Ch 1 · Carboxylic acids · AS & A Level Chemistry 9701
Topic 1 of 2 in Carboxylic acids and derivatives — AS & A Level Chemistry 9701 — 57 lessons.
Introduction
Carboxylic acids, -COOH, are weak acids whose chemistry links the alcohols and carbonyl compounds already met to esters and amides met later. This lesson covers how they are made, how they react as acids, and how the carboxyl group is converted into an ester or reduced back to an alcohol.
Key ideas — Preparation of carboxylic acids
Primary alcohols or aldehydes are oxidised by hot acidified potassium dichromate(VI) or manganate(VII), heated under reflux with an excess of oxidant, to give a carboxylic acid: CH_3CH_2OH + 2[O] arrow CH_3COOH + H_2O Nitriles are hydrolysed by heating under reflux with dilute acid (or with alkali, then acidifying) to give a carboxylic acid and ammonium ions: CH_3CN + H_2O + HCl arrow CH_3COOH + NH_4Cl Esters are also hydrolysed by dilute acid or alkali to give a carboxylic acid (or its salt) and an alcohol.
🧑🏫 Teacher's explanation
Think of a carboxylic acid as a chemical 'middle ground'. It is more oxidized than a primary alcohol or an aldehyde, but less oxidized than carbon dioxide. To make it, you need to add oxygen atoms or remove nitrogen-based groups.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of ethanoic acid produced from the complete oxidation of 4.6 g of ethanol (CH_3 extCH_2 extOH), assuming a 100% yield. A_r: C=12, H=1, O=16.
🔍 Worked example — Standard
A sample of propanenitrile (CH_3 extCH_2 extCN) is hydrolyzed using dilute HCl under reflux. Calculate the mass of propanoic acid (CH_3 extCH_2 extCOOH) produced from 0.10 mol of the nitrile. A_r: C=12, H=1, N=14, O=16.
🔍 Worked example — Exam-style
Explain why heating a primary alcohol with acidified potassium dichromate(VI) under reflux produces a carboxylic acid, whereas heating it with the oxidant without reflux produces an aldehyde. (Conceptual/Calculation focus: If 0.5 mol of propan-1-ol is oxidized to propanoic acid, how many moles of oxygen atoms are consumed?)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the type of reaction occurring. [1] (b) State the necessary conditions (reagents and technique) to ensure the reaction goes to completion. [2] (c) Write a balanced chemical equation for this reaction using [O] to represent the oxygen provided by the oxidant. [2] [5 marks]
Key ideas — Reactions with reactive metals, alkalis and carbonates
Carboxylic acids are weak acids, only partially dissociating in water, RCOOH leftharpoons RCOO^- + H^+, but they still show typical acid reactions. With a reactive metal such as magnesium, hydrogen gas is released: 2CH_3COOH + Mg arrow (CH_3COO)_2Mg + H_2. With an alkali, neutralisation forms a salt and water. With a carbonate, effervescence of carbon dioxide occurs: 2CH_3COOH + Na_2CO_3 arrow 2CH_3COONa + H_2O + CO_2; this last reaction distinguishes carboxylic acids from alcohols and phenols, neither of which release carbon dioxide from a carbonate.
🧑🏫 Teacher's explanation
Think of a carboxylic acid like a person who is polite but not overly eager to give away their belongings. In chemistry terms, they are 'weak acids.' This means that when you put them in water, they don't completely fall apart into ions; instead, they exist in a delicate balance, with only a small fraction of molecules releasing a hydrogen ion (H^+). This is why they are less reactive than strong acids like HCl.\n\nBecause they have that H^+ ion available, they behave like any other acid. If you give them a reactive metal, they'll swap that H^+ for a metal ion and release hydrogen gas. If you give them a base (alkali), they perform a standard neutralization. If you give them a carbonate, they react to produce salt, water, and carbon dioxide gas. Imagine a fizzy drink: that bubbling you see when you drop an antacid tablet into water is exactly what happens when an acid meets a carbonate.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of magnesium required to react completely with 46.0 g of ethanoic acid (CH_3 extCOOH) in the reaction: 2CH_3 extCOOH + Mg arrow (CH_3 extCOO)_2Mg + H_2. (Ar: C=12, H=1, O=16, Mg=24)
🔍 Worked example — Standard
A student reacts 0.100 mol of propanoic acid (C_2H_5COOH) with excess sodium carbonate (Na_2CO_3). Calculate the volume of CO_2 gas produced at RTP. (Assume molar volume of gas at RTP is 24 dm^3 mol^-1)
🔍 Worked example — Exam-style
Identify the products of the reaction between ethanoic acid and sodium hydroxide (NaOH), including state symbols, and determine if the resulting salt is acidic, basic, or neutral.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Esterification
Warmed with an alcohol in the presence of a concentrated sulfuric acid catalyst, a carboxylic acid undergoes a reversible condensation reaction to form an ester: CH_3COOH + C_2H_5OH leftharpoons CH_3COOC_2H_5 + H_2O
🧑🏫 Teacher's explanation
Think of esterification like a dance where two people (the acid and the alcohol) want to hold hands. However, they are currently too 'clunky' to connect. To make them stick, you need a 'matchmaker'—the concentrated sulfuric acid. This catalyst doesn't just help them meet; it also helps pull out a small molecule of water to allow the bond to form.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of ethyl ethanoate, CH_3 extCOOC_2 extH_5. (Relative atomic masses: C=12, H=1, O=16).
