[26.3] carboxylic acids

Question 1

describe the structure of carboxylic acids

Answer 1

• contain the carboxyl functional group COOH
• bonds are in a planar arrangement
• are functional group isomeric with esters (RCOOR)

Question 2

what does a carboxyl group include?

Answer 2

• carbonyl (C=O) group
• hydroxyl (O-H) group

Question 3

how to name a carboxylic acid

Answer 3

• find longest chain of C atoms containing COOH group
• remove the e and add oic acid after the basic name
• number the chains starting from the end nearer the COOH group
• prefix with alkyl substituents
• side chain positions are based on the C in COOH being 1

Question 4

physical properties: boiling point

Answer 4

• increases as size increases (due to increased london forces)
• carboxylic acids have high boiling points for their relative mass
• this arises from intermolecular hydrogen bonding due to polar O-H bonds
• extra intermolecular attraction means more energy is needed to separate molecules

Question 5

physical properties: solubility

Answer 5

• soluble in water due to hydrogen bonding
• small ones dissolve readily in water (C1-4)
• as mass increases, solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule
• benzoic acid is fairly insoluble in cold but soluble in hot water
• dicarboxylic acids readily dissolve in water

Question 6

chemical properties: acidity

Answer 6

• weak acids
• when dissolved in water, carboxylic acids partially dissociate
• RCOOH + H₂O (l) ⇌ RCOO⁻ (aq) + H₃O⁺ (aq)

Question 7

acid reactions of carboxylic acids

Answer 7

take place in:

• redox reactions with metals
• neutralisation reactions with bases (alkalis, metal oxides, and carbonates)

Question 8

redox reactions of carboxylic acids with metals

Answer 8

• form hydrogen gas an a carboxylate salt
• observe metal disappearing and effervescence as hydrogen gas is evolved
• 2RCOOH + Mg (s) ➞ (RCOO⁻)₂Mg²⁺ (aq) + H₂ (g)

Question 9

reactions of carboxylic acids with metal oxides

Answer 9

2RCOOH + CaO (s) ➞ (RCOO⁻)₂Ca²⁺ (aq) + H₂O (l)

Question 10

reactions of carboxylic acids with alkalis

Answer 10

• overall: RCOOH + NaOH (aq) ➞ RCOO⁻Na⁺ (aq) + H₂O (l)
• ionic: H⁺ (aq) + OH⁻ ➞ H₂O (l)

Question 11

reactions of carboxylic acids with carbonates

Answer 11

2RCOOH + Na₂CO₃ (aq) ➞ 2RCOO⁻Na⁺ (aq) + H₂O (l) + CO₂ (g)

Question 12

how can a carboxylic acid be liberated from its salt?

Answer 12

• by treatment with a stronger acid
• conversion of an acid to its water soluble salt followed by acidification of the salt to restore the acid is often used to separate acids from a mixture
• RCOO⁻Na⁺ (aq) + HCl (aq) ➞ RCOOH + NaCl (aq)

Question 13

test for carboxylic acids

Answer 13

• add sodium carbonate
• observation: bubbling
• 2RCOOH + Na₂CO₃ (aq) ➞ 2RCOO⁻Na⁺ (aq) + H₂O (l) + CO₂ (g)

Question 14

how can phenols and carboxylic acids be distinguished?

Answer 14

• they are both weak acids
• reaction with sodium carbonate
• phenol is too weak of an acid to react with carbonate