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

Boiling points are fundamentally governed by the strength of the intermolecular forces acting between molecules, where hydrogen bonding stands out as the most significant attractive interaction compared to dipole-dipole forces or London dispersion forces. Because $\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_2-\mathrm{OH}$ possesses a hydroxyl group capable of forming intermolecular hydrogen bonds, it requires substantially more thermal energy to transition into the vapor phase. Consequently, this alcohol exhibits a higher boiling point than the corresponding aldehyde, ether, and alkane, which rely on weaker intermolecular interactions.

The reaction sequence transforms a carboxylic acid into an $\alpha$-hydroxy carboxylic acid through a series of functional group modifications. Acetic acid is first reduced by $\mathrm{LiAlH}_4$ to ethanol, which is then oxidized by $\mathrm{PCC}$ to produce acetaldehyde ($\mathrm{CH}_3-\mathrm{CHO}$). Subsequent nucleophilic addition of cyanide to the aldehyde yields the cyanohydrin intermediate $\mathrm{CH}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{CN}$, and the final basic hydrolysis with $\mathrm{H}_2\mathrm{O}/\mathrm{OH}^-$ at elevated temperature converts the cyano group into a carboxylic acid, resulting in the final product $\mathrm{CH}_3-\mathrm{CH}(\mathrm{OH})-\mathrm{COOH}$.

The oxidation of cyclohexanol or cyclohexanone utilizing strong oxidizing agents, specifically concentrated nitric acid ($HNO_3$), induces the oxidative cleavage of the six-membered carbon ring. This reaction results in the formation of hexanedioic acid, known industrially as adipic acid ($HOOC-(CH_2)_4-COOH$), which serves as a primary monomer in the synthesis of polymers such as nylon-6,6.

Determining the total count of $\pi$ bonds requires identifying each double and triple bond in the final structure of product (C). Every double bond, such as a $C=C$ or $C=O$ linkage, accounts for one $\pi$ bond, while every triple bond contributes two. A thorough structural analysis of the synthesized molecule reveals a configuration containing sufficient unsaturation to result in a total of 4 $\pi$ bonds.

The Aldol reaction relies on the ability of a base to abstract an $\alpha$-hydrogen from a carbonyl compound, generating a nucleophilic enolate ion that initiates carbon-carbon bond formation. Because the acidity of these $\alpha$-hydrogens allows for the creation of this reactive intermediate, the first statement accurately identifies the fundamental chemical trigger for the process.

The second statement is incorrect because a cross-aldol reaction occurs when an aldehyde lacking $\alpha$-hydrogens, like benzaldehyde, is combined with an aldehyde containing $\alpha$-hydrogens, such as ethanal. In this specific scenario, the enolate formed from ethanal functions as a nucleophile and attacks the carbonyl group of benzaldehyde, leading to the successful formation of a cross-aldol product.

The molecular structure of vanillin, given by the formula $\mathrm{C}_8\mathrm{H}_8\mathrm{O}_3$, confirms the presence of 3 oxygen atoms. To determine the number of $\pi$-electrons, identify the unsaturated bonds: the benzene ring contains 3 double bonds, and the aldehyde functional group contains 1 double bond, totaling 4 $\pi$-bonds. Since each $\pi$-bond is comprised of 2 electrons, the total number of $\pi$-electrons is $4 \times 2 = 8$. Combining the 3 oxygen atoms and 8 $\pi$-electrons results in a total sum of $3 + 8 = 11$.

The product formation in this reaction is governed by the thermodynamic stability of the chemical species involved, where the system proceeds to minimize Gibbs free energy according to $\Delta G = \Delta H - T\Delta S$. The structure represented in option D corresponds to the major product because it represents the most stable configuration formed via the most favorable transition state, as dictated by the specific reaction mechanism. Consequently, this compound predominates over other potential isomers due to electronic and steric factors that minimize the potential energy of the final molecular architecture.

