Aldehydes Ketones And Carboxylic Acids – JEE Main ChemistryPractice Questions & PYQs

The effectiveness of an oxidation reaction depends heavily on the oxidative potential of the reagents chosen. Because $\mathrm{CrO}_3 \cdot \mathrm{H}_2\mathrm{SO}_4$ acts as a strong oxidizing agent, it facilitates the complete oxidation of primary alcohols directly to carboxylic acids rather than stopping at the aldehyde stage. Consequently, this specific reagent pathway is unable to isolate acetaldehyde as a product, whereas the other listed processes are chemically suited to generate it.

When ethyl pent-4-yn-oate interacts with $\mathrm{CH}_3\mathrm{MgBr}$, the ester carbonyl group undergoes two successive nucleophilic additions to form a tertiary alcohol, establishing the first statement as true. During this transformation, the acidic proton on the terminal alkyne also reacts with the reagent, consuming an additional mole of Grignard reagent beyond the two needed for the ester conversion. Consequently, the process utilizes three moles of $\mathrm{CH}_3\mathrm{MgBr}$ in total, which proves that the second statement regarding the consumption of only two moles is false.

Clemmensen reduction serves as the initial chemical transformation, effectively reducing the carbonyl group of the open-chain ketone 4-ethylhexan-2-one to a methylene group to yield the alkane 3-ethylhexane. This intermediate alkane subsequently undergoes aromatization, a form of catalytic reforming that triggers cyclization and dehydrogenation to rearrange the carbon structure into the stable aromatic compound ethylbenzene.

The Wolff-Kishner reduction is specifically designed to convert a carbonyl group into an alkane while selectively leaving carbon-carbon double bonds within the molecule unaffected. This transformation is achieved by using hydrazine ($\mathrm{NH}_2-\mathrm{NH}_2$) in the presence of a strong base like $\mathrm{C}_2\mathrm{H}_5\mathrm{O}^-\mathrm{Na}^+$, which provides the necessary driving force by releasing stable nitrogen gas. In contrast, catalytic hydrogenation with $\mathrm{Ni}/\mathrm{H}_2$ would reduce any available double bonds, and common reducing agents like $\mathrm{NaBH}_4$ would simply yield an alcohol rather than the desired alkane product.

The Hoffmann Bromamide degradation serves as the primary mechanism for transforming amides into amines, involving the formation of an isocyanate intermediate followed by hydrolysis to yield a primary amine (Product A) with one fewer carbon atom. The subsequent treatment of this primary amine with nitrous acid ($NaNO_2 + HCl$) at $273-278$ K initiates the diazotization reaction, which produces a diazonium salt (Product B) if the substrate is an aromatic amine, or releases nitrogen gas and generates alcohols if the substrate is aliphatic. This chemical sequence utilizes these distinct functional group transformations to systematically derive the structure of the intermediates at each stage of the reaction.

Synthesizing 3-nitrobenzoic acid from benzene requires first establishing a meta-directing nitro group through nitration using $\mathrm{HNO}_3 / \mathrm{H}_2\mathrm{SO}_4$, which yields nitrobenzene. This nitro group then directs the subsequent electrophilic substitution of bromine, facilitated by $\mathrm{Br}_2 / \mathrm{AlBr}_3$, to the meta position to form m-bromonitrobenzene. The final transformation involves converting the bromine into a carboxylic acid group by forming a Grignard reagent with $\mathrm{Mg} / \mathrm{ether}$, reacting it with $\mathrm{CO}_2$, and performing an acid workup with $\mathrm{H}_3\mathrm{O}^+$.

The reagent sodium hypochlorite ($NaOCl$) acts as a robust oxidizing agent, driving the transformation of ketone functional groups into carboxylic acids. Since the observed product is a carboxylic acid, we can conclude that the starting compound $P$ must be a ketone to satisfy this specific oxidation pathway.

The synthesis of carboxylic acids using a Grignard reagent involves a nucleophilic attack on $\text{CO}_2$ that adds a carboxyl group to the existing alkyl chain, thereby increasing the total carbon count by one. Starting with $\text{CH}_3\text{CH}_2\text{CH}_2\text{Br}$ introduces a three-carbon chain into this process, which inevitably leads to the formation of a four-carbon carboxylic acid known as butanoic acid, $\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}$. Because the length of the carbon backbone is extended by this reaction sequence, it cannot synthesize three-carbon propionic acid.

The reaction between acetophenone and a mixture of $\mathrm{Br}_2$ and $\mathrm{KOH}$ is a classic example of the haloform reaction, which occurs specifically in methyl ketones. In this transformation, the basic medium facilitates the cleavage of the carbon-carbon bond adjacent to the carbonyl group, effectively converting the terminal methyl group into bromoform ($\mathrm{CHBr}_3$). Simultaneously, the remainder of the molecule is oxidized to form the conjugate base of the corresponding carboxylic acid, resulting in the formation of a benzoate salt.

To identify which compounds contain a $-COOH$ group, one must analyze the functional groups present in the molecular structure of each molecule provided. While sulphanilic acid contains a sulfonic acid moiety, picric acid is characterized by its phenolic and nitro groups, and ascorbic acid comprises a cyclic structure with hydroxyl and lactone groups. Only aspirin, known as acetylsalicylic acid, incorporates the required $-COOH$ functional group, establishing that exactly 1 compound satisfies the condition.

