Describe the alcohols and carboxylic acids as homologous series, including the production and reactions of ethanol and ethanoic acid and the formation of esters

What this dot point is asking

SEAB wants you to describe alcohols and carboxylic acids as homologous series, focusing on ethanol and ethanoic acid: how ethanol is made (fermentation and hydration of ethene), how it is oxidised to ethanoic acid, the acid reactions of ethanoic acid, and how an alcohol and a carboxylic acid form an ester. The fermentation conditions and the oxidation link are common exam marks.

The answer

The alcohol and carboxylic acid families

• Alcohols are a homologous series with the $\text{-OH}$ (hydroxyl) functional group; the general formula is $\text{C}_n\text{H}_{2n+1}\text{OH}$. Ethanol is $\text{C}_2\text{H}_5\text{OH}$.
• Carboxylic acids are a homologous series with the $\text{-COOH}$ (carboxyl) functional group. Ethanoic acid is $\text{CH}_3\text{COOH}$.

Members of each series share chemical properties because they share a functional group.

Making ethanol

There are two routes:

• Fermentation: glucose with yeast, in the absence of air, at about $30\ \text{to}\ 40\ ^\circ\text{C}$: $\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$. It uses a renewable resource but is slow and gives impure, dilute ethanol.
• Hydration of ethene: ethene plus steam over a phosphoric acid catalyst at high temperature and pressure: $\text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_5\text{OH}$. It is fast and gives pure ethanol but uses ethene from finite crude oil.

Reactions of ethanol

• Combustion: ethanol burns in air to give carbon dioxide and water, releasing energy, so it is used as a fuel.
• Oxidation: ethanol is oxidised to ethanoic acid by an oxidising agent such as acidified potassium manganate(VII) (purple to colourless) or acidified potassium dichromate (orange to green). The same oxidation happens slowly in air, turning wine sour.

Reactions of ethanoic acid

Ethanoic acid is a typical weak acid, so it shows the usual acid reactions:

• with reactive metals it gives a salt (an ethanoate) and hydrogen,
• with carbonates it gives a salt, water and carbon dioxide,
• with bases (alkalis) it gives a salt and water.

For example, $2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$.

Esters

When a carboxylic acid reacts with an alcohol, warmed with a little acid catalyst, an ester forms (plus water). Ethanoic acid plus ethanol gives ethyl ethanoate. Esters are sweet-smelling liquids used in flavourings and perfumes.

Examples in context

Example 1. Alcohol as a renewable fuel. Ethanol made by fermenting sugar cane is blended into petrol in some countries because it burns cleanly and comes from a renewable crop. This uses the combustion of an alcohol and the sustainability advantage of fermentation over the crude-oil route.

Example 2. Vinegar and food flavours. Vinegar is dilute ethanoic acid, formed when the ethanol in wine or cider is oxidised by bacteria in air. The esters made from carboxylic acids and alcohols give many fruits and sweets their characteristic smells, linking these two homologous series to everyday food chemistry.

Try this

Q1. State the conditions needed for the fermentation of glucose to ethanol. [2 marks]

• Cue. Yeast, in the absence of air (anaerobic), at a warm temperature of about $30\ \text{to}\ 40\ ^\circ\text{C}$.

Q2. Name the product and the colour change when ethanol is oxidised by acidified potassium manganate(VII). [2 marks]

• Cue. The product is ethanoic acid; the colour changes from purple to colourless.

Q3. Name the type of compound formed when ethanoic acid reacts with ethanol, and state one use of such compounds. [2 marks]

• Cue. An ester; used in flavourings or perfumes (sweet-smelling).