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

The Wolff-Kishner reduction utilizes hydrazine ($NH_2-NH_2$) and potassium hydroxide ($KOH$) to convert carbonyls to methylene groups, while the Clemmensen reduction employs amalgamated zinc ($Zn-Hg$) and concentrated $HCl$ to achieve the same transformation. Tollen's reagent ($Ag(NH_3)_2OH$) is the diagnostic chemical used in Tollen's test to oxidize aldehydes, and Fehling's reagent, consisting of aqueous $CuSO_4$, sodium potassium tartrate, and $KOH$, serves the same analytical purpose in Fehling's test. Together, these pairings accurately associate the specific chemical reagents with their corresponding name reactions and functional group tests.

The iodoform test specifically identifies compounds containing a methyl ketone group ($CH_3-C(=O)-$) or a methyl carbinol group ($CH_3-CH(OH)-$) by producing a yellow precipitate of iodoform ($CHI_3$) upon reaction with iodine and base. Ethanol contains the required $CH_3CH(OH)-$ motif and reacts positively, unlike methanol, allowing for the differentiation of pair A. Likewise, acetone produces a positive result due to its methyl ketone group, while anisole does not, distinguishing pair E, whereas the other listed pairs do not provide such definitive chemical contrast.

The transformation of Compound (X) proceeds through a systematic sequence of functional group interconversions starting with the electrophilic addition of bromine to the double bond, which forms a vicinal dibromide. Subjecting this dibromide to an excess of sodium amide initiates a double dehydrohalogenation process, effectively converting the dibromide into a terminal alkyne by removing two equivalents of hydrogen bromide. In the presence of excess base, the terminal alkyne is deprotonated to form an acetylide anion, which then undergoes a nucleophilic substitution reaction with $CH_3I$ to append a methyl group, thereby extending the carbon chain. The final step involves the dissolving metal reduction of the resulting substituted alkyne using $Na/NH_3$, which stereoselectively produces a trans-alkene. This sequence of reactions modifies the original structure to yield the major product with a molar mass of 118 g mol⁻¹.

The reaction proceeds via the mechanism of electrophilic aromatic substitution, where a Lewis acid catalyst generates a reactive carbocation electrophile from the alkylating agent. This electrophile undergoes nucleophilic attack by the $\pi$-electron system of the aromatic ring to form a sigma complex, which subsequently loses a proton to restore aromaticity and yield the substituted product. Consequently, A represents the alkylated aromatic derivative formed through the attachment of the alkyl group to the benzene ring.

The compound $P$, identified as 4,4-dimethoxybutan-2-one ($CH_3COCH_2CH(OCH_3)_2$), contains a methyl ketone group that yields a positive iodoform test while the protected aldehyde functionality, in the form of an acetal, results in a negative Tollen's test. Upon treatment with dilute acid, the acetal moiety undergoes hydrolysis to regenerate the aldehyde group in compound $Q$ ($CH_3COCH_2CHO$), which possesses both a methyl ketone and an aldehyde group, leading to positive results for both the iodoform and Tollen's tests.

The condensation reaction between acetaldehyde, $CH_3-CH=O$, and semicarbazide, $H_2N-NH-CO-NH_2$, occurs through the nucleophilic attack of the more nucleophilic terminal amino group on the carbonyl carbon, resulting in the formation of a semicarbazone with the structure $CH_3-CH=N-NH-CO-NH_2$. Because the product specified in the first statement displays an incorrect connectivity that deviates from this standard chemical pathway, the assertion is invalid. In contrast, hydrazones are derivatives of carbonyl compounds that undergo facile hydrolysis in the presence of dilute acid, effectively cleaving the carbon-nitrogen double bond to regenerate the parent aldehyde, $Ph-CH=O$, which confirms that the second statement accurately describes this fundamental hydrolytic reactivity.

