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

The iodoform test confirms the presence of a methyl ketone group, while the utilization of the anhydride of compound $B$ in the synthesis of phenolphthalein identifies $B$ as $o$-phthalic acid ($\mathrm{C}_8\mathrm{H}_6\mathrm{O}_4$). Therefore, compound $A$ is $2$-methylacetophenone, as the acetyl group ($\mathrm{COCH}_3$) provides the positive iodoform test, and the subsequent oxidation of both the acetyl and the ortho-methyl substituents by $\mathrm{KMnO}_4/\mathrm{KOH}$ results in the dicarboxylic acid structure of $o$-phthalic acid.

Intramolecular aldol condensation is driven by the formation of stable cyclic structures, particularly five- or six-membered rings, which are both kinetically and thermodynamically favorable compared to other possible cyclizations. In this process, a base abstracts an $\alpha$-hydrogen to generate an enolate, which subsequently acts as a nucleophile to attack the intramolecular carbonyl group, resulting in a cyclic aldol intermediate. This intermediate then undergoes rapid dehydration to produce a conjugated $\alpha,\beta$-unsaturated cyclic ketone, which constitutes the major product of the reaction.

Bromination of $p$-hydroxybenzophenone proceeds via electrophilic aromatic substitution, where the regioselectivity is determined by the strongly activating and ortho/para-directing hydroxyl group. Although the benzoyl group ($-COC_6H_5$) is deactivating and meta-directing, the powerful activating effect of the $-OH$ group dominates the orientation of the substitution. Consequently, the electrophilic bromine atom replaces the hydrogen atom at the position ortho to the hydroxyl group to form 3-bromo-4-hydroxybenzophenone.

Lithium aluminum hydride, $LiAlH_4$, functions as a highly selective reducing agent that readily converts functional groups like carbonyls into alcohols while ignoring isolated carbon-carbon double bonds. Since this reagent lacks the ability to attack non-polar alkene linkages, the $C=C$ bond remains completely unaffected throughout the transformation. This chemoselectivity ensures that the final structure contains a reduced carbonyl group, while the original alkene geometry and position remain perfectly preserved in the product.

Aldehydes, such as propanal, are inherently more reactive toward nucleophilic addition than ketones like acetone because they experience less steric hindrance and have a more electrophilic carbonyl carbon. Since the transformation of a carbonyl compound into an acetal following the equilibrium $\text{RCHO} + 2\text{MeOH} \rightleftharpoons \text{RCH(OMe)}_2 + \text{H}_2\text{O}$ is reversible, applying Le Chatelier's principle by using an excess of methanol shifts the equilibrium position significantly toward the desired product. Combining the higher intrinsic reactivity of the aldehyde substrate with the concentration-driven shift provided by excess methanol yields the highest overall reaction rate.

The reaction proceeds via a nucleophilic addition mechanism where the hydride species acts as a nucleophile attacking the electrophilic carbonyl carbon to generate a tetrahedral alkoxide intermediate. Subsequent protonation by the solvent or acidic workup yields product A as the alcohol, while product B corresponds to the borate salt byproduct formed through the dissociation of the boron-containing species during the reduction. This transformation is governed by the stoichiometry R_2C=O + \[H^-]$ \rightarrow R_2CH-OH$, which efficiently reduces the carbonyl group to a hydroxyl moiety under the specified reaction conditions.

DIBAL-H, expressed as $AlH(i-Bu)_2$, is a selective reducing agent used to convert nitriles into aldehydes by effectively stopping the reduction process at the imine stage. Upon the addition of water in the second step, this imine intermediate undergoes hydrolysis to generate $RCHO$ as the final product, rather than proceeding further to a primary amine.

Aldehydes that lack $\alpha$-hydrogens undergo a cross-Cannizzaro reaction in the presence of a concentrated base, where one reactant is oxidized and the other is reduced. ${HCHO}$ specifically acts as the hydride donor in this process because it facilitates the formation of a less sterically hindered tetrahedral intermediate compared to other carbonyl compounds. This preference ensures that the ${HCHO}$ is oxidized into a formate while the other aldehyde is reduced into an alcohol.

The stability of a cyclic anhydride is dictated by the resulting ring size, where five- and six-membered structures are highly favored due to their minimal conformational strain. In contrast, the formation of a seven-membered ring from adipic acid, $\mathrm{CO_2H-(CH_2)_4-CO_2H}$, results in an extremely strained and unstable product that does not form readily under typical dehydrating conditions. Because of this significant energetic barrier and the resulting instability of the cyclic molecule, adipic acid is the least reactive candidate for anhydride formation when compared to shorter-chain dicarboxylic acids that yield more stable, smaller rings.

The stability of the intermediate carbocation formed during the rate-determining step is the primary factor governing the regioselectivity of electrophilic addition reactions. In these transformations, the electrophile attacks the double bond to generate the most stable cationic species, as this pathway possesses the lowest activation energy. According to the principle of stability, tertiary carbocations are more stable than secondary or primary ones due to increased inductive effects and hyperconjugation. Consequently, the nucleophile preferentially attacks this more substituted intermediate, leading to the formation of the major product as the kinetic and thermodynamic preference of the reaction.