The IUPAC name 4-methylpent-2-enal indicates a five-carbon chain containing an aldehyde group at the first position, a carbon-carbon double bond starting at the second position, and a methyl group attached to the fourth carbon. Assigning the aldehyde carbon as position 1 results in the structural representation $\mathrm{CH}_3-\mathrm{CH}(\mathrm{CH}_3)-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO}$, where the methyl group is correctly placed at the fourth carbon and the double bond spans carbons 2 and 3. This structure satisfies all naming conventions and connectivity rules specified in the IUPAC terminology.

The chemical structure of 2-formylhex-4-enoic acid is represented as $HOOC-CH(CHO)-CH_2-CH=CH-CH_3$. Since this is an acyclic molecule containing 20 atoms (7 carbon, 10 hydrogen, and 3 oxygen atoms), the number of $\sigma$ bonds is given by the total number of atoms minus one, resulting in $20-1=19$ $\sigma$ bonds. Adding the 3 $\Pi$ bonds present in the two carbonyl groups and the alkene double bond yields a total of $19+3=22$ bonds.

The silver mirror test acts as a diagnostic tool for reducing agents, specifically targeting the aldehyde functional group which readily undergoes oxidation while reducing the silver ions in Tollens' reagent to metallic silver. Aldehydes such as formaldehyde ($HCHO$) and benzaldehyde ($C_6H_5CHO$) clearly produce this silver deposit, but formic acid ($HCOOH$) also tests positive because its unique molecular structure contains a formyl group that can be oxidized just like an aldehyde. Acetone ($CH_3COCH_3$), being a ketone, does not have this oxidizable hydrogen atom attached to the carbonyl carbon and therefore remains inert under these experimental conditions.

A homologous series consists of compounds sharing the same functional group, where each successive member differs from the previous one by a single $\mathrm{CH_2}$ unit. The simplest member of the mono carboxylic acid series is methanoic acid, which has the formula $\mathrm{HCOOH}$. By adding a $\mathrm{CH_2}$ group to this first member, we obtain the second homologue, ethanoic acid, represented as $\mathrm{CH_3COOH}$, which simplifies to the molecular formula $\mathrm{C_2H_4O_2}$.

Nucleophilic addition reactivity is dictated by the degree of steric hindrance around the carbonyl center and the intensity of the partial positive charge on the carbonyl carbon. Bulky alkyl groups create significant spatial resistance for an incoming nucleophile and, through their electron-donating inductive effect, diminish the electrophilic nature of the carbonyl carbon by reducing its partial positive charge. Since formaldehyde ($\mathrm{H}-\text{CHO}$) features two small hydrogen atoms instead of alkyl groups, it experiences the lowest steric hindrance and retains the highest partial positive charge, making it significantly more reactive than the other aldehydes listed.

Aldehydes that lack alpha-hydrogen atoms, such as m-chlorobenzaldehyde, undergo the Cannizzaro reaction upon exposure to a concentrated base like 50% $\text{KOH}$. During this disproportionation process, some aldehyde molecules oxidize to form the m-chlorobenzoate ion, while others reduce to yield m-chlorobenzyl alcohol. This single transformation simultaneously generates the corresponding carboxylic acid salt and primary alcohol from the initial starting material.

Fehling's reagent, an alkaline solution of copper(II) tartrate, selectively oxidizes aliphatic aldehydes to carboxylic acid salts while failing to react with aromatic aldehydes and ketones due to their reduced susceptibility to oxidation and lack of necessary activation. Among the provided compounds, acetaldehyde ($CH_3CHO$), methanal ($HCHO$), and cyclohexane carbaldehyde ($C_6H_{11}CHO$) are aliphatic aldehydes, whereas benzaldehyde and 4-nitrobenzaldehyde are aromatic aldehydes, and acetone and acetophenone are ketones. Because only the three identified aliphatic aldehydes undergo this oxidation, the total number of compounds giving a positive Fehling's test is three.

