The reactions of alcohols (oxidation, dehydration, ester formation) and haloalkanes (nucleophilic substitution and elimination), and an introduction to mass spectrometry and infrared spectroscopy.

What this topic is asking

Edexcel Topic 11 wants you to describe and explain the reactions of alcohols and haloalkanes, including the nucleophilic substitution and elimination mechanisms with curly arrows, and to interpret simple mass spectra and infrared spectra to identify functional groups.

The answer

Reactions of alcohols

The oxidation is accompanied by an orange-to-green colour change as dichromate(VI) is reduced to chromium(III). Using $[\text{O}]$ to represent the oxidant keeps the equations simple, for example:

Reactions of haloalkanes

Haloalkanes undergo nucleophilic substitution with nucleophiles such as $\text{OH}^-$ (warm aqueous, giving alcohols), $\text{CN}^-$ (warm ethanolic, giving nitriles and adding a carbon) and ammonia (excess, in a sealed tube, giving amines). They also undergo elimination to alkenes with hot ethanolic hydroxide. The conditions decide the route: aqueous favours substitution, ethanolic favours elimination.

Reactivity depends on C-X bond enthalpy: $\text{C-I}$ ($238\ \text{kJ mol}^{-1}$) is weaker than $\text{C-Br}$ ($276$) which is weaker than $\text{C-Cl}$ ($338$). The weaker bond breaks more easily, so iodoalkanes react fastest and chloroalkanes slowest. This is confirmed experimentally by warming each haloalkane with aqueous silver nitrate in ethanol and timing the appearance of the silver halide precipitate.

The nucleophilic substitution mechanism

For a primary haloalkane such as bromoethane, the $\text{OH}^-$ lone pair attacks the $\delta+$ carbon while the $\text{C-Br}$ bond breaks heterolytically, releasing $\text{Br}^-$:

Introduction to analysis

Examples in context

Example 1. Hydrolysing haloalkanes with silver nitrate. A classic Edexcel practical compares 1-chlorobutane, 1-bromobutane and 1-iodobutane. Each is warmed with aqueous silver nitrate in ethanol; the haloalkane is hydrolysed, releasing a halide ion that reacts with $\text{Ag}^+$. The iodoalkane gives a yellow $\text{AgI}$ precipitate almost at once (weakest $\text{C-I}$ bond), the bromoalkane a cream $\text{AgBr}$ more slowly, and the chloroalkane a white $\text{AgCl}$ slowest of all. The order of rate directly demonstrates the trend in $\text{C-X}$ bond enthalpy.

Example 2. CFCs and the ozone layer. Chlorofluorocarbons such as $\text{CCl}_2\text{F}_2$ are haloalkanes that are very unreactive at ground level. In the stratosphere UV light breaks the weaker $\text{C-Cl}$ bond homolytically to give chlorine radicals, which catalyse the destruction of ozone. Recognising that the $\text{C-Cl}$ bond is broken (not $\text{C-F}$, which is far stronger) is the key chemistry, and it links the haloalkane bond-strength trend to a real environmental problem.

Try this

Q1. State the reagent and the product when a secondary alcohol is oxidised. [2 marks]

• Cue. Acidified potassium dichromate; the product is a ketone.

Q2. Explain why iodoalkanes react faster than chloroalkanes in nucleophilic substitution. [2 marks]

• Cue. The C-I bond is weaker than the C-Cl bond, so it breaks more easily.