Read the following passage given below and answer the question.
Aldehydes and ketones undergo nucleophilic addition reactions. A nucleophile attacks the electrophilic carbon atom of polar carbonyl group from perpendicular to plane of \(sp^2\) hybridized orbitals of carbonyl carbon. The hybridization of carbon changes from \(sp^2\) to \(sp^3\) and a tetrahedral alkoxide intermediate is formed. This captures \(H^+\) to give neutral product. In carboxylic acids, carbonyl carbon is less electrophilic than carbonyl carbon of aldehydes and ketones.
From the following reactions, identify the reactions which are correctly matched with the names:
Choose the correct answers from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A, B, C, E only
These named reactions fall into two families. Reactions that make an aldehyde and stop at that stage: the Rosenmund reduction (acyl chloride, \( \ce{H2} \), Pd on \( \ce{BaSO4} \)), the Etard reaction (toluene, chromyl chloride) and the Gattermann-Koch reaction (benzene, CO and HCl). Reactions that reduce C=O to CH2: the Clemmensen reduction (Zn-Hg and conc. HCl, acid medium) and the Wolff-Kishner reduction (hydrazine, then KOH in ethylene glycol on heating, basic medium). The Balz-Schiemann reaction belongs to neither family: it is a reaction of diazonium salts that gives aryl fluorides.
(A) Etard reaction — Correct. Chromyl chloride oxidises the methyl group of toluene to a chromium complex, which on hydrolysis gives benzaldehyde.
\( \displaystyle \ce{C6H5CH3} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{CrO2Cl2},\ \ce{CS2}} \ce{C6H5CHO} \)
(B) Rosenmund reduction — Correct. An acyl chloride is hydrogenated over palladium on barium sulphate. The poisoned catalyst stops the reduction at the aldehyde.
\( \displaystyle \ce{RCOCl + H2} \xrightarrow{\ce{Pd-BaSO4}} \ce{RCHO + HCl} \)
(C) Clemmensen reduction — Correct. Zinc amalgam and concentrated HCl reduce the carbonyl group of an aldehyde or ketone to a \( \ce{CH2} \) group.
\( \displaystyle \ce{R2C=O + 4[H]} \xrightarrow{\ce{Zn-Hg},\ \text{conc. }\ce{HCl}} \ce{R2CH2 + H2O} \)
(D) Balz-Schiemann reaction — Incorrect. The reaction shown (hydrazine, then KOH on heating, C=O reduced to \( \ce{CH2} \)) is the Wolff-Kishner reduction:
\( \displaystyle \ce{R2C=O} \xrightarrow[-\ce{H2O}]{\ce{NH2NH2}} \ce{R2C=NNH2} \xrightarrow[\text{ethylene glycol, heat}]{\ce{KOH}} \ce{R2CH2 + N2} \)
The real Balz-Schiemann reaction makes an aryl fluoride: an arenediazonium salt is treated with fluoroboric acid, and the diazonium fluoroborate is heated.
\( \displaystyle \ce{ArN2^+Cl^- + HBF4 -> ArN2^+BF4^- + HCl} \)
\( \displaystyle \ce{ArN2^+BF4^-} \xrightarrow{\Delta} \ce{ArF + BF3 + N2} \)
(E) Gattermann-Koch reaction — Correct. Benzene treated with carbon monoxide and HCl in the presence of anhydrous \( \ce{AlCl3} \) or CuCl gives benzaldehyde.
\( \displaystyle \ce{C6H6 + CO} \xrightarrow{\ce{HCl},\ \text{anhyd. }\ce{AlCl3/CuCl}} \ce{C6H5CHO} \)