🔍 Worked example — Standard
A student reacts 0.20 mol of ethanoic acid with 0.50 mol of ethanol. Calculate the maximum yield of ethyl ethanoate in moles, assuming the reaction goes to completion.
🔍 Worked example — Exam-style
Calculate the mass of ethyl ethanoate produced from 12.0 g of ethanoic acid reacted with excess ethanol. (Molar masses: CH_3COOH = 60, CH_3COOC_2H_5 = 88).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Reduction to primary alcohols
Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride, LiAlH_4, in dry ether — a stronger reducing agent than NaBH_4, which cannot reduce carboxylic acids: CH_3COOH + 4[H] arrow CH_3CH_2OH + H_2O
🧑🏫 Teacher's explanation
Think of a carboxylic acid as a molecule with a 'high-energy' functional group (the carboxyl group) that is quite stubborn. To turn it into an alcohol, you need to add hydrogen atoms to the carbon atom that is currently double-bonded to oxygen. This is like trying to force a heavy door open; a gentle push won't work, you need a powerful shove.
📖 Definitions
🔍 Worked example — Easy
Calculate the number of moles of ethanol produced when 0.10 mol of ethanoic acid is reduced using excess LiAlH_4. The equation is: CH_3 extCOOH + 4[H] arrow CH_3 extCH_2 extOH + H_2 extO
🔍 Worked example — Standard
A student reduces 4.4 g of propanoic acid (CH_3CH_2COOH) using excess LiAlH_4. Calculate the mass of propan-1-ol (CH_3CH_2CH_2OH) produced. (M_r: C=12, H=1, O=16)
🔍 Worked example — Exam-style
Explain why NaBH_4 cannot be used to reduce ethanoic acid to ethanol, whereas LiAlH_4 can.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Physical properties
Carboxylic acid molecules hydrogen-bond strongly to one another, often pairing up as hydrogen-bonded dimers in the liquid or vapour state, so short-chain carboxylic acids have higher boiling points than alcohols of similar relative molecular mass. The short-chain members, such as methanoic and ethanoic acid, are fully miscible with water because they can also hydrogen-bond to water molecules through both the mathrmC=O and mathrmO-H parts of the carboxyl group; as the length of the non-polar hydrocarbon chain increases, this water solubility decreases.
🧑🏫 Teacher's explanation
Think of carboxylic acids as the 'social butterflies' of organic chemistry. They have two different ways to connect with others: they can act as acids (donating a proton, H^+) or they can act as nucleophiles. Because they have both a carbonyl group (C=O) and a hydroxyl group (O-H), they are incredibly good at forming strong hydrogen bonds, often pairing up in 'dimers' like two people holding hands with both hands.
📖 Definitions
🔍 Worked example — Easy
Write a balanced equation for the reaction between ethanoic acid and sodium carbonate, including state symbols.
🔍 Worked example — Standard
Calculate the molar mass of ethanoic acid (CH_3 extCOOH) and determine how many moles are in 46.0g. (Use A_r: C=12, H=1, O=16)
🔍 Worked example — Exam-style
Complete the equation for the esterification of propanoic acid with methanol, including the catalyst required.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Write the equation for this reaction, including state symbols. [2] (b) Identify the type of reaction occurring. [1] [3 marks]
Worked example
Question. A student has two unlabelled bottles, one containing ethanoic acid and the other ethanol. (a) Describe a simple chemical test, including the observation, that distinguishes the two liquids. [2] (b) Write an equation for the reaction used in (a). [1] (c) Explain, in terms of acidity, why ethanoic acid reacts with sodium carbonate but ethanol does not. [2]
🔍 Worked example — Easy
Identify the products of the reaction between propanoic acid and sodium hydroxide.
🔍 Worked example — Exam-style
A student reacts 0.10 mol of ethanoic acid with 0.10 mol of ethanol using a concentrated H_2SO_4 catalyst. Write the balanced equation for the formation of the ester.
⚠️ Common errors
Common mistakes
Practice
Answers