Aldehydes, Ketones & Carboxylic Acids – Solved Board Questions & Numericals
Topic-wise solved board examination questions on aldehydes, ketones, and carboxylic acids with step-by-step scoring solutions and reaction mechanisms.
Topic-Wise Solved Board Examination Questions & Solutions
Curated board examination questions with step-by-step marking schemes for CBSE & ICSE board aspirants.
(a) What are ketones? State their general formula and write the IUPAC name of the simplest ketone.
(b) Explain why propanone does not give a silver mirror test with Tollens' reagent while propanal does.
(a) Definition & Formula:
- Ketones are organic compounds possessing a carbonyl group (>C=O) bonded to two hydrocarbon alkyl or aryl groups.
- General Formula: CnH2nO or R-CO-R'
- Simplest Ketone: Propanone (CH₃-CO-CH₃), commonly known as Acetone (3 Carbon atoms). [1 Mark]
(b) Explanation of Tollens' Test:
- Propanal (CH₃CH₂CHO) contains a hydrogen atom attached directly to the carbonyl group (C-H bond), which makes it readily oxidizable to propanoic acid by mild oxidizing agents like Tollens' reagent [Ag(NH₃)₂]⁺, forming metallic silver.
- Propanone (CH₃COCH₃) contains no hydrogen on the carbonyl carbon (it is bonded to two methyl carbon groups). Breaking carbon-carbon bonds requires strong oxidizing agents, so propanone cannot reduce Tollens' reagent and gives no silver mirror. [2 Marks]
Write the chemical equations for the following named conversions:
(i) Benzoyl chloride to Benzaldehyde (Rosenmund Reduction)
(ii) Ethanal to Propan-2-ol using Methyl magnesium bromide (Grignard Reagent)
(i) Rosenmund Reduction:
C₆H₅-CO-Cl + H₂ ──[ Pd / BaSO₄ , Quinoline ]──> C₆H₅-CHO + HCl Benzoyl chloride Benzaldehyde
(ii) Conversion of Ethanal to Propan-2-ol via Grignard Reagent:
CH₃-CHO + CH₃MgBr ──[ Dry Ether ]──> CH₃-CH(OMgBr)-CH₃
│ (Acid Hydrolysis, H₃O⁺)
▼
CH₃-CH(OH)-CH₃ + Mg(OH)Br
(Propan-2-ol, 2° Alcohol)
Arrange the following in increasing order of their boiling points with reasoning:
Ethane (C₂H₆), Methoxy methane (CH₃OCH₃), Ethanal (CH₃CHO), Ethanol (CH₃CH₂OH)
Order: Ethane < Methoxy methane < Ethanal < Ethanol
Reasoning:
- Ethane: Non-polar with weak London dispersion forces (Lowest boiling point).
- Methoxy methane: Weak dipole-dipole interactions.
- Ethanal: Stronger dipole-dipole attractions due to polar carbonyl group (>C=O).
- Ethanol: Intermolecular Hydrogen bonding between -OH groups (Highest boiling point).
Indexed Topics & Examination Keywords
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