Alcohols

Alcohols are a family of organic compounds that all contain the functional group $-\text{OH}$ (an oxygen and hydrogen joined to a carbon). Their names end in -ol, for example ethanol.

Recognising and naming alcohols

• Functional group: $-\text{OH}$
• General form: $\text{C}_n\text{H}_{2n+1}\text{OH}$ for simple alcohols
• Number shows the carbon that carries the $-\text{OH}$: propan-1-ol (end carbon), propan-2-ol (middle carbon)
• Examples: methanol ($\text{CH}_3\text{OH}$), ethanol ($\text{C}_2\text{H}_5\text{OH}$)

Properties and uses

• Small alcohols are colourless liquids that mix well with water.
• They are flammable and release energy when burned.
• Uses of ethanol: solvent (perfumes, inks, hand sanitisers) and fuel (spirit burners, biofuels).

Making ethanol

1) Fermentation (from plant sugars)

Aqueous glucose is converted to ethanol by yeast at 25–35 °C, without oxygen.

\[ \text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\;\text{yeast, 25\!–\!35^\circ\!\text{C}, no O}_2\;} 2\,\text{C}_2\text{H}_5\text{OH} + 2\,\text{CO}_2 \]

• Advantages: renewable raw materials (sugar), low temperature/pressure.
• Disadvantages: slow, batch process, impure ethanol (needs distillation), produces $\text{CO}_2$.

2) Addition of steam to ethene (from crude oil)

Ethene reacts with steam at 300 °C and 60 atm with an acid catalyst (e.g. phosphoric acid).

$$\text{C}_2\text{H}_4 + \text{H}_2\text{O} \xrightarrow[\;60\,\text{atm}\;]{\;300^\circ\!\text{C},\ \text{acid cat.}\;} \text{C}_2\text{H}_5\text{OH}$$

• Advantages: fast, continuous, high purity, no $\text{CO}_2$ by-product.
• Disadvantages: uses non-renewable ethene, high energy cost.

Key reactions of ethanol

Combustion (burning)

Complete combustion produces carbon dioxide and water.

$$\text{C}_2\text{H}_5\text{OH} + 3\,\text{O}_2 \rightarrow 2\,\text{CO}_2 + 3\,\text{H}_2\text{O}$$

Oxidation to ethanoic acid (vinegar)

• Chemical oxidation: acidified potassium manganate(VII) turns ethanol into ethanoic acid.
• Bacterial oxidation in air does the same during vinegar production.

$$\text{C}_2\text{H}_5\text{OH} + \text{[O]} \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$$

Common misconceptions

• Fermentation needs no oxygen; with oxygen, ethanol is oxidised to acid.
• Complete combustion gives only $\text{CO}_2$ and $\text{H}_2\text{O}$; limited oxygen makes carbon monoxide or soot.
• Methanol and ethanol are different; methanol is toxic.