6.1.3: Carboxylic acids and esters

Question 1

Carboxylic acid functional group

Answer 1

R — C — O

— OH

Question 2

Ester functional group

Answer 2

R — C — O

— O —- R

Question 3

Physical properties of carboxylic acids

Answer 3

• High MBP
∵ can form hydrogen bonds between molecules
• Soluble in water: solubility decreases w/ increasing chain length
∵ greater proportion of chain = hydrophobic
• Permanent dipole
⟶ δ⁻ on the oxygens, δ⁺ on the C and H

Question 4

Chemical properties of carboxylic acids: reaction with base

Answer 4

Acid + Base ⟶ Salt + Water
e.g.
CH₃COOH + NaOH ⟶ CH₃COONa + H₂O

Question 5

Chemical properties of carboxylic acids: reaction with carbonate

Answer 5

Acid + Carbonate ⟶ Salt + Water + Carbon dioxide

e.g. CH₃COOH + NaHCO₃ ⟶ CH₃COONa + H₂O + CO₂

Question 6

Chemical properties of carboxylic acids: reaction with PCl₅

Answer 6

CH₃COOH + PCl₅ ⟶ CH₃COCl + POCl₃ + HCl

Question 7

Chemical properties of carboxylic acids: reduction

Answer 7

CH₃COOH –[H]–> CH₃CH₂OH

Where [H] is LiAlH₄

Question 8

Why use LiAlH₄ rather than NaBH₄ to reduce carboxylic acids?

Answer 8

• LiAlH₄ is more aggressive than NaBH₄
• This is necessary because you need to go back two steps to reduce a carboxylic acid to an alcohol
1) Carboxylic acid reduced to aldehyde
2) Aldehyde reduced to primary alcohol

Question 9

Chemical properties of carboxylic acids: reaction with NH₃

Answer 9

CH₃COOH + NH₃ (aq) ⟶ CH₃CONH₂ + H₂O

Question 10

Give the two ways esters can be synthesised?

Answer 10

• Alcohol + Carboxylic acid ⇌ Ester + Water (H₂SO₄ catalyst, heat under reflux)
• Alcohol + Acid anhydride ⟶ Ester + Carboxylic acid

Question 11

Which method of synthesising esters is better?

Answer 11

• Alcohol + Acid anhydride
∵ This gives a better yield (not under reflux)
∵ No catalyst required

Question 12

An acid anhydride is the condensation reaction product of

Answer 12

Two carboxylic acids (which are dehydrated in the reaction, hence water molecule produced)

Question 13

What is the test for an OH group?

Answer 13

• Add PCl₅

* If white fumes of gaseous HCl are given off, OH group is present

Question 14

Uses of esters

Answer 14

• Perfumes, fragrances

* Solvents

Question 15

How to obtain a carboxylic acid from an ester?

Answer 15

• Hydrolysis

⟶ Hydrolysis is either with an acid or base catalyst hence “acid-catalysed hydrolysis” or “base-catalysed hydrolysis”

Question 16

Conditions for acid-catalysed ester hydrolysis

Answer 16

• Dilute but strong acid (therefore water and protons provided)
⟶ H₂SO₄ good ∵ diprotic
• Heat under reflux

Question 17

Downside of acid-catalysed ester hydrolysis

Answer 17

• An equilibrium is established ∴ some ester will always remain unhydrolysed

Question 18

Advantage of base-catalysed ester hydrolysis

Answer 18

• Hydrolysis goes to completion

* More efficient process

Question 19

Base-catalysed ester hydrolysis conditions

Answer 19

• Excess NaOH (aq)

* Heat under reflux

Question 20

Base-Catalysed ester hydrolysis produces

Answer 20

The carboxylate salt of the carboxylic acid (+ alcohol)

Question 21

How to obtain a carboxylic acid from its carboxylate salt?

Answer 21

Acidification

e.g. C₆H₅COO⁻Na⁺ + HCl (aq) ⟶ C₆H₅COOH + NaCl (aq)

Question 22

Acid-catalysed ester hydrolysis equation

Answer 22

Ester + Water ⇌H₂SO₄⇌ Carboxylic acid + Alcohol

Question 23

Base-catalysed ester hydrolysis equation

Answer 23

Ester + Base(aq) ⇌ Carboxylate salt + Alcohol

Question 24

What is the mechanism for the reaction between acyl chloride and water

Answer 24

Addition-elimination