CBSE 2022 · Region 1 · Set 2 · Q12 · 5 marks

Read the passage given below and answer the questions that follow : Aldehydes, ketones and carboxylic acids are some of the important classes of organic compounds containing carbonyl group. These are highly polar molecules due to higher electro-negativity of oxygen relative to carbon in the carbonyl group. Aldehydes are prepared by dehydrogenation or controlled oxidation of primary alcohols and controlled reduction of acyl halides. Ketones are prepared by oxidation of secondary alcohols and hydration of alkynes. Aldehydes and ketones undergo nucleophilic addition reactions onto the carbonyl group but carboxylic acid does not undergo nucleophilic addition reaction. The alpha $\displaystyle (\alpha)$ - hydrogens of aldehydes and ketones are acidic. Therefore aldehydes and ketones having at least one $\displaystyle \alpha$-hydrogen undergo Aldol condensation. Aldehydes are easily oxidised by mild oxidising agents such as Tollens' reagent and Fehling's reagent. Carboxylic acids are prepared by the oxidation of primary alcohols, aldehydes and by hydrolysis of nitriles. Aromatic carboxylic acids are prepared by side-chain oxidation of alkyl benzenes. Carboxylic acids are considerably more acidic than alcohols and most of simple phenols.

(a)

Arrange the following in the increasing order of their reactivity towards nucleophilic addition reaction. : \[\mathrm{CH}_{3} \mathrm{COCH}_{3}, \mathrm{CH}_{3} \mathrm{CHO}, \mathrm{HCHO}, \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCH}_{3} \]

(b)

Give a simple chemical test to distinguish between Ethanal and Propanone.

(c)

Why carboxylic acid does not give nucleophilic addition reactions like aldehydes and ketones ? $\displaystyle 1$

(i)

Why $\displaystyle \alpha$-hydrogen of aldehydes and ketones are acidic in nature ?

(ii)

Write the products in the following :

(ii)

$\displaystyle 2$ $\displaystyle \xrightarrow{\text{Conc. NaOH}}$

(ii)

OR Write the major products of the following reactions :

(i)

$\displaystyle \xrightarrow[\text{(b) } \mathrm{H}^{+}]{\text{(a) } \mathrm{KMnO}_4,\ \mathrm{KOH}}$

(ii)

$\displaystyle \xrightarrow{\mathrm{H}_2,\ \mathrm{Pd}\text{-}\mathrm{BaSO}_4}$

CBSE Class 12 Chemistry past-paper question from the 2022board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.