Organic Chemistry: Carboxylic Acids

Revise Organic Chemistry: Carboxylic Acids for Chemistry 4 (IAL) WCH14 (A2 Level) — revision notes and instant AI marking.

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Edexcel IAL Chemistry — Organic Chemistry

Carboxylic Acids

Big idea: carboxylic acids are the "–COOH" family — weak, hydrogen-bonded acids that can be made from alcohols, aldehydes or nitriles, and turned into salts, esters, acyl chlorides or amides depending on what you react them with.

Summary — the whole chapter in one scroll

What they are: compounds containing the –COOH functional group, general formula C_nH_2n+1COOH (shortened RCOOH).
Physical properties: high melting/boiling points because of hydrogen bonding; short-chain ones dissolve in water, but solubility drops as the carbon chain gets longer.
How to make them: (1) oxidise a primary alcohol or aldehyde with acidified KMnO₄ or K₂Cr₂O₇ under reflux, or (2) hydrolyse a nitrile with dilute acid or dilute alkali (+ acidification).
Reactions: weak acid behaviour with carbonates/bicarbonates (fizzes CO₂ — a classic test), reduction with LiAlH₄ to primary alcohols, reactions with metals/alkalis/carbonates to form salts, reaction with PCl₅ to form acyl chlorides, and condensation with alcohols to form esters.
Acyl chlorides & esters: acyl chlorides are super-reactive carboxylic acid derivatives (–COCl) that undergo nucleophilic addition–elimination with water (→ carboxylic acid), alcohols (→ esters), and ammonia/amines (→ amides + ammonium salt).
Ester hydrolysis: acid hydrolysis is reversible (equilibrium, doesn't go to completion); alkaline hydrolysis is irreversible (goes to completion, gives a carboxylate salt that needs acidifying to get the acid back).
Polyesters: condensation polymers made from a diol + a dicarboxylic acid (or a single hydroxycarboxylic acid), linked by ester bonds, releasing water at every link.

1. Carboxylic Acids — Structure & Physical Properties

Functional group Boiling points Solubility

Every carboxylic acid has the –COOH group sitting at the end of a carbon chain. Think of it as a carbon double-bonded to one oxygen (like a ketone) and single-bonded to an –OH group, both attached to the same carbon. That carbon is always numbered carbon 1 — which is why you never need locant numbers in the name (unlike "propan-2-ol" or similar).

Naming pattern

Carboxylic acids are named: alkan + oic acid. So the 2-carbon chain (ethane) becomes ethanoic acid, the 3-carbon chain (propane) becomes propanoic acid, and so on. Some countries call the whole family "alkanoic acids" instead of "carboxylic acids" — same thing.

Structure	Name	Molecular Formula
HCOOH	Methanoic acid (formic acid)	HCOOH
CH₃COOH	Ethanoic acid (acetic acid)	CH₃COOH
CH₃CH₂COOH	Propanoic acid	CH₃CH₂COOH

Why the boiling points are so high

Carboxylic acids contain two polarised groups: the C=O and the O–H. Because of the O–H bond, molecules can form hydrogen bonds with each other — and in fact, two carboxylic acid molecules can pair up into a "dimer," hydrogen-bonding to each other at both ends like a molecular handshake. That's a lot of intermolecular attraction to break, which is why carboxylic acids have unusually high melting and boiling points for their size — often higher than alcohols of similar molar mass.

Rule to remember More hydrogen bonding = more energy needed to separate molecules = higher boiling point. Carboxylic acids > alcohols > alkanes of similar chain length, in terms of boiling point.