Metamerism arises when structural isomers share the same functional group but differ in the distribution of alkyl groups attached to that functional group. In 2,6-diethylcyclohexanone, the carbonyl moiety is flanked by two ethyl substituents, whereas in 6-methyl-2-n-propylcyclohexanone, the arrangement consists of a methyl group and a propyl group; since both compounds share the same molecular formula but differ in the alkyl connectivity around the ketone, Statement-I is correct. Conversely, keto-enol tautomerism requires the presence of at least one $\alpha$-hydrogen-a hydrogen atom on a carbon directly adjacent to the carbonyl group-to facilitate the shift required to form the enol double bond. Because 2,2,6,6-tetramethylcyclohexanone has all four $\alpha$-positions fully occupied by methyl groups, it contains no $\alpha$-hydrogens and cannot undergo keto-enol tautomerism, which renders Statement-II incorrect.

Tollen's reagent, consisting of the diamminesilver(I) complex $[Ag(NH_3)_2]^+$, acts as a selective oxidizing agent that reacts with aldehydes to produce a characteristic silver mirror on the walls of the reaction vessel. In this redox process, the aldehyde group ($R-CHO$) is oxidized to a carboxylate ion, while $Ag^+$ ions are reduced to metallic silver, $Ag(s)$. Compound C is the only species among the choices that possesses the necessary aldehyde functional group to facilitate this reduction, thereby yielding a positive result in the Tollen's test.

The Hell-Volhard-Zelinsky (HVZ) reaction enables the selective $\alpha$-halogenation of carboxylic acids using chlorine gas and red phosphorus. In this reaction sequence, the intermediate carboxylic acid undergoes electrophilic substitution at the $\alpha$-carbon, where one of the hydrogen atoms is replaced by a chlorine atom. This transformation specifically converts the acid derivative into 2-chloropropanoic acid as the major product.

The transformation proceeds through an initial elimination reaction catalyzed by the strong base NaOEt to convert the starting substrate into an alkene. Subsequent treatment with $\mathrm{B_2H_6/H_2O_2}$ facilitates an anti-Markovnikov hydroboration-oxidation to yield a terminal primary alcohol. Finally, the use of Jones reagent ($\mathrm{CrO_3/H_2SO_4}$) selectively oxidizes this primary alcohol into a carboxylic acid, completing the required functional group modification.

Dehydration of secondary alcohols proceeds via the $E1$ mechanism where the stability of the intermediate carbocation dictates the regioselectivity of the double bond formation. The process begins with the protonation of the hydroxyl group to generate a good leaving group, followed by the departure of water to form a carbocation intermediate. This intermediate undergoes deprotonation from the most substituted $\beta$-carbon, yielding the most stable alkene isomer as the final product (P).

The hydration of prop-1-yne using mercuric sulphate in a dilute acidic medium follows Markovnikov's rule, proceeding via a vinyl alcohol intermediate to yield acetone ($CH_3COCH_3$) as the product 'x'. Similarly, the reaction of ethane nitrile ($CH_3CN$) with methyl magnesium bromide ($CH_3MgBr$) followed by acidic hydrolysis involves a nucleophilic attack on the nitrile carbon, generating an imine intermediate that upon hydrolysis forms acetone as the product 'y'. When these two molecules of acetone react in the presence of barium hydroxide, they undergo an aldol condensation to form the intermediate 4-hydroxy-4-methylpentan-2-one, which upon heating undergoes dehydration to yield the final product, 4-methylpent-3-en-2-one.

Determining the molecular identity of an organic compound begins with establishing its empirical formula from the provided molar ratio. Since carbon, oxygen, and hydrogen are present in an equal molar ratio, the empirical formula is identified as $\mathrm{CHO}$. Calculating the molar mass of this empirical unit yields the equation $12 + 16 + 1 = 29\,\mathrm{g\,mol}^{-1}$. By comparing this to the given molar mass, the molecular multiplier is found via the equation $58 / 29 = 2$. Applying this factor to the empirical formula results in the molecular formula $\mathrm{C_2H_2O_2}$, which leads to the identified chemical structure.

