Match List-I (reagent) with List-II (Transformation)
|
List-I Reducing Agent |
List-II Transformation |
|
(A) Sodium Borohydride |
(I) Aldehyde to alkane |
|
(B) \( SnCl_2 \) and \( HCl \) |
(II) Primary alcohols to aldehydes |
|
(C) \( Zn-Hg \) and conc. \( HCl \) |
(III) Nitriles to Imine |
|
(D) DIBAL-H |
(IV) Ester to aldehyde |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A)-(II), (B)-(III), (C)-(I), (D)-(IV)
Each reducing agent in List-I acts on a particular group and stops at a particular stage. Hydride reagents (\( \ce{NaBH4} \), DIBAL-H) deliver \( \ce{H-} \) to a polar multiple bond, \( \ce{SnCl2/HCl} \) reduces a nitrile only as far as the imine, and zinc amalgam with concentrated HCl removes the carbonyl oxygen completely.
(A) Sodium Borohydride — This matches (II) Primary alcohols to aldehydes. \( \ce{NaBH4} \) is the reagent in the list that links aldehydes with primary alcohols: it is a mild hydride donor that reduces an aldehyde to a primary alcohol (and a ketone to a secondary alcohol) without touching esters or acids.
\( \displaystyle \ce{RCHO} \xrightarrow{\ce{NaBH4}} \ce{RCH2OH} \)
(B) \(SnCl_2\) and \(HCl\) — This matches (III) Nitriles to Imine. In the Stephen reaction stannous chloride and HCl reduce a nitrile to an imine, which is then hydrolysed to an aldehyde.
\( \displaystyle \ce{R-CN} \xrightarrow{\ce{SnCl2 + HCl}} \ce{R-CH=NH} \xrightarrow{\ce{H3O+}} \ce{R-CHO} \)
(C) \(Zn-Hg\) and conc. \(HCl\) — This matches (I) Aldehyde to alkane. In the Clemmensen reduction the carbonyl group is reduced to a \( \ce{CH2} \) group (for an aldehyde, the CHO becomes \( \ce{CH3} \)).
\( \displaystyle \ce{RCHO} \xrightarrow{\ce{Zn-Hg}/\text{conc. }\ce{HCl}} \ce{RCH3 + H2O} \)
(D) DIBAL-H — This matches (IV) Ester to aldehyde. Diisobutylaluminium hydride is a bulky hydride that reduces an ester only to the aldehyde stage at low temperature.
\( \displaystyle \ce{RCOOR'} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) DIBAL-H}} \ce{RCHO + R'OH} \)
Shortcut: (C)-(I), the Clemmensen reduction, appears in Options (1) and (4); of these, only Option (1) has (B)-(III), the Stephen reaction.
Note: It seems that there is a mistake in the original NTA paper. List-II (II) is printed as “Primary alcohols to aldehydes”; it should be “Aldehydes to primary alcohols”, since sodium borohydride is a reducing agent and the change of a primary alcohol to an aldehyde is an oxidation.