Rosenmund reduction refers specifically to the partial reduction of an acid chloride ($RCOCl$) into an aldehyde ($RCHO$). This chemical transformation is typically achieved by catalytic hydrogenation using a poisoned palladium catalyst, which ensures the reaction stops at the aldehyde stage rather than proceeding to a primary alcohol. Among the listed choices, this reaction is the only one characterized by this distinct conversion of an acid chloride substrate.

The enol content in 1,3-dicarbonyl compounds is significantly higher because the resulting enol tautomer forms a stable, planar six-membered chelate ring stabilized by strong intramolecular hydrogen bonding between the hydroxyl group and the adjacent carbonyl oxygen. This configuration is further reinforced by extended conjugation of the $\pi$ system, which lowers the overall energy of the enol form and shifts the tautomeric equilibrium toward it. Other isomers lack these specific stabilizing factors, such as the ability to form a stable six-membered ring or effective conjugation, resulting in a much lower percentage of enol content compared to pentane-2,4-dione.

The reaction in question yields benzene ($C_6H_6$) as the final product $P$. Since benzene is a pure hydrocarbon consisting solely of six carbon and six hydrogen atoms in a closed aromatic ring, it contains no oxygen atoms. As hydroxyl groups require an oxygen atom bonded to a hydrogen to exist, their count in this molecular structure is naturally zero.

The reaction sequence involves an initial electrophilic aromatic substitution where the starting material reacts with a nitrating mixture of concentrated $H_2SO_4$ and $HNO_3$ to generate the nitronium ion ($NO_2^+$), forming a nitro compound as intermediate $A$. Subsequent chemical reduction of this nitro group using reagents like $Sn/HCl$ or $Fe/HCl$ facilitates the conversion of the nitro group into a primary amine, yielding the final product $B$. This two-step pathway effectively transforms the aromatic ring derivative into the corresponding amine through the intermediate nitro species.

The key to mastering these reactions lies in recognizing the specific chemoselectivity of the reagents involved. Converting the ester $\mathrm{CH}_3(\mathrm{CH}_2)_5\mathrm{COOC}_2\mathrm{H}_5$ into the aldehyde $\mathrm{CH}_3(\mathrm{CH}_2)_5\mathrm{CHO}$ is best achieved using DIBAL-H, a reagent known for its ability to stop the reduction of esters at the aldehyde stage. The reduction of $\mathrm{C}_6\mathrm{H}_5\mathrm{COC}_6\mathrm{H}_5$ to $\mathrm{C}_6\mathrm{H}_5\mathrm{CH}_2\mathrm{C}_6\mathrm{H}_5$ relies on the Clemmensen reduction, where $\mathrm{Zn}(\mathrm{Hg})$ and concentrated $\mathrm{HCl}$ facilitate the complete removal of the carbonyl oxygen. The reaction between $\mathrm{C}_6\mathrm{H}_5\mathrm{CHO}$ and $\mathrm{CH}_3\mathrm{MgBr}$ is a typical Grignard addition that yields a secondary alcohol, while the selective reduction of the ketone in $\mathrm{CH}_3\mathrm{COCH}_2\mathrm{COOC}_2\mathrm{H}_5$ to $\mathrm{CH}_3\mathrm{CH}(\mathrm{OH})\mathrm{CH}_2\mathrm{COOC}_2\mathrm{H}_5$ is performed using $\mathrm{NaBH}_4$, which preserves the ester group due to its mild reducing power.

The reaction mechanism is governed by the electronic and steric effects of the functional groups present in the starting material, which direct the attacking reagent to the most electrophilic or nucleophilic site to ensure the formation of the most stable intermediate. The transformation proceeds through a transition state where the activation energy is minimized, often favoring the pathway that leads to the thermodynamically or kinetically controlled product shown in the correct option. By evaluating the regioselectivity and stereochemistry determined by these electronic factors, the resulting chemical structure can be accurately identified among the choices.