Ch 1 · Carboxylic acids · AS & A Level Chemistry 9701

Topic 1 of 3 in Carboxylic acids and derivatives — AS & A Level Chemistry 9701 — 56 lessons.

Introduction

A2 extends AS carboxylic acid chemistry in two directions: a new preparation from the side chain of an alkylbenzene, and a deeper, comparative treatment of acidity that explains why carboxylic acids, phenols and alcohols differ so much in acid strength, and why substituents on the carbon chain change that strength further.

Key ideas — Benzoic acid from an alkylbenzene

Any alkylbenzene with a side chain containing at least one hydrogen on the carbon attached to the ring is oxidised by hot acidified potassium manganate(VII) under reflux to benzoic acid, regardless of the length of the original side chain, since the whole chain except the ring carbon is removed as carbon dioxide and water: C_6H_5CH_3 + 3[O] arrow C_6H_5COOH + H_2O

🧑‍🏫 Teacher's explanation

Think of an alkylbenzene like a tree with a long branch. When you use a powerful chemical 'chainsaw' like hot acidified potassium manganate(VII), it doesn't just trim the branch; it chops off everything on that branch except for the single point where it was attached to the trunk. This single point becomes the carboxylic acid group (-COOH).

📖 Definitions

🔍 Worked example — Standard

A sample of ethylbenzene is oxidized using hot acidified KMnO_4. If 0.500 moles of ethylbenzene react completely, how many moles of benzoic acid are formed?

🔍 Worked example — Exam-style

Explain why the oxidation of toluene (C_6H_5CH_3) yields benzoic acid, but the oxidation of chlorobenzene (C_6H_5Cl) does not yield a carboxylic acid under the same conditions.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Identify the type of reaction occurring. [1] (b) Draw the structure of X if it has the molecular formula C_9H_12. [2] [3 marks]

Key ideas — Forming acyl chlorides with phosphorus and sulfur chlorides

A carboxylic acid reacts with phosphorus(V) chloride, phosphorus(III) chloride, or sulfur dichloride oxide (thionyl chloride, SOCl_2) to replace the -OH with -Cl, forming an acyl chloride: CH_3COOH + PCl_5 arrow CH_3COCl + POCl_3 + HCl CH_3COOH + SOCl_2 arrow CH_3COCl + SO_2 + HCl SOCl_2 is often preferred, since both by-products are gases that escape the mixture, leaving the acyl chloride easy to purify.

🧑‍🏫 Teacher's explanation

Think of a carboxylic acid like a person holding a heavy, bulky handle (the -OH group). While the acid is stable, that -OH group is actually a mediocre 'leaving group'—it doesn't want to leave easily. To make the acid more reactive, we want to swap that -OH for something much more 'aggressive' and eager to depart, like a chlorine atom.

📖 Definitions

🔍 Worked example — Easy

Write a balanced chemical equation for the reaction between ethanoic acid and phosphorus pentachloride (PCl_5) to produce ethanoyl chloride.

🔍 Worked example — Standard

Explain why SOCl_2 is preferred over PCl_5 for the synthesis of an acyl chloride in a laboratory setting.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Write a balanced chemical equation for this reaction, including state symbols for the gaseous by-products. [2] (b) State one reason why this method is preferred over using PCl_5. [1] [3 marks]

Key ideas — Further oxidation of methanoic and ethanedioic acid

Methanoic acid, HCOOH, is unusual: the carbon bearing -COOH also carries a hydrogen atom, so, unlike other carboxylic acids, it can be oxidised further, to carbon dioxide and water, and it gives a positive result with Fehling's and Tollens' reagents. Ethanedioic acid, mathrmHOOC-COOH, is similarly oxidised further, by hot acidified potassium manganate(VII), to carbon dioxide; this reaction is used to standardise manganate(VII) solutions in titrations.

🧑‍🏫 Teacher's explanation

Most carboxylic acids are 'dead ends' for oxidation because the carbon in the carboxyl group (-COOH) is already bonded to two oxygens and one carbon/hydrogen, meaning there is no C-H bond to break. However, methanoic acid is the 'rebel' of the group; because it has a hydrogen atom attached directly to the carboxyl carbon, it can be oxidized further into CO_2 and H_2O. Think of it like a piece of wood that is already partially charred; while most wood just burns to ash, this specific piece is so reactive it turns completely into gas.

📖 Definitions

🔍 Worked example — Easy

Calculate the moles of methanoic acid in 25.0 cm^3 of a 0.100 mol dm^-3 solution.

