CBSE Class 12 Chemistry Aldehydes Ketones And Carboxylic Acids Board Exam Notes

ASSERTION - REASON TYPE QUESTIONS
A statement of assertion is followed by a statement of reason. Mark the correct choice from the options given below.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Both assertion and reason are false.

1. Assertion : Carbonyl compounds take part in nucleophilic addition reactions.
Reason : These reactions are initiated by nucleophilic attack at the electron deficient carbon atom. ( Ans - a)

2. Assertion : All aldehydes do not take part in aldol condensation.
Reason : In the aldol condensation, cabanion is generated by the abstraction of α - H atom by base. (Ans - b)

3. Assertion : Acetone is less reactive towards nucleophilic addition than acetaldehyde.
Reason : The alkyl groups hinder the nucleophilic attack on carbonyl carbon atom. ( Ans - a)

4. Assertion : The α - H atom in carbonyl compounds is less acidic.
Reason : The anion formed after the loss of α - H atom is not resonance stabilized. ( Ans - d)

5. Assertion : Aldehydes and ketones both react with Tollen’s reagent to form silver mirror.
Reason : Both aldehydes and ketones contain a carboxylic group. ( Ans - d)

One - word answer
1. Name the reagent used to distinguish between methanoic acid and ethanoic acid.
( Ans - Tollen’ s reagent)

2. Name the product obtained by reaction of hydroxylamine with carbonyl compound?
( Ans - Oxime)

SHORT ANSWER TYPE QUESTIONS (2‐ MARKS)
Q1. Write chemical equation to illustrate following name reactions:‐
(a) Cannizzaro's reaction
(b) Hell‐Volhard‐ Zelinsky reaction
Ans. (a) Cannizzaro's reaction

Q2. Givea chemical test to distinguish between thefollowing pairs:‐

(i) Phenol and benzoic acid
(ii) Benzaldehyde and Acetophenone

Ans. (i) Benzoic acid reacts with NaHCO₃ giving CO₂ gas with effervescence where as phenol does not . C₆H₅ COOH + NaHCO₃ →C₆H₅COONa + H₂O + CO₂
(ii) Acetophenone on reacting with hot NaOH / I2 gives yellow ppt of CHI3while Benzophenone does not .CH3COC6H5 NaOH/I2 CHI3 + C6H5COONa

Q3. Account for the following :‐
(i) Chloroacetic acid Cl‐CH2COOH is a stronger acid than acetic acid CH3COOH.
(ii) Carboxylic acids do not give the reaction of carbonyl group.
Ans. (i) The –I effect of Cl atom in ClCH₂ COOH stabiles the ClCH COO^O ion while +I e ffect of CH ‐ group in CH₃ COOH destabilizes the CH₃COO⁰.
(ii) In the resonating structures of carboxylic acid and carbonyls, the carbonyl carbon of carboxylic acid is less electro‐positive (less electrophile) than carbonyl carbon in aldehydes and ketones. Therefore carboxylic acids do not give the reaction of carbonyl group

Q4. Arrange the following.
(i) C6H5COOH, FCH2COOH, NO2CH2COOH( decreasing order of their acidic character)
(ii) Ethanal, Propanal, Propanone, Butanone reaction (increasing order of their nucleophilic addition reaction)
Ans. (i) NO₂‐CH₂‐COOH> F‐CH₂‐COOH >C₆H₅‐COOH
(ii) Butanone < Propanone < Propanal < Ethanal

Q5. An organic compound 'A' with molecular formula C8H8O gives positive DNP and iododorm test. It does not reduce Tollens' or Fehling's reagent and does not decolourise bromine water also . On oxidation with chromic acid, it gives a carboxylic acid 'B' with molecular formula C₇H₆O₂. Deduce the structures A and B.
Ans. Since A does not give Fehling's or Tollen‘s test but gives iodoform test and 2,4‐ DNP test so it has CH3 CO‐ group. Hence: