In the Etard reaction, chromyl chloride ($\text{CrO}_2\text{Cl}_2$) oxidises the methyl group of toluene in non-polar solvent ($\text{CS}_2$ or $\text{CCl}_4$) to a brown chromium complex, which on aqueous hydrolysis gives benzaldehyde, an aromatic aldehyde:

$$\text{C}_6\text{H}_5\text{CH}_3 + 2\text{CrO}_2\text{Cl}_2 \xrightarrow{\text{CS}_2} \text{C}_6\text{H}_5\text{CH}(\text{OCrOHCl}_2)_2 \xrightarrow{\text{H}_3\text{O}^+} \text{C}_6\text{H}_5\text{CHO}$$

Reagents and their standard chemical compositions:

• Tollens' reagent: Ammoniacal silver nitrate $[\text{Ag}(\text{NH}_3)_2]\text{OH}$ $\rightarrow$ (III)
• Jones reagent: Chromium trioxide in aqueous sulphuric acid / acetone ($\text{CrO}_3 / \text{H}_2\text{SO}_4$) $\rightarrow$ (IV)
• Lucas reagent: Equimolar mixture of concentrated $\text{HCl}$ and anhydrous $\text{ZnCl}_2$ $\rightarrow$ (II)
• Fehling's solution B: Alkaline solution of sodium potassium tartrate (Rochelle salt) $\rightarrow$ (I)

Correct match: (A)-(III), (B)-(IV), (C)-(II), (D)-(I).

Acidic strength is inversely related to $\text{p}K_a$ (lower $\text{p}K_a$ means stronger acid):

• $\text{FCH}_2\text{COOH}$: $\text{p}K_a = 2.66$ (strongest acid due to highest electronegativity of F)
• $\text{ClCH}_2\text{COOH}$: $\text{p}K_a = 2.86$
• $\text{BrCH}_2\text{COOH}$: $\text{p}K_a = 2.90$
• $\text{HCOOH}$: $\text{p}K_a = 3.75$ (weakest acid among these)

Increasing order of acidity: $\text{HCOOH (D)} \lt \text{BrCH}_2\text{COOH (A)} \lt \text{ClCH}_2\text{COOH (B)} \lt \text{FCH}_2\text{COOH (C)}$.

Reaction analysis:

• Hydration of unsymmetrical or non-ethyne alkynes in presence of $\text{H}_2\text{SO}_4/\text{HgSO}_4$ forms ketones (e.g., propyne gives acetone).
• Ozonolysis of tetra-substituted or tri-substituted alkenes produces ketones.
• Treatment of alkyl nitriles ($\text{R-CN}$) with Grignard reagents followed by acid hydrolysis yields ketones ($\text{R-CO-R'}$).
• Stephen's reaction involves the reduction of nitriles with stannous chloride ($\text{SnCl}_2$) in the presence of $\text{HCl}$ followed by hydrolysis to specifically yield aldehydes (not ketones):
  $$\text{R-CN} + \text{SnCl}_2 + \text{HCl} \rightarrow \text{R-CH=NH}\cdot\text{HCl} \xrightarrow{\text{H}_3\text{O}^+} \text{R-CHO}$$