Preparation and Physical Properties of Carboxylic Acids - UNSOLVED PRACTICE SET
Chapter: Aldehydes Ketones and Carboxylic Acids | Topic: Preparation and Physical Properties
PREPARATION AND PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS - UNSOLVED PRACTICE SET
Topic: Preparation and Physical Properties
Multiple Choice Questions
Q1. Carboxylic acids are prepared by oxidation of:
- Primary alcohols
- Aldehydes
- Alkyl benzenes
- All of the above
Q2. Hydrolysis of nitriles gives:
- Amides
- Carboxylic acids
- Alcohols
- Amines
Q3. The boiling points of carboxylic acids are higher than corresponding alcohols because of:
- Higher molecular mass
- Stronger hydrogen bonding (dimer formation)
- Dipole-dipole interactions
- Ionic bonding
Q4. Lower carboxylic acids are soluble in water because:
- They form hydrogen bonds with water
- They are polar
- They ionize in water
- All of the above
Q5. Which of the following is NOT a method for preparing carboxylic acids?
- Oxidation of primary alcohols
- Hydrolysis of esters
- Reduction of aldehydes
- Carbonation of Grignard reagents
Q6. The smell of vinegar is due to:
- Formic acid
- Acetic acid
- Propionic acid
- Butyric acid
Short Answer Questions
Q7. Write the reaction for the oxidation of ethanol to acetic acid. What oxidizing agent is used?
Q8. What is the hydrolysis of nitriles? Write the reaction for the preparation of acetic acid from acetonitrile.
Q9. Why do carboxylic acids have higher boiling points than alcohols of comparable molecular mass?
Q10. Explain why formic acid and acetic acid are miscible with water in all proportions, while higher acids are only partially soluble.
Q11. Write the reaction for the carbonation of Grignard reagent (CH₃MgBr) followed by acid hydrolysis. What is the product?
Q12. Your mother uses vinegar (acetic acid) in cooking. Why does vinegar have a sour taste? What is the typical concentration of acetic acid in household vinegar?
Long Answer Questions
Q13. (a) Describe the preparation of carboxylic acids from:
(i) Primary alcohols
(ii) Aldehydes
(iii) Alkyl benzenes
(iv) Nitriles
(v) Grignard reagents
Write the reactions for each method.
Q14. (a) Discuss the physical properties of carboxylic acids with respect to:
(i) Boiling points
(ii) Solubility in water
(iii) Odour
(iv) Melting points
Explain each property in terms of intermolecular forces.
(b) Why do carboxylic acids exist as dimers in the liquid state and in non-aqueous solvents?
Q15. (a) Compare the physical properties of carboxylic acids with those of alcohols, aldehydes, and ketones of comparable molecular mass.
(b) Explain why:
(i) The melting points of even-numbered carboxylic acids are higher than odd-numbered ones
(ii) Benzoic acid has a higher melting point than acetic acid
Numerical / Application-Based Problems
Q16. The following table gives boiling point data for some compounds:
| Compound | Molecular Mass (g/mol) | Boiling Point (°C) |
|---|---|---|
| CH₃OH | 32 | 65 |
| HCOOH | 46 | 101 |
| CH₃CH₂OH | 46 | 78 |
| CH₃COOH | 60 | 118 |
| CH₃CH₂CH₂OH | 60 | 97 |
| CH₃CH₂COOH | 74 | 141 |
(a) Compare the boiling points of HCOOH and CH₃CH₂OH despite similar molecular masses. Explain the difference.
(b) Compare the boiling points of CH₃COOH and CH₃CH₂CH₂OH despite similar molecular masses. Explain.
(c) Plot a graph of boiling point vs. molecular mass for carboxylic acids and alcohols.
(d) Calculate the average increase in boiling point per CH₂ group for carboxylic acids.
(e) A student claims that hydrogen bonding is the only reason for the higher boiling points of carboxylic acids. Is this correct? Explain. (Hint: Consider dimer formation.)
Q17. The following data relates to the solubility of carboxylic acids in water:
| Acid | Solubility (g/100 mL water at 20°C) |
|---|---|
| HCOOH | Miscible |
| CH₃COOH | Miscible |
| CH₃CH₂COOH | Miscible |
| CH₃(CH₂)₂COOH | 5.6 |
| CH₃(CH₂)₃COOH | 1.0 |
| CH₃(CH₂)₄COOH | 0.2 |
| C₆H₅COOH | 0.34 |
(a) Explain why solubility decreases as the carbon chain length increases.
(b) Why is benzoic acid much less soluble than butanoic acid despite having a similar number of carbon atoms?
(c) Calculate the number of carbon atoms at which the carboxylic acid becomes practically insoluble in water.
(d) The sodium salt of benzoic acid (sodium benzoate) is highly soluble in water. Why is it used as a food preservative instead of benzoic acid?
(e) A pharmaceutical company wants to formulate a water-soluble drug containing a carboxylic acid group. What modification would you suggest to improve water solubility?
Q18. In a school science project, students investigate the preparation and uses of carboxylic acids.
(a) A student prepares acetic acid by oxidation of ethanol using acidified K₂Cr₂O₇. Write the reaction. Why is the reaction carried out under reflux?
(b) Another student prepares benzoic acid by oxidation of toluene using KMnO₄. Write the reaction. Why does the benzene ring remain intact while the methyl group is oxidized?
(c) A third student prepares lactic acid (2-hydroxypropanoic acid) from acetaldehyde. Write the reactions involved. (Hint: Cyanohydrin formation followed by hydrolysis.)
(d) The students learn that citric acid is used in soft drinks and as a cleaning agent. Why does citric acid have a sour taste? How many carboxylic acid groups does it contain?
(e) The teacher asks: "If you were designing a biodegradable plastic from carboxylic acids, which acid would you choose and why?" Give your answer with reasoning. (Consider lactic acid and polylactic acid PLA.)