Aldehyde ketones & carboxylic Acid MCQs for NEET — Chemistry Questions with Answers

The NCERT text states: 'The above methods (Grignard reagents and nitriles from alkyl halides) are useful for converting alkyl halides into corresponding carboxylic acids having one carbon atom more than that present in alkyl halides (ascending the series).'

The NCERT text mentions: 'Aromatic carboxylic acids are also prepared by side-chain oxidation of alkylbenzenes.' And 'Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate.'

The NCERT states: 'Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate ($KMnO_4$) in neutral, acidic or alkaline media or by potassium dichromate ($K_2Cr_2O_7$) and chromium trioxide ($CrO_3$) in acidic media (Jones reagent).' Using $K_2Cr_2O_7/H^+$ on Butan-1-ol will oxidize it to Butanoic acid. While Butyronitrile hydrolysis will give butanoic acid, option o1 is a direct method mentioned in the text for converting alcohol to acid, which is specifically highlighted with an example (Butan-1-ol to butanoic acid).

The reaction of Grignard reagents with carbon dioxide (dry ice) forms salts of carboxylic acids, which upon acidification, yield the carboxylic acid. To form 2-phenylethanoic acid ($C_6H_5CH_2COOH$), the Grignard reagent must be Benzylmagnesium bromide ($C_6H_5CH_2MgBr$). This allows for the addition of one carbon atom to the benzyl group.

Bromoethane ($CH_3CH_2Br$) reacts with KCN to form ethyl nitrile ($CH_3CH_2CN$). Hydrolysis of nitriles ($CH_3CH_2CN$) in the presence of $H^+$ catalyst yields carboxylic acids. In this case, ethyl nitrile will be hydrolyzed to propanoic acid ($CH_3CH_2COOH$). The method from nitriles increases the carbon chain by one.

The NCERT states: 'Primary and secondary alkyl groups are oxidised in this manner while tertiary group is not affected.' referring to the vigorous oxidation of alkylbenzenes to aromatic carboxylic acids. Therefore, tertiary alkylbenzenes are not readily oxidized.

The NCERT explicitly states: 'Aldehydes are easily oxidised to carboxylic acids by mild oxidising reagents such as Tollens’ reagent and Fehling’s reagent.' Butanal is an aldehyde, and butanoic acid is its corresponding carboxylic acid. $NaBH_4$ and $LiAlH_4$ are reducing agents, not oxidizing agents. Concentrated $H_2SO_4$ is an acid catalyst but not an oxidizing agent for this conversion.

The context highlights: 'The above methods (Grignard reagents and nitriles from alkyl halides) are useful for converting alkyl halides into corresponding carboxylic acids having one carbon atom more than that present in alkyl halides (ascending the series).'

The NCERT states: 'Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid... The entire side chain is oxidised to the carboxyl group irrespective of length of the side chain.' To get benzene-1,4-dicarboxylic acid, the starting material must have two methyl groups at positions 1 and 4 on the benzene ring, which is p-xylene.

The NCERT specifies: 'Nitriles are hydrolysed to amides and then to acids in the presence of $H^+$ or $OH^-$ as catalyst.'