into the stratosphere. The overall effect, $2O_3 \rightarrow 3O_2$, depletes the ozone layer that screens harmful UV radiation, and AQA expects you to be able to identify the chlorine radical as the catalyst and write the two propagation steps.

Try this

Q1. Give the reagent and product when 1-bromopropane reacts with potassium cyanide. [2 marks]

• Cue. $KCN$ in ethanol gives butanenitrile, $CH_3CH_2CH_2CN$.

Q2. Explain why iodoalkanes hydrolyse faster than chloroalkanes. [2 marks]

• Cue. The $C-I$ bond enthalpy is lower than $C-Cl$, so it breaks more easily.