Carboxylic acids Revision Notes

Ethanoic acid reacts with certain metals, such as sodium (Na), magnesium (Mg), and zinc (Zn), to produce a salt and hydrogen gas (H₂). This is a typical acid-metal reaction where the metal displaces hydrogen from the acid.

The salt formed is a metal ethanoate, named by replacing the hydrogen in ethanoic acid with the metal ion.

For example, with sodium: sodium ethanoate (CH₃COONa) is formed along with hydrogen gas.

• General equation: 2CH₃COOH (aq) + 2M (s) → 2CH₃COOM (aq) + H₂ (g), where M is a metal such as Na.
• Example with sodium: 2CH₃COOH (aq) + 2Na (s) → 2CH₃COONa (aq) + H₂ (g).
• Observation: Effervescence due to hydrogen gas bubbles.
• Reaction is usually vigorous with reactive metals like sodium and slower with less reactive metals like zinc.

Ethanoic acid reacts with bases, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), or ammonia (NH₃), to form a salt and water. This is a neutralisation reaction where the acid donates a proton (H⁺) to the base.

The salt formed is a metal ethanoate if the base is a metal hydroxide, or an ammonium ethanoate if ammonia is used.

For example, with sodium hydroxide, sodium ethanoate and water are produced.

• General equation: CH₃COOH (aq) + MOH (aq) → CH₃COOM (aq) + H₂O (l), where M is a metal ion.
• Example with sodium hydroxide: CH₃COOH (aq) + NaOH (aq) → CH₃COONa (aq) + H₂O (l).
• Observation: The solution becomes neutralised; no gas is evolved.
• Reaction is exothermic and complete when equivalent amounts of acid and base react.

Ethanoic acid reacts with carbonates, such as calcium carbonate (CaCO₃) or sodium carbonate (Na₂CO₃), to produce a salt, water, and carbon dioxide gas (CO₂).

This reaction occurs because the carbonate ion (CO₃²⁻) reacts with the acid to release CO₂ gas.

The salt formed is a metal ethanoate, with the metal from the carbonate replacing the hydrogen in ethanoic acid.

• General equation: 2CH₃COOH (aq) + MCO₃ (s) → CH₃COOM (aq) + H₂O (l) + CO₂ (g), where M is a metal ion.
• Example with calcium carbonate: 2CH₃COOH (aq) + CaCO₃ (s) → Ca(CH₃COO)₂ (aq) + H₂O (l) + CO₂ (g).
• Observation: Effervescence due to carbon dioxide gas bubbles.
• Reaction is often used to test for carbonates by observing CO₂ evolution.

The salts formed in these reactions are called ethanoates (acetates). Their names depend on the metal or cation present.

For example: sodium ethanoate (CH₃COONa), calcium ethanoate (Ca(CH₃COO)₂), and ammonium ethanoate (CH₃COONH₄).

The ethanoate ion is CH₃COO⁻, which combines with metal ions to form ionic compounds.

• Sodium ethanoate: CH₃COONa
• Calcium ethanoate: Ca(CH₃COO)₂
• Ammonium ethanoate: CH₃COONH₄
• Ethanoate ion formula: CH₃COO⁻

Balanced chemical equations show the stoichiometric relationships in these reactions.

Example: Reaction of ethanoic acid with magnesium metal.

Magnesium reacts with ethanoic acid to form magnesium ethanoate and hydrogen gas.

• Equation: 2CH₃COOH (aq) + Mg (s) → (CH₃COO)₂Mg (aq) + H₂ (g)
• Magnesium ethanoate formula: (CH₃COO)₂Mg
• Observation: Bubbles of hydrogen gas form; magnesium gradually dissolves.
• This reaction illustrates the typical acid-metal reaction producing salt and hydrogen.

Ethanol can be oxidised to ethanoic acid by heating it with acidified potassium permanganate (KMnO4) solution. The potassium permanganate acts as a strong oxidising agent in acidic conditions, converting the primary alcohol group (-CH2OH) in ethanol to a carboxylic acid group (-COOH).

The reaction requires heating under reflux to prevent loss of volatile substances and to ensure complete oxidation.

• Reagents: Ethanol, acidified potassium permanganate (KMnO4 in dilute sulfuric acid).
• Conditions: Heat under reflux to maintain reaction mixture and prevent evaporation.
• Observation: The purple colour of KMnO4 fades to colourless or brown as it is reduced.
• Equation: C2H5OH + 2[O] → CH3COOH + H2O (where [O] represents oxygen from KMnO4).
• Ionic equation for KMnO4 reduction in acidic medium: MnO4⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H2O.

The overall balanced equation for the oxidation of ethanol to ethanoic acid using acidified potassium permanganate is:

• 3 C2H5OH + 2 KMnO4 + 4 H2SO4 → 3 CH3COOH + 2 MnSO4 + K2SO4 + 7 H2O

Vinegar is produced by the bacterial oxidation of ethanol to ethanoic acid. Specific bacteria, such as Acetobacter species, oxidise ethanol in the presence of oxygen from air. This process is slower than chemical oxidation but occurs naturally and is used industrially to produce vinegar.

