Introduction: Meet the Carboxylic Acids

Welcome to one of the most important chapters in Organic Chemistry! You have probably encountered carboxylic acids more often than you think. Ethanoic acid is what gives vinegar its sharp smell and sour taste, while benzoic acid is a common preservative used in the food industry. In this chapter, we will explore how to make these molecules, how they behave as acids, and the cool ways they transform into other useful chemicals.

Prerequisite Check: Before we dive in, remember that the functional group for carboxylic acids is the carboxyl group, written as –COOH. It consists of a carbonyl group (C=O) and a hydroxyl group (–OH) attached to the same carbon atom.


1. How to Make Carboxylic Acids

There are three main "recipes" you need to know for the GCE A-Level syllabus to synthesize these acids.

A. From Primary Alcohols or Aldehydes

We can oxidise primary alcohols or aldehydes to get carboxylic acids. Think of this as adding "oxygen" to the molecule.

  • Reagents: Acidified \(KMnO_{4}\) (Potassium Manganate(VII)) OR acidified \(K_{2}Cr_{2}O_{7}\) (Potassium Dichromate(VI)).
  • Conditions: Heat under reflux.
  • Observation: If using \(KMnO_{4}\), the purple solution turns colourless. If using \(K_{2}Cr_{2}O_{7}\), the orange solution turns green.

Example: \(CH_{3}CH_{2}OH + 2[O] \rightarrow CH_{3}COOH + H_{2}O\)

B. From Nitriles (The "Carbon-Climbing" Method)

This is a very important reaction because it allows you to increase the number of carbon atoms in a chain. When you hydrolyse a nitrile (–CN), the nitrogen is replaced, and you get a –COOH group.

  • Reagents: Dilute acid (e.g., \(HCl\)) or dilute alkali (e.g., \(NaOH\)).
  • Conditions: Heat.
  • Note: If you use an alkali, you will initially get a salt. You must add dilute acid at the end (acidification) to get the actual carboxylic acid.

Quick Review: Oxidation adds oxygen or removes hydrogen. Hydrolysis uses water (often with an acid or base catalyst) to break a bond.


2. The Acidity of Carboxylic Acids

Why are they called "acids"? Because they can donate a proton (\(H^{+}\))! Don't worry if this seems tricky; just think of the molecule "letting go" of the hydrogen atom attached to the oxygen.

Why are they acidic?

When a carboxylic acid loses an \(H^{+}\), it forms a carboxylate ion (\(RCOO^{-}\)). This ion is very stable because the negative charge isn't stuck on one oxygen; it "spreads out" (delocalises) over both oxygen atoms.

Analogy: Imagine carrying a very heavy backpack. If you share the weight with a friend, it’s much easier to carry. In the same way, sharing the negative charge over two oxygens makes the ion more stable!

Comparing Strength: Chlorine-Substituted Ethanoic Acids

Not all carboxylic acids are equal. If we swap a hydrogen on the carbon chain for a Chlorine atom, the acid becomes stronger.

  • The Reason: Chlorine is electron-withdrawing. It pulls electrons away from the \(COO^{-}\) group.
  • The Effect: This further "spreads out" the negative charge, making the carboxylate ion even more stable.
  • The Rule: The more Chlorine atoms you add, the stronger the acid becomes!

Ranking: Trichloroethanoic acid > Dichloroethanoic acid > Chloroethanoic acid > Ethanoic acid.

Key Takeaway: Stability of the anion (\(RCOO^{-}\)) determines acid strength. Anything that pulls electrons away from the negative charge stabilizes it!


3. Chemical Reactions of Carboxylic Acids

Carboxylic acids are quite reactive. Here are the four transformations you must master:

A. Formation of Salts

Just like inorganic acids, they react with bases.

  • With Metals: Forms salt + Hydrogen gas (\(H_{2}\)).
  • With Alkalis (e.g., NaOH): Forms salt + Water (\(H_{2}O\)).
  • With Carbonates: Forms salt + Water + Carbon Dioxide (\(CO_{2}\)).

Pro-Tip: If you see bubbles when adding an unknown organic liquid to sodium carbonate, it’s almost certainly a carboxylic acid!

B. Formation of Esters (Condensation)

This is how we make many fruity smells!

  • Reagents: Alcohol + Carboxylic Acid.
  • Conditions: Concentrated \(H_{2}SO_{4}\) (catalyst) and heat.
  • Process: A water molecule is removed. The –OH comes from the acid, and the –H comes from the alcohol.

Example: Ethanoic acid + Ethanol \(\rightleftharpoons\) Ethyl ethanoate + Water.

C. Formation of Acyl Chlorides

To turn a –COOH group into a more reactive –COCl group:

  • Reagent: \(PCl_{5}\) (Phosphorus pentachloride).
  • Observation: Misty white fumes of \(HCl\) gas are evolved.

D. Reduction to Primary Alcohols

If oxidation turns an alcohol into an acid, reduction does the opposite.

  • Reagent: \(LiAlH_{4}\) (Lithium Aluminium Hydride) in dry ether.
  • Result: Ethanoic acid (\(CH_{3}COOH\)) turns back into Ethanol (\(CH_{3}CH_{2}OH\)).

Did you know? We must use "dry ether" because \(LiAlH_{4}\) is so powerful it will react violently with even a tiny drop of water!


Common Mistakes to Avoid

  • Wrong Catalyst: For esterification, you must specify concentrated \(H_{2}SO_{4}\). Dilute won't work!
  • Missing Reflux: When oxidizing alcohols to acids, you must state "reflux." If you just "distill," you might only get the aldehyde.
  • Benzoic Acid Solubility: Remember that benzoic acid is not very soluble in cold water but dissolves well in hot water. Its salt (sodium benzoate), however, is very soluble!

Quick Review Box

Make it: Oxidise alcohols/aldehydes or hydrolyse nitriles.
Test it: Add \(Na_{2}CO_{3}\) and look for \(CO_{2}\) bubbles.
Change it: Use \(PCl_{5}\) for acyl chlorides, alcohol for esters, or \(LiAlH_{4}\) for alcohols.
Acid Strength: Adding electronegative groups like Chlorine makes the acid stronger.