Prepare for exams with solved exercises on Carboxylic Acids, designed for the PTB curriculum. Cover key concepts such as properties, reactions, and applications of carboxylic acids to reinforce your understanding.
Q. 4 Write down the structural formulae of the followings:
(i) Valeric acid (Pentanoic acid):
Structural formula: CH₃(CH₂)₃COOH
(ii) Propionic acid (Propanoic acid):
Structural formula: CH₃CH₂COOH
(iii) Oxalic acid:
Structural formula: HOOC-COOH
(iv) Benzoic acid:
Structural formula: C₆H₅COOH
(v) Acetic anhydride:
Structural formula: (CH₃CO)₂O
(vi) Acetyl chloride:
Structural formula: CH₃COCl
Q. 5 Write down the names of the following compounds by IUPAC system.
(i) 2-Aminopropanoic acid
(ii) 3-Phenylpropanoic acid
(iii) Ethanoic anhydride
(iv) Ethyl ethanoate
(v) 2-Aminopropanoic acid (This structure seems to repeat from part (i))
(vi) Propyl methanoate
Q. 6 (a) How is acetic acid manufactured? What is glacial acetic acid?
- Manufacture of Acetic Acid:
Acetic acid can be manufactured through methanol carbonylation, where methanol reacts with carbon monoxide to form acetic acid. The reaction is usually catalyzed by a metal complex such as rhodium or iridium. CH₃OH + CO → CH₃COOH - Glacial Acetic Acid:
Glacial acetic acid is concentrated, anhydrous (water-free) acetic acid, which freezes below 16.7°C (63°F). It is called “glacial” because it solidifies just below room temperature, resembling ice.
(b) How would you convert acetic acid into the following compounds?
(i) Methane:
By decarboxylation of sodium acetate with sodium hydroxide (soda lime method), methane can be obtained.
CH₃COONa + NaOH → CH₄ + Na₂CO₃
(ii) Acetyl chloride:
By reacting acetic acid with thionyl chloride (SOCl₂) or phosphorus trichloride (PCl₃).
CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl
(iii) Acetamide:
By reacting acetic acid with ammonia (NH₃) followed by dehydration.
CH₃COOH + NH₃ → CH₃CONH₂ + H₂O
(iv) Acetic anhydride:
By heating acetic acid with a dehydrating agent such as phosphorus pentoxide (P₂O₅).
2 CH₃COOH → (CH₃CO)₂O + H₂O
Q. 7 (a) What are fatty acids?
Fatty acids are long-chain carboxylic acids that are found in fats and oils. They have the general formula CH₃(CH₂)ₙCOOH, where ‘n’ typically ranges from 2 to 28. Saturated fatty acids have no double bonds, while unsaturated fatty acids contain one or more double bonds.
(b) What is vinegar? Describe how is vinegar prepared from ethanol?
- Vinegar is an aqueous solution of acetic acid (CH₃COOH) and trace chemicals. It is primarily used in cooking and food preservation.
- Preparation of Vinegar from Ethanol:
Ethanol undergoes aerobic oxidation in the presence of acetic acid bacteria (Acetobacter). The ethanol is converted into acetic acid through this microbial process: C₂H₅OH + O₂ → CH₃COOH + H₂O
Q. 8 How would you carry out the following conversions?
(i) Acetic acid into acetamide:
Acetic acid can be converted into acetamide by reacting it with ammonia and removing water.
CH₃COOH + NH₃ → CH₃CONH₂ + H₂O
(ii) Acetic acid into acetone:
Acetic acid is first converted into calcium acetate by reacting it with calcium hydroxide. Calcium acetate is then heated to produce acetone via dry distillation.
(CH₃COO)₂Ca → CH₃COCH₃ + CaCO₃
Q. 9 Write down the mechanisms of the following reactions.
(i) Between acetic acid and ethanol:
This is an esterification reaction where acetic acid reacts with ethanol to form ethyl acetate and water.
Mechanism:
- Protonation of acetic acid, followed by nucleophilic attack by ethanol.
- Water is eliminated, and deprotonation yields ethyl acetate.
(ii) Between acetic acid and ammonia:
Acetic acid reacts with ammonia to form ammonium acetate, which on heating loses water to give acetamide.
