Carboxylic acids

Full Notes

Carboxylic acids and their reactions have been outlined in more detail here.
This page is just what you need to know for CIE A-level Chemistry :)

Carboxylic acids contain a -COOH functional group and are weak acids, due to a weak O–H bond that can easily break, releasing a H⁺ ion and leaving behind a negatively charged carboxylate ion (COO⁻).

Preparation of Carboxylic Acids

A common method for making carboxylic acids is the Oxidation of Primary Alcohols or Aldehydes (covered in more detail for CIE here).

• Reagents: Acidified K₂Cr₂O₇ or KMnO₄
• Conditions: Heat under reflux
Reflux conditions ensures complete oxidation; aldehyde is not lost by evaporation.
Examples:
• CH₃CH₂OH → CH₃COOH
• CH₃CHO → CH₃COOH

Hydrolysis of Nitriles

• Reagents: Dilute HCl or NaOH, followed by acidification

Example: CH₃CH₂CN → CH₃CH₂COOH

Hydrolysis of Esters

Esters can be broken apart in hydrolysis reactions. The products obtained depend on the conditions used.

Acid Hydrolysis:

• Reagent: Dilute H₂SO₄ or HCl
• Reaction: Ester + Water ⇌ Carboxylic Acid + Alcohol

Example: CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

Alkaline Hydrolysis:

• Reagent: NaOH
• Reaction: Ester + NaOH → Carboxylate Salt + Alcohol

Example: CH₃COOCH₂CH₃ + NaOH → CH₃COO⁻Na⁺ + CH₃CH₂OH

Matt’s exam tip

If alkaline hydrolysis is used, a carboxylic acid can still be obtained. You just need to remember to add H⁺ ions (from a dilute acid) to acidify the carboxylate ion in the salt. (For example CH₃COO⁻ + H⁺ → CH₃COOH)

Reactions of Carboxylic Acids

Carboxylic acids are weak acids and can donate H⁺ ions in reactions, producing a carboxylate ion (COO⁻) that forms a salt. Because they are weak acids they only partially dissociate in solution.

Redox Reaction with Reactive Metals

• Metal displaces hydrogen from the –COOH group, producing hydrogen gas and a carboxylate salt.

Example: 2CH₃COOH + Zn → (CH₃COO)₂Zn + H₂(g)

Neutralisation with Alkalis

• OH⁻ accepts the H⁺ from the –COOH group, forming a salt and water.

Example: CH₃COOH + NaOH → CH₃COONa + H₂O

Reaction with Carbonates

• Typical acid–carbonate reaction producing carbon dioxide, water, and a salt.

Example: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂(g) + H₂O

Esterification with Alcohols

• Reagents: Alcohol + conc. H₂SO₄ (catalyst)
• Conditions: Heat under reflux
• The acid and alcohol react to form an ester and water; the reaction is reversible and slow, so a catalyst and heat are used.

Example: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

Reduction by LiAlH₄

• Reagent: LiAlH₄ in dry ether
• LiAlH₄ is a more powerful reducing agent than NaBH₄ (which isn’t powerful enough to reduce a carboxylic acid), because of this no water can be present and the reaction must be carried out in dry ether.

Example: CH₃COOH + 4[H] → CH₃CH₂OH + H₂O

Matt’s exam tip

Remember reduction in organic chemistry is the gaining of a carbon-hydrogen bond. To provide the hydrogen needed, we use reducing agents (such as NaBH₄ and LiAlH₄) and show hydrogen from a reducing agent in equations as [H].

Summary

• Carboxylic acids (–COOH) are weak acids that form carboxylate ions (COO⁻) and salts.
• They can be prepared by oxidation (primary alcohols/aldehydes), nitrile hydrolysis (then acidify), and ester hydrolysis (acid or alkaline).
• They react with reactive metals, alkalis, and carbonates; they undergo esterification with alcohols (conc. H₂SO₄, heat).
• LiAlH₄ in dry ether reduces carboxylic acids to primary alcohols; NaBH₄ is not strong enough.