Esters are inherently susceptible to nucleophilic acyl substitution when placed in an aqueous basic medium, a process commonly referred to as saponification. The hydroxide ion acts as a strong nucleophile that attacks the electrophilic carbonyl carbon of the ester group ($RCOOR'$), triggering the cleavage of the molecule into a carboxylate anion and an alcohol. Because this reaction proceeds readily under basic conditions, esters are chemically unstable in this environment compared to other more robust alcohol derivatives like ethers.

The synthesis of benzaldehyde from these precursors relies on specific name reactions in organic chemistry. Benzene is converted to benzaldehyde via the Gattermann-Koch reaction using $\mathrm{CO}$, $\mathrm{HCl}$, and anhydrous $\mathrm{AlCl}_3$. Benzonitrile undergoes the Stephen reduction, where reaction with $\mathrm{SnCl}_2$ and $\mathrm{HCl}$ followed by acid hydrolysis yields benzaldehyde. Finally, benzoyl chloride is selectively reduced to benzaldehyde through the Rosenmund reduction, which utilizes $\mathrm{H}_2$ in the presence of a $\mathrm{Pd}-\mathrm{BaSO}_4$ catalyst poisoned by sulfur or quinoline to prevent further reduction to a primary alcohol.

Nucleophilic addition reactivity depends on the steric accessibility of the carbonyl carbon and the electronic environment surrounding it. Aldehydes are inherently more reactive than ketones because ketones contain two alkyl groups that create greater steric hindrance and donate more electron density to the electrophilic carbon, diminishing its positive charge. Within this hierarchy, the presence of an ethyl group in butanone ($CH_3COCH_2CH_3$) compared to the two methyl groups in propanone ($CH_3COCH_3$) makes butanone the least reactive due to enhanced bulkiness, while benzaldehyde ($C_6H_5CHO$) is less reactive than propanal ($CH_3CH_2CHO$) because resonance stabilization from the phenyl ring reduces the electrophilicity of the carbonyl center.

The reaction of propanal (\text{CH}_3\text{CH}_2\text{CHO}$) with methylmagnesium bromide produces butan-2-ol, a secondary alcohol, while the reaction of acetone (\text{CH}_3\text{COCH}_3$) with the same reagent yields 2-methylpropan-2-ol, a tertiary alcohol. The Lucas test differentiates these alcohols by reaction rate, showing immediate turbidity for the tertiary alcohol due to the rapid formation of a stable carbocation, whereas the secondary alcohol reacts more slowly, typically within five minutes. Additionally, the iodoform test specifically identifies the $\text{CH}_3\text{CH(OH)-}$ structural fragment present in butan-2-ol but absent in 2-methylpropan-2-ol, confirming that compound C is butan-2-ol and compound D is 2-methylpropan-2-ol.

Nucleophilic addition of HCN to a carbonyl compound is governed by the electrophilicity of the carbonyl carbon, where a higher positive charge density at this site significantly increases the rate of nucleophilic attack. Because electron-donating groups reduce this positive charge density and electron-withdrawing groups enhance it, the specific electronic environment around the $C=O$ bond determines the reactivity, leading to the established order of (iii) < (i) < (iv) < (ii).

Intermolecular forces in a compound are dictated by its chemical structure, with London dispersion forces serving as the universal attractive interaction present in all molecules. Ethyl acetate further exhibits dipole-dipole attractions because its carbonyl group creates a permanent molecular polarity. Because the hydrogen atoms in ethyl acetate are only bonded to carbon rather than highly electronegative atoms like oxygen or nitrogen, hydrogen bonding cannot occur, identifying London dispersion and dipole-dipole forces as the active interactions.

The reaction sequence relies on the principle of regioselective electrophilic substitution followed by a reduction step, where the initial reagent dictates the directing effects on the aromatic ring. The presence of the electron-donating group introduced in the first step activates the ring towards further substitution, while the subsequent transformation converts the intermediate into a more stable functional group via a concerted mechanism. This pathway ensures that the final product [C] is the thermodynamically favored isomer, minimizing steric hindrance and maximizing hyperconjugative stabilization represented by the interaction between the substituents and the aromatic $\pi$-system.

The reaction involves the electrophilic aromatic substitution of aniline with aqueous bromine, where the strong electron-donating $-NH_2$ group activates the benzene ring toward electrophilic attack at all ortho and para positions. Consequently, the reaction proceeds rapidly without a Lewis acid catalyst to form a white precipitate of $C_6H_2Br_3NH_2$. The amino group is highly activating, increasing the electron density on the ring to such an extent that polybromination occurs at every available activated site.

The acidity of $\alpha$-hydrogens is primarily governed by the resonance stability of the conjugate base, where the electron-withdrawing effect of the adjacent carbonyl group is the critical factor. Resonance donation from the nitrogen in amides (D) and the oxygen in esters (C) into the carbonyl reduces its ability to stabilize the negative charge, making these compounds less acidic than ketones (A), which lack such resonance donation. The compound (B), featuring two electron-withdrawing carbonyl groups, provides the most effective resonance stabilization for the conjugate base, resulting in the highest $\alpha$-hydrogen acidity.

The chemical transformation yields 1-phenylpropan-2-one, a compound characterized by a benzyl group attached to the carbonyl carbon of a methyl ketone. This results in the specific molecular structure $\mathrm{C}_6\mathrm{H}_5-\mathrm{CH}_2-\text{overset}{\mathrm{O}}{\text{overset}{\parallel}{\mathrm{C}}}-\mathrm{CH}_3$, where the methylene bridge creates a distinct separation between the aromatic ring and the ketone functional group. This arrangement differentiates the product from isomers such as acetophenone, where the carbonyl is directly bonded to the benzene ring, and propiophenone, which contains an ethyl chain attached to the carbonyl group.