Organic reaction pathways: WACE Year 12 Chemistry

What this dot point is asking

Each organic functional group reacts in characteristic ways. Knowing these reactions lets you predict products and design reaction pathways, sequences of steps that convert a starting material into a target compound.

Substitution (alkanes and haloalkanes)

Alkanes are unreactive but undergo substitution with halogens in ultraviolet light: an H atom is replaced by a halogen atom, e.g. $\text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV}} \text{CH}_3\text{Cl} + \text{HCl}$. This is a free-radical reaction that gives a mixture of products.

Haloalkanes undergo substitution with aqueous hydroxide to form alcohols: $\text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Br}^-$. This is a useful way to introduce an OH group.

Addition (alkenes)

Alkenes are reactive because of the C=C double bond and undergo addition, where the double bond opens and atoms add across it:

• Hydrogenation: $\text{CH}_2=\text{CH}_2 + \text{H}_2 \rightarrow \text{CH}_3\text{CH}_3$ (nickel catalyst).
• Halogenation: addition of $\text{Br}_2$ (the rapid decolourising of bromine water is the test for a C=C).
• Hydration: addition of water (steam, acid catalyst) gives an alcohol: $\text{CH}_2=\text{CH}_2 + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{CH}_2\text{OH}$.
• Hydrohalogenation: addition of $\text{HX}$ gives a haloalkane.

Oxidation (alcohols)

Primary alcohols are oxidised (by acidified dichromate or permanganate) first to an aldehyde and then to a carboxylic acid. Secondary alcohols oxidise to a ketone. Tertiary alcohols are not readily oxidised, because the carbon bearing the OH has no H to remove. For example:

The orange dichromate turning green is the visual sign of oxidation.

Esterification (alcohol plus carboxylic acid)

A carboxylic acid reacts with an alcohol, with a concentrated sulfuric acid catalyst, to form an ester and water. This is a reversible condensation reaction:

Esters have characteristic sweet, fruity smells.

Building a pathway

A reaction pathway links these single steps. To plan one, identify the functional group of the start and target, then choose reactions that step from one family to the next. For example, to convert ethene to ethyl ethanoate, you would hydrate ethene to ethanol, oxidise some ethanol to ethanoic acid, then esterify the two.

When answering pathway questions in the WACE examination, give the reagents and conditions for every step, write balanced equations, and identify the functional group formed at each stage.