C25: Alcohols, Carboxylic Acids and Esters
The functional groups, key reactions and properties of alcohols, carboxylic acids and esters, and how these important organic compounds are made and used.
Functional Groups
A functional group is the part of an organic molecule responsible for its characteristic chemical reactions.
| Homologous Series | Functional Group | General Formula | Example |
|---|---|---|---|
| Alcohols | −OH (hydroxyl) | CₙH₂ₙ₊₁OH | Ethanol: C₂H₅OH |
| Carboxylic acids | −COOH (carboxyl) | CₙH₂ₙ₊₁COOH | Ethanoic acid: CH₃COOH |
| Esters | −COO− (ester link) | Varies | Ethyl ethanoate: CH₃COOC₂H₅ |
Alcohols
Alcohols contain the −OH functional group. They are named by replacing the "e" of the corresponding alkane with "ol".
The first four alcohols:
| Name | Formula |
|---|---|
| Methanol | CH₃OH |
| Ethanol | C₂H₅OH |
| Propanol | C₃H₇OH |
| Butanol | C₄H₉OH |
Properties of Alcohols
- Alcohols are flammable — they burn in air to produce carbon dioxide and water
- The first three alcohols are soluble in water and produce neutral solutions
- Alcohols are used as fuels, solvents, and in alcoholic drinks
Alcohols dissolve in water to give neutral solutions. They also dissolve many organic compounds that water cannot dissolve, making them useful as solvents.
Reactions of Alcohols
Combustion
Alcohols burn in oxygen to produce carbon dioxide and water (complete combustion).
Worked Example: Combustion of ethanol
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
This reaction is exothermic, making alcohols useful as fuels.
Reaction with Sodium
Alcohols react with sodium metal to produce sodium alkoxide and hydrogen gas. The reaction is less vigorous than the reaction of sodium with water.
Worked Example: Ethanol with sodium
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂
Ethanol + sodium → sodium ethoxide + hydrogen
Observations: sodium sinks, gentle effervescence (hydrogen gas), sodium gradually disappears.
Oxidation of Alcohols
Alcohols can be oxidised to carboxylic acids using an oxidising agent such as acidified potassium dichromate(VI). The solution changes from orange to green.
Worked Example: Oxidation of ethanol
C₂H₅OH + 2[O] → CH₃COOH + H₂O
Ethanol is oxidised to ethanoic acid
The [O] represents the oxygen from the oxidising agent (acidified potassium dichromate(VI)).
Alcohols can also be oxidised by microbial action when exposed to air. This is how wine turns into vinegar (ethanoic acid) when left open.
Dehydration of Alcohols (Higher)
Alcohols can be dehydrated to form alkenes by passing the alcohol vapour over a hot catalyst (aluminium oxide, Al₂O₃) at about 300 °C. Water is eliminated.
Worked Example: Dehydration of ethanol
C₂H₅OH → C₂H₄ + H₂O
Ethanol → ethene + water
This is the reverse of the hydration of ethene.
Carboxylic Acids
Carboxylic acids contain the −COOH functional group. They are weak acids because they only partially ionise in water.
The first four carboxylic acids:
| Name | Formula |
|---|---|
| Methanoic acid | HCOOH |
| Ethanoic acid | CH₃COOH |
| Propanoic acid | C₂H₅COOH |
| Butanoic acid | C₃H₇COOH |
Carboxylic acids are weak acids because they partially dissociate in water: only some of the −COOH molecules release H⁺ ions. This means their pH is higher (less acidic) than a strong acid of the same concentration.
Reactions of Carboxylic Acids
Reaction with Carbonates
Carboxylic acids react with carbonates to produce a salt, carbon dioxide and water. This is the typical reaction of an acid with a carbonate.
Worked Example: Ethanoic acid with sodium carbonate
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O
Ethanoic acid + sodium carbonate → sodium ethanoate + carbon dioxide + water
Observation: effervescence (carbon dioxide gas produced).
Reaction with Metals
Worked Example: Ethanoic acid with magnesium
2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
Ethanoic acid + magnesium → magnesium ethanoate + hydrogen
Reaction with Bases (Alkalis)
Worked Example: Ethanoic acid with sodium hydroxide
CH₃COOH + NaOH → CH₃COONa + H₂O
Ethanoic acid + sodium hydroxide → sodium ethanoate + water
This is a neutralisation reaction producing a salt and water.
Reaction with Alcohols (Esterification)
Carboxylic acids react with alcohols in the presence of an acid catalyst (usually concentrated sulfuric acid) to form an ester and water.