Tollen's reagent, an alkaline solution containing the complex $[Ag(NH_3)_2]^+$, selectively oxidizes aldehyde groups to carboxylic acids but does not react with acetals, ketals, or ketones that lack $\alpha$-hydroxy substituents. Cyclic acetals and ketals are stable under the basic conditions of this test because their structure prevents the formation of an open-chain aldehyde via ring opening, making them incapable of reducing the silver ions. Therefore, structures B and C are identified as the non-reactive acetal or ketal derivatives, distinguishing them from the reactive aldehyde or hemiacetal configurations found in structures A and D.

The Cannizzaro reaction occurs in aldehydes lacking $\alpha$-hydrogens when treated with a concentrated base, while cross-Aldol condensation typically utilizes an aldehyde as an electrophile and a ketone with $\alpha$-hydrogens as the enolate donor. 4-Methoxybenzaldehyde, which has the molecular formula $C_8H_8O_2$, satisfies these criteria because it lacks $\alpha$-hydrogens, allowing it to undergo the Cannizzaro reaction to produce 4-methoxybenzyl alcohol and the corresponding salt, while simultaneously acting as the exclusive electrophile in a cross-Aldol reaction with acetophenone. Other candidates are excluded because they are not aldehydes, possess $\alpha$-hydrogens, or fail to match the given molecular formula.

Systematic analysis of chemical reaction sequences involves tracing functional group transformations from reactants to products, often represented as $P \xrightarrow{\text{reagents}} \text{Intermediate} \dots \rightarrow \text{Product}$. By examining the electronic and steric requirements of the reagents in each step, one can deduce the specific structure of the starting material P that satisfies the entire reaction pathway. This logical deduction identifies the molecule that contains the necessary functional groups to undergo the first transformation, confirming the structural identity of the compound as depicted in the chosen option.

The molar mass of a substance is determined by summing the atomic masses of all constituent atoms based on their chemical formulas, while stoichiometric coefficients define the molar relationships between products in a reaction. For the products identified as $x$ ($CO$) and $y$ ($H_2O$), the molar mass is calculated individually by adding the atomic weights of their elements. The molar mass of $CO$ is $12 + 16 = 28 \text{ g/mol}$, and the molar mass of $H_2O$ is $2(1) + 16 = 18 \text{ g/mol}$. Combining these two values provides the final result of $28 + 18 = 46 \text{ g/mol}$.

Starting with the optically active isomer of $C_4H_9Br$, dehydrohalogenation using alcoholic potassium hydroxide yields but-2-ene, which subsequently reacts with bromine to form the vicinal dihalide 2,3-dibromobutane. Treatment of this dibromide with strong bases like alcoholic potassium hydroxide and sodamide facilitates a double elimination to produce but-2-yne. Hydration of this alkyne in the presence of $HgSO_4$ and $H_2SO_4$ at 333 K involves the nucleophilic addition of water to form butan-2-one.

The resulting product, butan-2-one, contains a methyl ketone group, represented as $CH_3-C(=O)-$, which serves as the diagnostic structural feature for the haloform test. This reaction specifically identifies methyl ketones by producing a characteristic precipitate in the presence of a halogen and an alkali, thereby confirming the identity of the functional group in the final compound.

A carbohydrate functions as a reducing sugar if it possesses a free anomeric carbon capable of forming an aldehyde or ketone in its open-chain form, allowing it to act as a reducing agent. Because maltose contains an available hemiacetal group at its anomeric carbon, the claim that it is a non-reducing sugar is false. Conversely, lactose is correctly identified as a reducing sugar because its structure features a free anomeric carbon on the glucose moiety, confirming the validity of the second statement.

The 2,4-dinitrophenylhydrazine (2,4-DNP) test is a diagnostic reaction for carbonyl functional groups, producing a precipitate with both aldehydes and ketones, which explains why compounds Q, R, and S show a positive result. Tollen's reagent contains the $[\text{Ag(NH}_3)_2]^+$ complex, which is specifically reduced to metallic silver by aldehydes, isolating Q as the only aldehyde among the isomers. Furthermore, aromatic compounds undergo incomplete combustion resulting in a characteristic sooty flame due to their high carbon-to-hydrogen ratio, confirming the aromatic nature of Q and R.