(i) Draw structure of the 2, 4-dinitrophenyl hyzdrazone derivative of benzaldehyde.
(ii) Arrange the following in increasing order of their reactivity towards HCN :
Di-tert. butyl ketone, Acetaldehyde, Acetone
(iii) Give a simple chemical test to distinguish between benzoic acid and ethyl benzoate.
(iv) Write the name of the reagent to convert Ethanenitrile to Ethanal.
(v) Draw the structure of 'X' in the following reaction :
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Use NaHCO3 for the acid test, DIBAL-H for the nitrile reduction, and note that CrO3 oxidises cyclohexanol to cyclohexanone. Reactivity towards HCN falls as crowding rises.
Step 1: Part (i), the 2,4-DNP derivative of benzaldehyde: Benzaldehyde reacts with 2,4-dinitrophenylhydrazine by addition followed by loss of water. The C=O group becomes C=N. \[ \mathrm{C_6H_5CHO + H_2N{-}NH{-}C_6H_3(NO_2)_2 \rightarrow C_6H_5CH{=}N{-}NH{-}C_6H_3(NO_2)_2 + H_2O} \] The structure is a phenyl ring joined to CH=N-NH, and the NH is attached to a benzene ring that carries nitro groups at the 2 and 4 positions. It is a yellow or orange solid.
Step 2: Part (ii), reactivity towards HCN: HCN adds by nucleophilic attack of \( \mathrm{CN^-} \) on the carbonyl carbon. Large alkyl groups block the attack (steric hindrance) and push electrons to the carbon (+I effect), which lowers reactivity. Acetaldehyde has one methyl group and one hydrogen, so it is the least crowded and most reactive. Acetone has two methyl groups. Di-tert-butyl ketone has two very bulky groups and is hardly reactive. \[ \text{Di-tert-butyl ketone} < \text{Acetone} < \text{Acetaldehyde} \]
Step 3: Part (iii), benzoic acid and ethyl benzoate: Add sodium bicarbonate solution to each sample. Benzoic acid gives brisk effervescence of \( \mathrm{CO_2} \). Ethyl benzoate, an ester, gives no gas. \[ \mathrm{C_6H_5COOH + NaHCO_3 \rightarrow C_6H_5COONa + H_2O + CO_2\uparrow} \]
Step 4: Part (iv), ethanenitrile to ethanal: A nitrile is partly reduced to an aldehyde by DIBAL-H, that is diisobutylaluminium hydride, in a cold solvent, followed by water. \[ \mathrm{CH_3CN \xrightarrow[(ii)\ H_2O]{(i)\ DIBAL\text{-}H} CH_3CHO} \] The same change is possible with SnCl2 and HCl, followed by hydrolysis (Stephen reaction).
Step 5: Part (v), oxidation of cyclohexanol: The reagent \( \mathrm{CrO_3} \) is a strong oxidising agent. A secondary alcohol is oxidised to a ketone. Cyclohexanol has the -OH on a carbon with one hydrogen, so that carbon becomes C=O. X is cyclohexanone, a six-membered ring with a C=O group. \[ \mathrm{C_6H_{11}OH \xrightarrow{CrO_3} C_6H_{10}O} \]
Final Answer: (i) \( \mathrm{C_6H_5CH{=}N{-}NH{-}C_6H_3(NO_2)_2} \) (ii) Di-tert-butyl ketone < Acetone < Acetaldehyde (iii) NaHCO3 test (iv) DIBAL-H (v) Cyclohexanone. \[ \boxed{\text{(v) } X = \text{cyclohexanone}} \]
