Study Notes: Carboxylic Acids (IGCSE Chemistry 0620)

Welcome to the world of Carboxylic Acids! These are some of the most common organic compounds you will meet—in fact, the distinctive sour taste of vinegar comes from a carboxylic acid. Don't worry if this seems tricky at first; we will break down the structure, reactions, and uses step-by-step.

Key Takeaway: Carboxylic acids are organic compounds that behave as weak acids.

1. Understanding Carboxylic Acids: Structure and Formula

1.1 The Carboxyl Functional Group

The chemical properties of any homologous series are determined by its functional group. For carboxylic acids, this group is the carboxyl group.

Definition: The carboxyl group is the functional group that characterises carboxylic acids. It consists of a carbonyl group (\(C=O\)) bonded directly to a hydroxyl group (\(-OH\)).

  • The general formula is \(-\mathbf{COOH}\).
  • The general formula for the homologous series is \(C_n H_{2n+1}COOH\).

Analogy: Think of the carboxyl group (\(COOH\)) as having a double personality: part ketone/aldehyde (\(C=O\)) and part alcohol (\(-OH\)), but together they create a unique acidic property.

1.2 Naming and Formulae (Focus on Ethanoic Acid)

Carboxylic acids are named based on the number of carbon atoms in the chain, ending with the suffix -oic acid.

Ethanoic Acid (Acetic Acid)

This is the most important carboxylic acid you need to know, as it is the acid found in vinegar.

  • Molecular Formula: \(CH_3COOH\)
  • It has two carbon atoms (hence eth-).

Remember: The carbon atom within the carboxyl group (\(C\) in \(COOH\)) is counted as part of the total carbon chain!

The syllabus requires you to know up to four carbons (unbranched) in the extended section:

  • 1 Carbon: Methanoic acid
  • 2 Carbons: Ethanoic acid
  • 3 Carbons: Propanoic acid
  • 4 Carbons: Butanoic acid
Quick Review: How is the name determined?

Find the longest carbon chain. If it has 3 carbons, it’s prop-. Add the ending -oic acid. Result: Propanoic acid.

2. Chemical Properties: Reactions of Ethanoic Acid (Core)

Carboxylic acids are considered weak acids. This means they only partially dissociate (ionise) in aqueous solution to produce hydrogen ions (\(H^+\)).

Example: Ethanoic acid partially dissociates:

\(CH_3COOH (aq) \rightleftharpoons H^+ (aq) + CH_3COO^- (aq)\)

Since they are acids, they will undergo the characteristic reactions of acids, but often more slowly than strong mineral acids like hydrochloric acid.

2.1 Reaction with Metals

Carboxylic acids react with reactive metals to produce a salt and hydrogen gas.

Acid + Metal \(\rightarrow\) Salt + Hydrogen

When ethanoic acid reacts, the salt produced is called an ethanoate (or acetate).

Example: Ethanoic acid with Magnesium

\(2CH_3COOH (aq) + Mg (s) \rightarrow (CH_3COO)_2Mg (aq) + H_2 (g)\)

Salt Name: Magnesium ethanoate

Common Mistake to Avoid: Not all metals react. You must use a metal above hydrogen in the reactivity series (like Mg, Zn, Fe, but not Cu or Ag).

2.2 Reaction with Bases (Metal Oxides and Hydroxides)

Carboxylic acids react with bases (metal oxides or metal hydroxides) in a neutralisation reaction to produce a salt and water.

Acid + Base \(\rightarrow\) Salt + Water

Example: Ethanoic acid with Sodium Hydroxide

\(CH_3COOH (aq) + NaOH (aq) \rightarrow CH_3COONa (aq) + H_2O (l)\)

Salt Name: Sodium ethanoate

2.3 Reaction with Carbonates

Carboxylic acids react with carbonates to produce a salt, water, and carbon dioxide gas. This effervescence is the standard test for identifying an acid.

Acid + Carbonate \(\rightarrow\) Salt + Water + Carbon Dioxide

Example: Ethanoic acid with Sodium Carbonate

\(2CH_3COOH (aq) + Na_2CO_3 (s) \rightarrow 2CH_3COONa (aq) + H_2O (l) + CO_2 (g)\)

Salt Name: Sodium ethanoate

Key Takeaway Summary (Reactions): Carboxylic acids behave like normal acids. They produce ethanoate salts in all these reactions, releasing \(H_2\) with reactive metals, and \(CO_2\) with carbonates.

