Describe the properties and reactions of ethanol and ethanoic acid, including the production of ethanol, the oxidation of ethanol to ethanoic acid, and the formation of an ester

What this dot point is asking

SEAB wants you to describe the properties and reactions of ethanol (an alcohol) and ethanoic acid (a carboxylic acid): how ethanol is made, how it burns, how it is oxidised to ethanoic acid, the acid reactions of ethanoic acid, and how the two combine to form an ester. This completes organic chemistry and ties together oxidation (from the redox topic) and acid reactions (from the acids topic).

The answer

Ethanol and how it is made

Ethanol ($\text{C}_2\text{H}_5\text{OH}$) is an alcohol: it contains the $-\text{OH}$ (hydroxyl) group. It is a colourless liquid used as a fuel, a solvent, and in alcoholic drinks. There are two ways to make it:

• Fermentation: glucose (sugar) is broken down by yeast at a warm temperature (about $30\ ^\circ\text{C}$) in the absence of air, giving ethanol and carbon dioxide. This uses a renewable resource but is slow and gives impure ethanol.
• From ethene: ethene reacts with steam at high temperature and pressure with a catalyst, adding water across the double bond (hydration) to give ethanol. This is fast and gives pure ethanol but uses crude oil.

Combustion of ethanol

Ethanol burns completely in air to give carbon dioxide and water, releasing energy, which is why it is used as a fuel:

Oxidation of ethanol to ethanoic acid

Ethanol can be oxidised to ethanoic acid ($\text{CH}_3\text{COOH}$). This happens slowly when ethanol is left open to the air (with bacteria), turning wine sour into vinegar, or quickly using an oxidising agent such as acidified potassium manganate(VII) (the purple colour fades as it is reduced). Ethanoic acid is the acid in vinegar.

Ethanoic acid as a weak acid

Ethanoic acid is a carboxylic acid containing the $-\text{COOH}$ group. It is a weak acid (only partially ionised), but it still shows the typical acid reactions:

• With a reactive metal: gives a salt (an ethanoate) and hydrogen (effervescence).
• With a carbonate: gives a salt, water and carbon dioxide (effervescence).
• With a base or alkali: gives a salt and water (neutralisation).

These mirror the characteristic acid reactions from the acids and salts topic.

Esterification

When ethanoic acid reacts with ethanol (with an acid catalyst), they form an ester (ethyl ethanoate) and water:

ethanoic acid + ethanol gives ethyl ethanoate + water.

Esters are sweet-smelling liquids used in flavourings and perfumes. The formation of an ester from a carboxylic acid and an alcohol is called esterification, and it links the two organic compounds of this dot point.

Examples in context

Example 1. Brewing and baking. Fermentation of sugars by yeast produces ethanol in brewing and the carbon dioxide that makes bread rise in baking. The same reaction, valued for different products, shows how a single piece of organic chemistry underpins two everyday industries.

Example 2. Vinegar and esters in food. Vinegar is dilute ethanoic acid made by oxidising ethanol, and the pleasant smells of many fruits and the flavours added to sweets are esters formed from acids and alcohols. The chemistry of this dot point appears directly in the kitchen, from souring to fragrance.

Try this

Q1. State the conditions needed for the fermentation of glucose to ethanol. [1 mark]

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

Q2. Name the product when ethanol is oxidised, and one way this oxidation can be carried out. [2 marks]

• Cue. Ethanoic acid; by leaving ethanol open to the air with bacteria, or by using acidified potassium manganate(VII).

Q3. Name the type of compound formed when ethanoic acid reacts with ethanol, and the other product. [2 marks]

• Cue. An ester (ethyl ethanoate); the other product is water.