The bacteria oxidise ethanol first to ethanal (acetaldehyde) and then further to ethanoic acid. This is an aerobic process requiring oxygen.

The reaction conditions are mild, typically at room temperature or slightly warmer, and the process is carried out in fermentation vessels with aeration.

• Microorganisms: Acetobacter bacteria.
• Conditions: Aerobic, moderate temperature, aqueous solution.
• Process: Ethanol → Ethanal → Ethanoic acid.
• Equation: C2H5OH + O2 → CH3COOH + H2O (simplified overall).
• Used industrially to produce vinegar, which contains about 5–8% ethanoic acid by volume.

Ethanol has the structural formula CH3–CH2–OH, containing a primary alcohol group (-CH2OH). Oxidation converts this group first to an aldehyde (-CHO) and then to a carboxylic acid (-COOH) in ethanoic acid (CH3–COOH).

This change involves the loss of hydrogen atoms and the addition of oxygen atoms, increasing the oxidation state of the carbon atom bonded to oxygen.

The oxidation can be summarised as: Primary alcohol → Aldehyde → Carboxylic acid.

• Ethanol: CH3–CH2–OH
• Ethanal (acetaldehyde): CH3–CHO
• Ethanoic acid: CH3–COOH

In the laboratory, the oxidation of ethanol with acidified potassium permanganate is monitored by the colour change of the solution from purple to colourless or brown, indicating reduction of MnO4⁻ ions.

Heating under reflux is necessary to prevent loss of ethanol and products by evaporation.

The reaction mixture should be acidic, commonly using dilute sulfuric acid, to maintain the oxidising power of potassium permanganate and to allow the reduction of MnO4⁻ to Mn²⁺ ions, which are colourless or pale pink in solution.

• Use of reflux apparatus to heat safely and prevent evaporation.
• Colour change: Purple KMnO4 → colourless/brown Mn²⁺ ions.
• Acidic medium required (dilute H2SO4).
• Safety: Potassium permanganate is a strong oxidiser and should be handled with care; wear gloves and eye protection.

Esterification is a chemical reaction in which a carboxylic acid reacts with an alcohol to form an ester and water. This reaction requires an acid catalyst, commonly concentrated sulfuric acid (H₂SO₄), to proceed at a practical rate. Esters are organic compounds characterised by the functional group –COO–, linking the acid and alcohol parts.

The general reaction can be represented as: Carboxylic acid + Alcohol ⇌ Ester + Water. This is a reversible reaction, meaning the products can react to reform the reactants under certain conditions.

• Esterification produces esters and water.
• An acid catalyst is necessary to speed up the reaction.
• The reaction is reversible.

The general balanced equation for esterification is:

R–COOH + R'–OH ⇌ R–COO–R' + H₂O, where R and R' are alkyl groups.

For example, ethanoic acid (CH₃COOH) reacts with ethanol (CH₃CH₂OH) to form ethyl ethanoate (CH₃COOCH₂CH₃) and water: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O.

• Write balanced equations including state symbols when relevant.
• Use structural formulae to show the ester functional group.
• Example: Ethanoic acid + ethanol ⇌ ethyl ethanoate + water.

Concentrated sulfuric acid acts as a catalyst by donating protons (H⁺ ions) which protonate the carbonyl oxygen of the carboxylic acid, increasing its electrophilicity. This facilitates nucleophilic attack by the alcohol’s oxygen, promoting ester bond formation.

The reaction is typically heated under reflux to increase the rate without losing volatile reactants or products. The acid catalyst is not consumed and remains unchanged at the end of the reaction.

Because the reaction is reversible, removing water as it forms or using an excess of one reactant can drive the equilibrium towards ester formation (Le Châtelier’s principle).

• Sulfuric acid protonates the carboxylic acid to increase reactivity.
• Heating under reflux is used to maintain reaction temperature and prevent loss.
• Removing water or using excess reactant shifts equilibrium to produce more ester.

Esters are usually colourless liquids with distinctive, often pleasant, fruity smells. For example, ethyl ethanoate smells like pear drops.

They are less dense than water and generally insoluble or only slightly soluble in water due to their non-polar alkyl groups, but soluble in organic solvents.

In practical preparation, the ester layer can be separated by using a separating funnel after the reaction mixture is diluted with water.

• Esters have characteristic fruity odours.
• They are less dense than water and form a separate layer.
• Esters are used in flavourings and perfumes.

Propanoic acid (CH₃CH₂COOH) reacts with methanol (CH₃OH) in the presence of concentrated sulfuric acid to form methyl propanoate and water.

Balanced equation: CH₃CH₂COOH + CH₃OH ⇌ CH₃CH₂COOCH₃ + H₂O

This shows one molecule of propanoic acid reacts with one molecule of methanol to form one molecule of methyl propanoate and one molecule of water.

• Identify reactants and products with correct formulae.
• Balance the equation with one-to-one molar ratio.
• Include the acid catalyst and reaction conditions in practical contexts.