3. Formation of Ethanoic Acid (Supplement)

Ethanoic acid is produced by the oxidation of ethanol. This involves adding oxygen to the ethanol molecule, and there are two main ways this happens.

3.1 Method 1: Chemical Oxidation (Lab Method)

Ethanol can be oxidised using a strong oxidising agent, such as acidified aqueous potassium manganate(VII).

  • Reactants: Ethanol (\(CH_3CH_2OH\)) and Acidified Potassium Manganate(VII) (\(KMnO_4\))
  • Conditions: Gentle heating.

Process: Ethanol is oxidised to ethanoic acid.

\(CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O\)

(where \([O]\) represents the oxygen supplied by the oxidising agent)

Observation: The purple colour of the potassium manganate(VII) solution changes to colourless as it is used up (reduced) in the reaction.

3.2 Method 2: Bacterial Oxidation (Vinegar Production)

Ethanoic acid is produced commercially through the fermentation of alcoholic liquids (like wine or beer) to make vinegar.

  • Process: Ethanol reacts with oxygen from the air, in the presence of bacteria (such as Acetobacter).
  • This is often referred to as "souring" wine.

Reaction:

\(CH_3CH_2OH (aq) + O_2 (g) \xrightarrow{\text{Bacteria}} CH_3COOH (aq) + H_2O (l)\)

Did you know? Vinegar is typically 4-8% ethanoic acid in water. The process of making it is ancient—if wine is left exposed to air, the bacteria naturally present will turn the ethanol into acid!

4. Esterification: Making Esters (Supplement)

This is one of the most important reactions of carboxylic acids. They react with alcohols to form a group of compounds called esters. Esters are famous for their sweet, fruity smells and are used in flavourings and perfumes.

4.1 The Esterification Reaction

Esterification is a type of condensation reaction, meaning a small molecule (water) is removed as the two main molecules join together.

Carboxylic Acid + Alcohol \(\rightleftharpoons\) Ester + Water

Conditions:

  • Heating (usually gently, under reflux)
  • In the presence of an acid catalyst (typically concentrated sulfuric acid, \(H_2SO_4\)).

The reaction is reversible, which is why we use the equilibrium arrow (\(\rightleftharpoons\)).

4.2 Example: Forming Ethyl Ethanoate

Let’s combine our key acid (Ethanoic acid) with our key alcohol (Ethanol):

Ethanoic acid + Ethanol \(\rightleftharpoons\) Ethyl ethanoate + Water

\(CH_3COOH + CH_3CH_2OH \rightleftharpoons CH_3COOCH_2CH_3 + H_2O\)

The ester functional group is \(-\mathbf{COO-}\) (where the O-H from the acid and the H from the alcohol combine to form water).

The resulting ester, Ethyl Ethanoate, smells a bit like nail polish remover or pear drops.

4.3 Naming Esters: A Simple Trick

Naming esters can look intimidating, but it follows a strict pattern:

(Alcohol part) + (Acid part)

1. The name comes from the alcohol (the first part, ending in -yl).

2. The second part comes from the carboxylic acid (the second part, ending in -oate).

If you use Methanol and Propanoic acid, the product is Methyl Propanoate.

Memory Aid for Naming Esters:

The alcohol gives the Y(L) (e.g., Ethyl).

The acid gives the OATE (e.g., Ethanoate).

Quick Study Checklist for Carboxylic Acids (11.7)

Structure and Properties:
  • The functional group is the carboxyl group, \(-COOH\).
  • Carboxylic acids are weak acids (partially dissociated).
Core Reactions (Ethanoic Acid):
  • Metal (e.g., Mg) \(\rightarrow\) Ethanoate + Hydrogen
  • Base (e.g., NaOH) \(\rightarrow\) Ethanoate + Water
  • Carbonate (e.g., Na\(_2\)CO\(_3\)) \(\rightarrow\) Ethanoate + Water + Carbon Dioxide
Supplement Reactions/Preparation:
  • Formation: Oxidation of Ethanol (using acidified \(KMnO_4\)).
  • Esterification: Carboxylic acid + Alcohol \(\rightleftharpoons\) Ester + Water (needs acid catalyst, heat).
  • Esters: Named (Alcohol-yl) (Acid-oate), known for fruity smells.