Grignard reagents act as both potent nucleophiles and exceptionally strong bases, meaning their reactivity is determined by the fastest available pathway. Because the acid-base reaction of G.R. is fast, any aldehyde containing an acidic hydrogen atom-such as those found in hydroxyl groups-will immediately undergo proton transfer, consuming the reagent before it can target the electrophilic carbonyl center. Consequently, in cases where an acidic group is present, the desired nucleophilic addition to the $C=O$ bond does not take place because the reagent is depleted by the competing acid-base reaction.

Adsorption chromatography operates by partitioning substances based on their differential interactions between a solid stationary phase and a flowing solvent. Alumina serves as the solid medium, functioning as the adsorbent where the physical interaction takes place. Within this system, benzaldehyde is the component that concentrates on the surface of the alumina, establishing its role as the adsorbate. Consequently, acetonitrile acts as the mobile phase, moving across the stationary phase to transport the sample mixture.

The formation of the major product is primarily controlled by the thermodynamic stability of the resulting alkene, which follows Zaitsev's rule, favoring the most highly substituted double bond due to the electronic stabilization provided by hyperconjugation. This specific pathway minimizes the potential energy of the transition state, $\Delta G^{\ddagger}$, allowing the reaction to proceed preferentially through the most stable intermediate toward the target product. The product illustrated in option B is favored because it possesses the maximum number of alkyl substituents, thereby minimizing the steric strain and optimizing the electronic configuration of the $\pi$-system.

The industrial production of phenol is achieved through the aerobic oxidation of cumene using $O_2$ to form an unstable intermediate known as cumene hydroperoxide. When this intermediate is subsequently treated with dilute $HCl$, an acid-catalyzed rearrangement occurs, which causes the cleavage of the molecule to yield phenol and acetone as the final products. This efficient two-step synthesis remains the primary method used to manufacture these two significant organic chemicals on a large scale.

The formation of the major product is governed by the stability of the carbocation intermediate, where the mechanism proceeds through the most stable pathway to minimize the activation energy. Following the initial electrophilic addition, 1,2-hydride or alkyl shifts occur to rearrange the intermediate into a more stable tertiary carbocation, which then facilitates the nucleophilic attack to yield the final structure presented in option D.

Electrophilic aromatic substitution is governed by the electronic and steric effects of existing substituents on the aromatic ring, which determine the regioselectivity of the nitronium ion ($NO_2^+$) attack. Electron-donating groups activate the ring and direct the electrophile to ortho or para positions, whereas electron-withdrawing groups deactivate the ring and direct it to the meta position. The major product arises from the most stable transition state, typically favoring the para-isomer over the ortho-isomer due to minimized steric repulsion between bulky groups.

Acid-catalyzed dehydration of butan-2-ol using concentrated $H_2SO_4$ proceeds via an $E1$ mechanism and adheres to Zaitsev's rule, yielding the more substituted and stable alkene, but-2-ene. In the subsequent step, the addition of $HBr$ to the symmetric but-2-ene molecule results in the formation of 2-bromobutane as the major product, as the symmetry of the alkene ensures that both Markovnikov and anti-Markovnikov additions yield the same outcome.

The rate of acid-catalyzed dehydration is determined by the stability of the carbocation intermediate and the presence of pathways that lower the activation energy, such as neighboring group participation. In 4-hydroxypentan-2-one, the carbonyl oxygen at the 2-position acts as an internal nucleophile, assisting the departure of the protonated hydroxyl group at the 4-position by forming a stable five-membered cyclic oxonium intermediate. This intramolecular mechanism drastically lowers the activation energy compared to the other structures, which lack such a favorable arrangement for neighboring group assistance, thus allowing 4-hydroxypentan-2-one to undergo dehydration most readily.

The acidity of carboxylic acids is fundamentally governed by the stability of the resulting carboxylate anion ($R-COO^-$), which is enhanced by electron-withdrawing groups that disperse the negative charge through inductive or resonance effects, while electron-donating groups destabilize the anion and decrease acidity. The overall acid strength follows the trend where stability of the conjugate base increases with the electron-withdrawing nature of the substituents attached to the carboxylic group. Therefore, the arrangement III $<$ II $<$ IV $<$ I represents the progressive increase in acid strength directly correlated with the net electronic effects exerted by the specific substituents present in each respective molecule.

The conversion of a nitrile ($-CN$) to an aldehyde ($-CHO$) is effectively achieved using the Stephen reduction, which employs stannous chloride ($\mathrm{SnCl_2}$) in the presence of hydrochloric acid ($\mathrm{HCl}$) to form an imine hydrochloride intermediate. Subsequent hydrolysis of this imine intermediate with aqueous acid ($\mathrm{H_3O^{+}}$) yields the final aldehyde, while the inclusion of $\mathrm{B_2H_6}$ allows for the chemoselective reduction of other functional groups, such as carboxylic acids or esters, that may be present in the substrate.