CH₃COOH + NH₃ → CH₃CONH₂ + H₂O
(iii) Between acetic acid and thionyl chloride:
Acetic acid reacts with thionyl chloride to form acetyl chloride, sulfur dioxide, and hydrogen chloride.
CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl
Q. 10 What happens when the following compounds are heated?
(i) Calcium acetate:
On heating, calcium acetate decomposes to form acetone and calcium carbonate.
(CH₃COO)₂Ca → CH₃COCH₃ + CaCO₃
(ii) Sodium formate:
Sodium formate decomposes upon heating to give sodium oxalate and hydrogen gas.
2 HCOONa → Na₂C₂O₄ + H₂
Q. 11 What are amino acids? Explain their different types with one example in each case.
Amino acids are organic compounds that contain both an amino group (-NH₂) and a carboxyl group (-COOH). They are the building blocks of proteins.
- Non-polar amino acids: These amino acids have hydrophobic side chains. Example: Glycine.
- Polar amino acids: These have hydrophilic side chains. Example: Serine.
- Acidic amino acids: These contain an extra carboxyl group. Example: Aspartic acid.
- Basic amino acids: These contain an extra amino group. Example: Lysine.
Q. 12 Write a short note on acidic and basic characters of an amino acid.
Amino acids exhibit both acidic and basic properties due to the presence of an amino group (basic) and a carboxyl group (acidic). In acidic solutions, the amino group is protonated, while in basic solutions, the carboxyl group loses a proton. This amphoteric nature allows amino acids to act as buffers in biological systems.
Q. 13 What is a peptide bond? Write down the formula of a dipeptide.
A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another amino acid. The bond results in the release of a water molecule (condensation reaction).
- Formula of a dipeptide (example: Glycine + Alanine):
NH₂-CH₂-CONH-CH(CH₃)-COOH
Q. 14 What are zwitter ions?
A zwitterion is a molecule that contains both positive and negative charges but is overall neutral. In the case of amino acids, the carboxyl group loses a proton and becomes negatively charged, while the amino group gains a proton and becomes positively charged in neutral pH conditions.
Q. 15 What are amino acids, proteins, and peptides? How are they related?
- Amino acids: These are the building blocks of proteins and contain both amino and carboxyl functional groups.
- Peptides: Chains of two or more amino acids linked by peptide bonds.
- Proteins: Large molecules made up of one or more long chains of amino acids. They are polymers of peptides and fold into complex structures to perform specific biological functions.
The relationship: Proteins are composed of long sequences of peptides, which in turn are made up of amino acids linked by peptide bonds.
Q. 16 Study the facts given in (a), (b), and (c) and then answer questions which follow.
(a) A is an organic compound made up of C, H, and O. It has a vapour density of 15.
(Hint: Molecular mass = 2 × vapour density)
(b) On reduction, A gives a compound X which has the following properties:
(i) X is a colourless liquid miscible with water.
(ii) X is neutral to litmus.
(iii) When X is warmed with a few drops of conc. H₂SO₄, followed by a little salicylic acid, a characteristic smell is produced.
(c) When X is subjected to strong oxidation, it gives compound B, which has the following properties:
(i) B is a pungent smelling mobile liquid.
(ii) It is miscible with water, alcohol, or ether.
(iii) It is corrosive and produces blisters on contact with skin.
(iv) B can be obtained by passing the vapours of A with air over platinum black catalyst.
(i) B liberates H₂ with sodium.
(ii) B gives CO₂ with NaHCO₃.
Questions:
- What is the molecular mass of A?
- Vapour density of A = 15, so its molecular mass = 2 × 15 = 30. Hence, the molecular formula of A is CH₃OH (methanol).
- Identify A, X, and B.
- A is methanol (CH₃OH).
- X is formaldehyde (HCHO).
- B is formic acid (HCOOH).
- Give appropriate reactions to confirm the identities of A, X, and B.
- Methanol (A) on partial oxidation forms formaldehyde (X):
CH₃OH → HCHO + H₂ - Formaldehyde (X) on strong oxidation forms formic acid (B):
HCHO + [O] → HCOOH
- State one large-scale use of either A, X, or B.
- Methanol (A) is widely used as an industrial solvent and antifreeze.
- Formaldehyde (X) is used in the production of plastics and resins.
- Formic acid (B) is used in leather processing and as a preservative in livestock feed.
