Classify alcohols and describe their oxidation, dehydration, substitution and combustion reactions.

What this dot point is asking

TASC expects you to classify alcohols and describe their characteristic reactions with correct equations and observations.

Classifying alcohols

The class depends on how many carbon atoms are attached to the carbon that carries the hydroxyl group:

• Primary: the hydroxyl carbon is attached to one other carbon.
• Secondary: attached to two other carbons.
• Tertiary: attached to three other carbons.

This matters because the class controls the oxidation products.

Oxidation

Oxidising agents such as acidified potassium dichromate or acidified potassium permanganate oxidise alcohols.

The orange dichromate ion ($\text{Cr}_2\text{O}_7^{2-}$) turns green ($\text{Cr}^{3+}$) as it is reduced, and the purple permanganate ion ($\text{MnO}_4^-$) fades, so a colour change is a useful test for an oxidisable alcohol. To isolate an aldehyde from a primary alcohol you distil it off as it forms (before it is oxidised further); to make the carboxylic acid you heat under reflux with excess oxidiser.

Dehydration

Heating an alcohol with a concentrated acid catalyst (or passing its vapour over hot aluminium oxide) removes a molecule of water to form an alkene. For ethanol:

This is an elimination reaction, the reverse of the hydration of an alkene. With longer alcohols, more than one alkene can form depending on which neighbouring hydrogen is removed.

Substitution and combustion

Alcohols react with hydrogen halides (or with phosphorus halides) to replace the hydroxyl group with a halogen, forming a haloalkane: $\text{CH}_3\text{CH}_2\text{OH} + \text{HBr} \rightarrow \text{CH}_3\text{CH}_2\text{Br} + \text{H}_2\text{O}$. This is a substitution reaction. Alcohols also burn in oxygen; complete combustion gives carbon dioxide and water and releases energy, which is why ethanol is used as a fuel: $\text{CH}_3\text{CH}_2\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$. Incomplete combustion in limited oxygen produces carbon monoxide and soot.

Why class controls the oxidation product

The oxidation of an alcohol removes the hydrogen on the carbon that carries the hydroxyl group (the alpha hydrogen) along with a hydrogen from the $\text{O}-\text{H}$, forming a carbon-oxygen double bond. A primary alcohol has two alpha hydrogens, so after forming the aldehyde a further hydrogen remains that can be removed (with addition of oxygen) to give a carboxylic acid. A secondary alcohol has only one alpha hydrogen, so once the ketone forms there is no alpha hydrogen left and oxidation stops. A tertiary alcohol has no alpha hydrogen at all, which is why it cannot be oxidised by these reagents. Seeing oxidation this way makes the class rule easy to reconstruct rather than memorise.

Alcohols in context

Alcohols sit at the centre of organic synthesis because they can be made from alkenes (by hydration) and converted into aldehydes, ketones, carboxylic acids, esters, alkenes and haloalkanes. This versatility is why the reactions of alcohols recur throughout multistep synthesis questions. Ethanol in particular is both a major industrial solvent and a renewable fuel produced by fermentation, and the controlled oxidation of alcohols underpins the manufacture of many flavour and fragrance compounds.

In the exam, classify the alcohol first, then state the oxidation product for its class, write the dehydration product as an alkene, show substitution giving a haloalkane, and use the dichromate or permanganate colour change as evidence of oxidation.