Functional groups and properties of alcohols and carboxylic acids, fermentation and industrial hydration of ethene, reactions of ethanol (combustion, oxidation, dehydration, esterification), reactions of ethanoic acid, ester formation and hydrolysis, saponification, and comparison of soapy and soapless detergents.
Alcohols and carboxylic acids each carry a specific functional group that controls their chemistry. Both series are water-soluble at shorter chain lengths, weakly acidic or reacting as acids, and connected to each other through oxidation — ethanol oxidises to ethanoic acid. Esters, formed from the reaction between these two families, are responsible for the fragrances of fruits, flowers, and many food flavourings.
The alcohol functional group is the hydroxyl group: –OH.
General formula:
| Name | Formula | Condensed formula |
|---|---|---|
| Methanol | CH₄O | CH₃OH |
| Ethanol | C₂H₆O | C₂H₅OH or CH₃CH₂OH |
| Propan-1-ol | C₃H₈O | CH₃CH₂CH₂OH |
| Propan-2-ol | C₃H₈O | CH₃CH(OH)CH₃ |
The hydroxyl group allows alcohols to form hydrogen bonds with water, making short-chain alcohols completely miscible with water. As chain length increases, the non-polar carbon chain dominates and solubility decreases.
Ethanol is produced industrially (and in winemaking and rum manufacture) by the fermentation of sugars by yeast. Yeast contains enzymes that catalyse the anaerobic breakdown of glucose:
Conditions required:
Carbon dioxide is produced as a by-product — the bubbling seen during fermentation is CO₂ escaping. The process stops when the alcohol concentration reaches about 15%, because yeast is inhibited by higher concentrations. Distillation is used to purify and concentrate the ethanol.
In the Caribbean, rum is produced from the fermentation of sugarcane molasses. Wine is produced from the fermentation of grape juice.
Ethanol is also produced on a large scale by the addition of steam to ethene at high temperature and pressure with a phosphoric acid catalyst:
This method is faster and produces purer ethanol than fermentation, but requires ethene from petroleum and is therefore not renewable.
Ethanol burns completely in excess oxygen to produce carbon dioxide and water. The flame is blue:
Ethanol is used as a fuel (bioethanol) and as a fuel additive.
Ethanol reacts with sodium metal to produce hydrogen gas and sodium ethoxide:
The reaction is less vigorous than sodium with water, but the same hydrogen gas is produced. This demonstrates the weakly acidic nature of the –OH group.
When ethanol is heated with excess concentrated sulfuric acid at about 170 °C, water is eliminated and ethene is formed:
This is the reverse of hydration and shows that ethanol and ethene are interconvertible.
Ethanol is oxidised to ethanoic acid when treated with acidified potassium dichromate(VI) or potassium manganate(VII). In this oxidation, the orange dichromate turns green:
This reaction is also what occurs naturally when wine is left open to air — bacteria oxidise the ethanol to ethanoic acid, producing vinegar.
Ethanol reacts with carboxylic acids in the presence of a concentrated sulfuric acid catalyst (and heat) to form esters. This is covered in detail below.
The carboxylic acid functional group is: –COOH (the carboxyl group).
General formula:
| Name | Formula | Condensed formula |
|---|---|---|
| Methanoic acid | HCOOH | HCOOH |
| Ethanoic acid | C₂H₄O₂ | CH₃COOH |
| Propanoic acid | C₃H₆O₂ | CH₃CH₂COOH |
Carboxylic acids are weak acids — they ionise only partially in water:
Despite being weak acids, they display all the characteristic acid reactions:
| Reaction | Equation | Products |
|---|---|---|
| With metals | 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂ | Salt + hydrogen |
| With metal oxides | 2CH₃COOH + CuO → (CH₃COO)₂Cu + H₂O | Salt + water |
| With hydroxides | CH₃COOH + NaOH → CH₃COONa + H₂O | Salt + water |
| With carbonates | 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ | Salt + water + CO₂ |
| With alcohols | CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O | Ester + water |
An ester is the compound formed when a carboxylic acid reacts with an alcohol, with the elimination of water. The reaction is reversible and requires a concentrated sulfuric acid catalyst and heat:
The product here is ethyl ethanoate (ethyl acetate). The ester is named from the alcohol first (ethyl, from ethanol) then the acid (ethanoate, from ethanoic acid).
Esters have characteristic sweet, fruity smells and are widely used as:
The functional group of an ester is –COO–, which links the two carbon chains.
Esterification is reversible — esters can be broken down back into the alcohol and acid by hydrolysis (reaction with water), catalysed by acid or alkali:
Acid hydrolysis:
Saponification is the alkaline hydrolysis of fats (which are esters of glycerol and long-chain carboxylic acids called fatty acids). When fats or oils are heated with sodium hydroxide solution, they break down to produce soap (sodium salt of the fatty acid) and glycerol:
This is the basis of traditional soap manufacture.
Both soaps and soapless detergents are cleaning agents with a similar molecular structure: a long non-polar hydrocarbon tail (attracted to grease) and a polar ionic head (attracted to water). This allows them to surround and emulsify grease particles.
| Feature | Soap | Soapless detergent |
|---|---|---|
| Source | Made from natural fats/oils (saponification) | Synthesised from petroleum products |
| Behaviour in hard water | Forms scum (insoluble calcium/magnesium salts) | Does not form scum — effective in hard water |
| Behaviour in soft water | Effective | Effective |
| Biodegradability | Generally biodegradable | Some are not biodegradable; can cause foam in rivers |
| Environmental impact | Lower | Higher (persistent foam, aquatic toxicity) |
The key distinction for exams: soaps are ineffective in hard water because Ca²⁺ and Mg²⁺ ions react with soap molecules to form an insoluble scum. Soapless detergents do not react with these ions, so they lather and clean effectively in hard water.