JEE Challenger
More from Aldehydes, Ketones and Carboxylic Acids
ChemHardAdv 2018 • Paper 2

Preparation of Substituted Carboxylic Acid via Rearrangement and Reagents

The target product X\mathbf{X} can be produced by treating the primary product of the reactions specified in LIST-I with one or more suitable reagents from LIST-II.

(Note: Migratory aptitude follows aryl>alkyl>hydrogen\text{aryl} > \text{alkyl} > \text{hydrogen})

LIST-I

P.\mathbf{P.} Reactant mixture +H2SO4+ \text{H}_2\text{SO}_4

Q.\mathbf{Q.} Reactant mixture +HNO2+ \text{HNO}_2

R.\mathbf{R.} Reactant mixture +H2SO4+ \text{H}_2\text{SO}_4

S.\mathbf{S.} Reactant mixture +AgNO3+ \text{AgNO}_3

LIST-II

  1. I2,NaOH\text{I}_2, \text{NaOH}
  2. [Ag(NH3)2]OH[\text{Ag}(\text{NH}_3)_2]\text{OH}
  3. Fehling solution
  4. HCHO,NaOH\text{HCHO}, \text{NaOH}
  5. NaOBr\text{NaOBr}

Select the correct option:

Question Diagram 1

Options

A

P→1;Q→2,3;R→1,4;S→2,4\mathbf{P} \rightarrow 1; \quad \mathbf{Q} \rightarrow 2,3; \quad \mathbf{R} \rightarrow 1,4; \quad \mathbf{S} \rightarrow 2,4

B

P→1,5;Q→3,4;R→4,5;S→3\mathbf{P} \rightarrow 1,5; \quad \mathbf{Q} \rightarrow 3,4; \quad \mathbf{R} \rightarrow 4,5; \quad \mathbf{S} \rightarrow 3

C

P→1,5;Q→3,4;R→5;S→2,4\mathbf{P} \rightarrow 1,5; \quad \mathbf{Q} \rightarrow 3,4; \quad \mathbf{R} \rightarrow 5; \quad \mathbf{S} \rightarrow 2,4

D

P→1,5;Q→2,3;R→1,5;S→2,3\mathbf{P} \rightarrow 1,5; \quad \mathbf{Q} \rightarrow 2,3; \quad \mathbf{R} \rightarrow 1,5; \quad \mathbf{S} \rightarrow 2,3

Correct

Topics & Concepts

Aldehydes, Ketones and Carboxylic AcidsAldehydes and KetonesCarboxylic Acids and DerivativesOxidation and Reduction (Organic)

Step-by-Step Solution

To determine the correct set of reagents required to produce the target product X\mathbf{X} from the primary products of reactions in LIST-I, let us analyze the target molecule and each reaction step by step.

1. Target Molecule (X\mathbf{X})

The target compound X\mathbf{X} is 2,2-diphenylpropanoic acid: X=Ph2C(CH3)COOH\mathbf{X} = \text{Ph}_2\text{C}(\text{CH}_3)\text{COOH}

2. Analysis of Reaction P

  • Reactant: Ph2C(OH)−C(CH3)2OH\text{Ph}_2\text{C}(\text{OH})-\text{C}(\text{CH}_3)_2\text{OH} (1,1-diphenyl-2,2-dimethylethane-1,2-diol)
  • Reagent: H2SO4\text{H}_2\text{SO}_4 (Pinacol-Pinacolone Rearrangement)
  1. Protonation occurs preferentially at the −OH-\text{OH} group attached to the diphenyl-substituted carbon because loss of water yields a highly stable dibenzylic carbocation: Ph2C(OH)−C(CH3)2OH→H+Ph2C+−C(CH3)2OH+H2O\text{Ph}_2\text{C}(\text{OH})-\text{C}(\text{CH}_3)_2\text{OH} \xrightarrow{\text{H}^+} \text{Ph}_2\text{C}^+ - \text{C}(\text{CH}_3)_2\text{OH} + \text{H}_2\text{O}
  2. A methyl group migrates from the adjacent carbon to the carbocation carbon to form a resonance-stabilized oxocarbocation: Ph2C+−C(CH3)2OH→∼CH3−Ph2C(CH3)−C+(OH)CH3→−H+Ph2C(CH3)COCH3\text{Ph}_2\text{C}^+ - \text{C}(\text{CH}_3)_2\text{OH} \xrightarrow{\sim \text{CH}_3^-} \text{Ph}_2\text{C}(\text{CH}_3) - \text{C}^+(\text{OH})\text{CH}_3 \xrightarrow{-\text{H}^+} \text{Ph}_2\text{C}(\text{CH}_3)\text{COCH}_3
  3. The primary product is the methyl ketone, 3,3-diphenylbutan-2-one (Ph2C(CH3)COCH3\text{Ph}_2\text{C}(\text{CH}_3)\text{COCH}_3).
  4. To convert this methyl ketone into the carboxylic acid X\mathbf{X}, a haloform reaction is required:
    • Reagent 1: I2,NaOH\text{I}_2, \text{NaOH}
    • Reagent 5: NaOBr\text{NaOBr}

Thus, P→1,5\mathbf{P} \rightarrow 1, 5.