Worked Example: Ester formation
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Ethanoic acid + ethanol ⇌ ethyl ethanoate + water
Conditions: concentrated sulfuric acid catalyst, heated under reflux
Esters have pleasant, fruity smells and are used in flavourings and perfumes.
Esters
Esters are formed when a carboxylic acid reacts with an alcohol. They contain the −COO− functional group (the ester link). Esters are named as "alkyl carboxylate".
How to name esters:
- The first part comes from the alcohol (replace "-ol" with "-yl")
- The second part comes from the carboxylic acid (replace "-oic acid" with "-oate")
Worked Example: Naming esters
Methanol + ethanoic acid → methyl ethanoate + water
Propanol + butanoic acid → propyl butanoate + water
Butanol + propanoic acid → butyl propanoate + water
Properties and Uses of Esters
- Esters have pleasant, fruity smells — they are used in perfumes and flavourings
- Esters are volatile (evaporate easily) — this helps them carry scent
- Esters are used as solvents for paints, inks and adhesives
- Esters are used as plasticisers to make polymers more flexible
The esterification reaction is a reversible reaction. It reaches dynamic equilibrium. The concentrated sulfuric acid catalyst also absorbs water, shifting the equilibrium to the right and increasing the ester yield.
| Property | Alcohols | Carboxylic Acids | Esters |
|---|---|---|---|
| Functional group | −OH | −COOH | −COO− |
| pH in water | Neutral (7) | Weakly acidic (4–6) | Neutral |
| Smell | Sharp / no smell | Pungent / vinegar | Pleasant, fruity |
| Solubility in water | Soluble (small alcohols) | Soluble (small acids) | Insoluble (do not dissolve) |
| Reaction with sodium | Produces H₂ | Produces H₂ (slow) | No reaction |
| Reaction with carbonates | No reaction | Produces CO₂ | No reaction |
Production of Ethanol
There are two main methods of producing ethanol:
Fermentation
Fermentation uses yeast to convert sugar (glucose) into ethanol and carbon dioxide. It occurs at 25–35 °C in the absence of air (anaerobic conditions).
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Glucose → ethanol + carbon dioxide
Advantages: uses renewable resources (sugar cane), low energy input, simple equipment.
Disadvantages: slow process, produces dilute ethanol (needs distillation), batch process.
Hydration of Ethene
Hydration of ethene uses ethene (from cracking) and steam at high temperature and pressure with a phosphoric acid catalyst.
C₂H₄ + H₂O ⇌ C₂H₅OH
Conditions: 300 °C, 60–70 atm, phosphoric acid catalyst
Advantages: fast, continuous process, produces pure ethanol.
Disadvantages: uses non-renewable crude oil, requires high energy input, reversible reaction so not all ethene converts.
When comparing fermentation and hydration, consider: raw materials (renewable vs non-renewable), energy requirements, speed, purity of product, and type of process (batch vs continuous).
Practice Questions
1. Write an equation for the complete combustion of methanol (CH₃OH). (2 marks)
2CH₃OH + 3O₂ → 2CO₂ + 4H₂O
2. Describe the observations when sodium is added to ethanol. (2 marks)
The sodium sinks in the ethanol. Bubbles of hydrogen gas are produced (gentle effervescence). The sodium gradually dissolves and disappears, forming sodium ethoxide solution.
3. Ethanoic acid is described as a weak acid. Explain what this means. (2 marks)
Ethanoic acid partially dissociates in water. Only some of the CH₃COOH molecules release H⁺ ions, so it produces a lower concentration of H⁺ ions than a strong acid of the same concentration.
4. Write the equation for the formation of ethyl ethanoate from ethanoic acid and ethanol. Name the catalyst used. (3 marks)
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The catalyst is concentrated sulfuric acid.
5. Compare the advantages and disadvantages of producing ethanol by fermentation and by hydration of ethene. (4 marks)
Fermentation uses renewable resources (sugar) whereas hydration uses non-renewable crude oil. Fermentation requires low energy (30 °C) whereas hydration needs high temperature and pressure (300 °C, 70 atm). Fermentation is slow and produces impure ethanol requiring distillation, while hydration is fast and produces pure ethanol. Fermentation is a batch process; hydration is a continuous process.
Required Practical
Investigating the Combustion of Alcohols
Aim: To investigate how the number of carbon atoms in an alcohol affects the energy released per gram during combustion.
Method: Use a spirit burner containing a known alcohol. Measure 100 cm³ of water into a copper calorimeter. Record the initial temperature of the water. Weigh the spirit burner with its lid on. Place the burner under the calorimeter, remove the lid and light the wick. Heat the water for 3 minutes, stirring continuously. Replace the lid on the burner to extinguish the flame and reweigh the burner. Record the final temperature of the water. Repeat for different alcohols (methanol, ethanol, propanol, butanol).
Independent variable: Type of alcohol (number of carbon atoms)
Dependent variable: Temperature rise of water per gram of fuel burned
Control variables: Volume of water, distance from wick to calorimeter, type of calorimeter, time heated, starting temperature
Energy calculation: Energy (J) = mass of water (g) × specific heat capacity of water (4.18 J/g°C) × temperature change (°C)
Improvements: Use a draught shield to reduce heat loss to surroundings. Use a lid on the calorimeter to reduce heat loss by evaporation. Use a copper calorimeter rather than a glass beaker (better conductor).
Maths Skills
Temperature Change Calculations from the Combustion Practical
Calculate the energy transferred to the water and the energy per gram or per mole of fuel.
Step 1: Calculate temperature change: ΔT = T_final − T_initial
Step 2: Calculate energy transferred: Q = m × c × ΔT
where m = mass of water (g), c = 4.18 J/g°C, ΔT = temperature change
Step 3: Calculate energy per gram: Energy per gram = Q ÷ mass of fuel burned (g)
Example: 100 g of water heated from 20 °C to 45 °C. Mass of ethanol burned = 0.82 g.
Q = 100 × 4.18 × 25 = 10 450 J
Energy per gram = 10 450 ÷ 0.82 = 12 744 J/g = 12.7 kJ/g
Note: The actual energy released is higher because significant heat is lost to the surroundings. This practical always gives lower values than data book values.
Common Misconceptions
Misconceptions About Alcohols
All alcohols are drinkable like ethanol.
Only ethanol is found in alcoholic drinks. Methanol is highly toxic — even small amounts can cause blindness or death. Other alcohols (propanol, butanol) are also not safe for consumption. Ethanol is the only alcohol used in drinks.
Carboxylic acids are strong acids like hydrochloric acid.
Carboxylic acids are weak acids — they only partially dissociate in water. They react more slowly than strong acids and produce a lower concentration of H⁺ ions at the same concentration. For example, 0.1 mol/dm³ ethanoic acid has a higher pH than 0.1 mol/dm³ HCl.
Esterification is the same as neutralisation.
Esterification is a reaction between an alcohol and a carboxylic acid to form an ester and water. Neutralisation is a reaction between an acid and a base/alkali to form a salt and water. Both produce water, but the products and reactants are completely different.
6-Mark Question
Describe the reactions of alcohols and compare them to the reactions of water with sodium.
Alcohols burn in air, producing carbon dioxide and water — this is combustion, and they can be used as fuels. Alcohols react with sodium metal to produce an alkoxide and hydrogen gas (e.g. 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂). This is similar to the reaction of water with sodium (2H₂O + 2Na → 2NaOH + H₂) — both produce hydrogen gas. However, the reaction of sodium with water is extremely vigorous and potentially dangerous, while the reaction with alcohols is much slower and less violent. Alcohols can also be oxidised to carboxylic acids (e.g. ethanol → ethanoic acid) using an oxidising agent such as acidified potassium dichromate(VI). Alcohols react with carboxylic acids in the presence of an acid catalyst to form esters and water. Water does not undergo oxidation to form a carboxylic acid or react with carboxylic acids to form esters — these are reactions specific to the alcohol functional group (−OH).
AO3: Analysis and Evaluation
Evaluating the Combustion Practical
A student carried out the combustion of alcohols practical and obtained energy values much lower than data book values. The student's results for methanol were 6.2 kJ/g compared to the data book value of 22.7 kJ/g.
Evaluate the practical method and explain why the student's value is so much lower. Suggest specific improvements.
Answer: The student's value is much lower because a large proportion of the heat energy is lost to the surroundings rather than being transferred to the water. Heat is lost through: the calorimeter being open-topped (evaporation), draughts carrying heat away, heat radiating from the flame sideways and upwards rather than going into the water, and the calorimeter absorbing some heat. Improvements: use a draught shield around the burner and calorimeter, place a lid on the calorimeter, use a wider calorimeter to capture more heat, reduce the distance between the wick and the calorimeter base, insulate the system, and use a spirit burner with a wick that produces a cleaner flame.
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