🔍 Worked example — Standard

A 25.0 cm^3 sample of ethanedioic acid (C_2H_2O_4) is titrated against 0.0200 mol dm^-3 KMnO_4. The reaction is: 5C_2H_2O_4 + 2MnO_4^- + 6H^+ arrow 10CO_2 + 2Mn^2+ + 7H_2O. Calculate the concentration of the acid.

🔍 Worked example — Exam-style

Explain why chloroethanoic acid (ClCH_2COOH) is a stronger acid than ethanoic acid (CH_3COOH).

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Relative acidities of carboxylic acids, phenols and alcohols

The order of acid strength is carboxylic acid > phenol > water > alcohol. When a carboxylic acid loses a proton, the resulting carboxylate ion's negative charge is delocalised equally over two oxygen atoms, giving strong, symmetrical stabilisation. A phenoxide ion's charge is delocalised into the ring, but less effectively and less symmetrically, so phenol is a far weaker acid. An alkoxide ion has no delocalisation at all, and the electron-donating alkyl group further increases electron density on oxygen, so alcohols are the weakest acids of the three, weaker even than water.

🧑‍🏫 Teacher's explanation

Acidity is essentially a measure of how 'comfortable' a molecule feels after it has lost a proton (H^+). When an acid loses a proton, it becomes a negatively charged ion (the conjugate base). If that negative charge can be spread out over several atoms through resonance, the ion is stable, and the original molecule is more willing to give up its proton. Think of it like a group of people sharing the cost of a bill; if one person has to pay the whole thing, they are stressed (unstable), but if the cost is split among many people, everyone is much more relaxed (stable).\n\nIn carboxylic acids, the negative charge is shared equally between two oxygen atoms. In phenols, the charge is spread into the benzene ring, but it is not shared between two electronegative atoms, making it less stable. In alcohols, the charge is stuck on a single oxygen atom, and the nearby carbon group actually pushes even more negative charge toward it, making the ion very unstable and the alcohol very weak.

📖 Definitions

🔍 Worked example — Easy

Arrange the following species in order of increasing acidity: CH_3CH_2OH, C_6H_5OH, CH_3COOH.

🔍 Worked example — Standard

Explain why chloroethanoic acid (ClCH_2COOH) is a stronger acid than ethanoic acid (CH_3COOH).

🔍 Worked example — Exam-style

Calculate the pK_a of a weak acid if its K_a is 1.8 imes 10^-5.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(b) Predict whether 2,2,2-trichloroethanoic acid is stronger or weaker than chloroethanoic acid and explain your answer. [2] [5 marks]

Key ideas — Relative acidities of chlorine-substituted carboxylic acids

Replacing a hydrogen atom on the carbon chain with the highly electronegative chlorine atom withdraws electron density inductively, further stabilising the negative charge on the carboxylate ion and increasing acid strength: chloroethanoic acid is a stronger acid than ethanoic acid. Adding further chlorine atoms strengthens this inductive effect, so trichloroethanoic acid is a stronger acid than dichloroethanoic acid, which is stronger than chloroethanoic acid. The effect weakens as the substituent moves further from the carboxyl group, since inductive withdrawal is transmitted less effectively over more bonds.

🧑‍🏫 Teacher's explanation

Think of a carboxylic acid as a molecule trying to let go of a hydrogen ion (H^+). When that hydrogen leaves, the remaining part of the molecule (the carboxylate ion) carries a negative charge. For a molecule to be a 'strong' acid, it needs to be very happy/stable holding that negative charge. Imagine you are carrying a heavy backpack; if someone helps you pull the weight away, you feel more stable. In chemistry, electronegative atoms like chlorine act like that helper, pulling electron density away from the negative charge.

📖 Definitions

🔍 Worked example — Easy

Arrange the following acids in order of increasing acidity: CH_3 extCOOH, ClCH_2 extCOOH, Cl_2 extCHCOOH

🔍 Worked example — Standard

Explain why ClCH_2 extCOOH is a stronger acid than CH_3 extCOOH using the concept of the carboxylate ion.

🔍 Worked example — Exam-style

Predict the relative acidity of ClCH_2 extCOOH and ClCH_2 extCH_2 extCOOH.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Question. K_a(CH_3COOH) = 1.74 × 10^-5\ mathrmmol\ dm^-3 and K_a(CH_2ClCOOH) = 1.30 × 10^-3\ mathrmmol\ dm^-3. (a) Calculate the pH of a 0.100\ mathrmmol\ dm^-3 solution of each acid. [4] (b) Explain, in terms of structure, why chloroethanoic acid has the larger K_a value. [2]

Ch 1 · Carboxylic acids · AS & A Level Chemistry 9701

All chapters in Carboxylic acids and derivatives