3. Analysis of Reaction Q

  • Reactant: Ph2C(NH2)−CH(CH3)OH\text{Ph}_2\text{C}(\text{NH}_2)-\text{CH}(\text{CH}_3)\text{OH} (1-amino-1,1-diphenylpropan-2-ol)
  • Reagent: HNO2\text{HNO}_2 (Diazotization-driven semi-pinacol rearrangement)
  1. Reaction of −NH2-\text{NH}_2 with HNO2\text{HNO}_2 forms a diazonium ion −N2+-\text{N}_2^+, which departs immediately to generate a carbocation at the C-1 carbon: Ph2C(NH2)−CH(CH3)OH→HNO2Ph2C+−CH(CH3)OH+N2+H2O\text{Ph}_2\text{C}(\text{NH}_2)-\text{CH}(\text{CH}_3)\text{OH} \xrightarrow{\text{HNO}_2} \text{Ph}_2\text{C}^+ - \text{CH}(\text{CH}_3)\text{OH} + \text{N}_2 + \text{H}_2\text{O}
  2. According to the given migratory aptitude (aryl>alkyl>hydrogen\text{aryl} > \text{alkyl} > \text{hydrogen}), the alkyl (CH3\text{CH}_3) group migrates in preference to hydrogen: Ph2C+−CH(CH3)OH→∼CH3−Ph2C(CH3)−CH+(OH)→−H+Ph2C(CH3)CHO\text{Ph}_2\text{C}^+ - \text{CH}(\text{CH}_3)\text{OH} \xrightarrow{\sim \text{CH}_3^-} \text{Ph}_2\text{C}(\text{CH}_3) - \text{CH}^+(\text{OH}) \xrightarrow{-\text{H}^+} \text{Ph}_2\text{C}(\text{CH}_3)\text{CHO}
  3. The primary product is an aldehyde, 2,2-diphenylpropanal (Ph2C(CH3)CHO\text{Ph}_2\text{C}(\text{CH}_3)\text{CHO}).
  4. To oxidize this aliphatic aldehyde to carboxylic acid X\mathbf{X}, standard mild oxidizing agents can be used:
    • Reagent 2: [Ag(NH3)2]OH[\text{Ag}(\text{NH}_3)_2]\text{OH} (Tollens' reagent)
    • Reagent 3: Fehling solution

Thus, Q→2,3\mathbf{Q} \rightarrow 2, 3.

4. Analysis of Reaction R

  • Reactant: Ph(CH3)C(OH)−C(OH)(CH3)Ph\text{Ph}(\text{CH}_3)\text{C}(\text{OH})-\text{C}(\text{OH})(\text{CH}_3)\text{Ph} (2,3-diphenylbutane-2,3-diol)
  • Reagent: H2SO4\text{H}_2\text{SO}_4 (Pinacol-Pinacolone Rearrangement)
  1. Protonation of −OH-\text{OH} and loss of water gives carbocation Ph(CH3)C+−C(OH)(CH3)Ph\text{Ph}(\text{CH}_3)\text{C}^+ - \text{C}(\text{OH})(\text{CH}_3)\text{Ph}.
  2. Due to migratory aptitude (aryl>alkyl\text{aryl} > \text{alkyl}), the phenyl (aryl) group migrates to the adjacent carbon: Ph(CH3)C+−C(OH)(CH3)Ph→∼Ph−Ph2C(CH3)−C+(OH)CH3→−H+Ph2C(CH3)COCH3\text{Ph}(\text{CH}_3)\text{C}^+ - \text{C}(\text{OH})(\text{CH}_3)\text{Ph} \xrightarrow{\sim \text{Ph}^-} \text{Ph}_2\text{C}(\text{CH}_3) - \text{C}^+(\text{OH})\text{CH}_3 \xrightarrow{-\text{H}^+} \text{Ph}_2\text{C}(\text{CH}_3)\text{COCH}_3
  3. The primary product is the same methyl ketone (Ph2C(CH3)COCH3\text{Ph}_2\text{C}(\text{CH}_3)\text{COCH}_3) as in Reaction P.
  4. Haloform oxidation using reagents 1 (I2,NaOH\text{I}_2, \text{NaOH}) and 5 (NaOBr\text{NaOBr}) yields the carboxylic acid X\mathbf{X}.

Thus, R→1,5\mathbf{R} \rightarrow 1, 5.

5. Analysis of Reaction S

  • Reactant: Ph2C(Br)−CH(CH3)OH\text{Ph}_2\text{C}(\text{Br})-\text{CH}(\text{CH}_3)\text{OH} (1-bromo-1,1-diphenylpropan-2-ol)
  • Reagent: AgNO3\text{AgNO}_3
  1. Ag+\text{Ag}^+ precipitates Br−\text{Br}^- as AgBr\text{AgBr}, generating the exact same carbocation intermediate as formed in Reaction Q: Ph2C(Br)−CH(CH3)OH→Ag+Ph2C+−CH(CH3)OH+AgBr\text{Ph}_2\text{C}(\text{Br})-\text{CH}(\text{CH}_3)\text{OH} \xrightarrow{\text{Ag}^+} \text{Ph}_2\text{C}^+ - \text{CH}(\text{CH}_3)\text{OH} + \text{AgBr}
  2. Following migratory aptitude (alkyl>hydrogen\text{alkyl} > \text{hydrogen}), the methyl group migrates to yield the aldehyde Ph2C(CH3)CHO\text{Ph}_2\text{C}(\text{CH}_3)\text{CHO}.
  3. Oxidation using reagents 2 ([Ag(NH3)2]OH[\text{Ag}(\text{NH}_3)_2]\text{OH}) and 3 (Fehling solution) gives X\mathbf{X}.

Thus, S→2,3\mathbf{S} \rightarrow 2, 3.

Conclusion

Matching the results:

  • P→1,5\mathbf{P} \rightarrow 1, 5
  • Q→2,3\mathbf{Q} \rightarrow 2, 3
  • R→1,5\mathbf{R} \rightarrow 1, 5
  • S→2,3\mathbf{S} \rightarrow 2, 3

This matches Option D.

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