Aldehydes Ketones And Carboxylic Acids – NEET UG ChemistryPractice Questions & PYQs

This reaction sequence exemplifies the haloform reaction, where a methyl ketone undergoes halogenation in a basic medium to produce a sodium salt of a carboxylic acid and a haloform. When the salt designated as P is treated with an acid, it yields the corresponding carboxylic acid. Meanwhile, the haloform designated as Q, such as $CHCl_3$, slowly oxidizes upon exposure to atmospheric oxygen and light to generate $COCl_2$, which is a severely poisonous gas.

Carboxylic acid derivatives undergo specific stepwise transformations where esterification is followed by catalytic reduction to yield primary alcohols. When $\mathrm{CH_3COOH}$ reacts with $\mathrm{CH_3OH}$ under acidic conditions, it forms methyl acetate via Fischer esterification, which is subsequently converted into $\mathrm{CH_3CH_2OH}$ and methanol through hydrogenolysis using hydrogen gas and a catalyst. Tracing each individual reaction pathway in this manner allows us to map every reactant set in the first list to its exact product in the second list. Connecting these established chemical outcomes yields the complete pairing sequence where the reaction in item B correctly corresponds to the formation of ethanol.

Aldol condensation relies on the generation of carbon enolates from carbonyl compounds possessing alpha-hydrogens, which subsequently attack other carbonyl groups to form conjugated products through dehydration. In mixed cross-aldol systems, every feasible product is strictly limited by the availability of valid enolate donors and electrophilic acceptors. Because the structure depicted in option B demands an enolate from a reactant lacking the necessary alpha-hydrogen or relies on an impossible elimination sequence, this specific compound cannot be formed under standard reaction conditions.

Calculating the degree of unsaturation using the relation $\text{Degree of unsaturation} = \frac{2C + 2 - H}{2}$ for the molecular formula $\mathrm{C}_8\mathrm{H}_8\mathrm{O}$ yields a value of $5$, which points toward a benzene ring combined with a carbonyl unsaturation. The positive test with 2,4-DNP producing a red-orange precipitate verifies the existence of a carbonyl group, leaving open the possibility of either an aldehyde or a ketone.

Because the substance fails to reduce Fehling's reagent, the aldehyde classification is ruled out, establishing a ketone as the structural identity for compound $\mathrm{P}$. Subjecting this ketone to severe oxidation using chromic acid cleaves the structure to form an aromatic product designated as $\mathrm{Q}$. The observation that $\mathrm{Q}$ produces effervescence upon treatment with aqueous $\mathrm{NaHCO}_3$ proves it to be a carboxylic acid. Reconciling these specific chemical behaviors with an eight-carbon framework establishes that $\mathrm{P}$ is acetophenone ($\mathrm{C}_6\mathrm{H}_5\mathrm{COCH}_3$) and $\mathrm{Q}$ is benzoic acid ($\mathrm{C}_6\mathrm{H}_5\mathrm{COOH}$).

The addition of a dilute strong acid to an acetate salt liberates volatile acetic acid ($\mathrm{CH}_3\mathrm{COOH}$), which is readily recognized by its characteristic pungent odor resembling household vinegar. Because acetic acid is a weak organic acid, its vapors release hydronium ions upon contact with moisture on litmus paper, successfully shifting the indicator from blue to red. Other common anions like carbonates yield odorless carbon dioxide, whereas sulphides produce toxic hydrogen sulfide with a distinct rotten egg stench, making this acidic displacement uniquely diagnostic for the acetate group.

The reaction sequence identifies compound P as phenol, which features a strongly activating hydroxyl group directly attached to the aromatic ring. Due to this high activation, treating phenol with bromine water results in an immediate electrophilic substitution that forms a white precipitate of 2,4,6-tribromophenol. Meanwhile, the alternative choices are invalid because hydrogenating benzoyl chloride over palladium and barium sulfate produces benzaldehyde, phenol lacks sufficient acidity to react with $\text{NaHCO}_3$ with effervescence, and reacting benzene with acetyl chloride and anhydrous $\text{AlCl}_3$ yields acetophenone via Friedel-Crafts acylation.

The selective reduction of esters directly to aldehydes requires a specialized mild reducing agent that halts the reaction at the intermediate oxidation state before further reduction can occur. Diisobutylaluminium hydride, represented as $\mathrm{AlH}(\mathrm{iBu})_2$, performs this exact transformation by forming a stable intermediate complex that subsequently yields the aldehyde upon aqueous workup. In contrast, stronger hydrides lack this controlled selectivity and drive the starting material all the way to an alcohol.

The acidity of carboxylic acids is governed by the stability of their conjugate base carboxylate anions, which is significantly diminished by electron-donating alkyl substituents exhibiting a positive inductive effect. As the number of methyl groups attached to the alpha carbon increases, electron density is pushed toward the reaction center, intensifying negative charge localization and decreasing acidic strength. Formic acid ($\mathrm{HCOOH}$) lacks any alkyl group entirely, allowing maximum charge dispersion and establishing it as the strongest acid in the series, followed sequentially by acetic acid, isobutyric acid, and pivalic acid.

Specific reducing or oxidizing reagents possess characteristic limitations regarding the functional groups they can transform, making certain attempted conversions chemically unviable under standard conditions. The reagent specified in the incorrect reaction lacks the requisite chemical potential or selectivity to convert the given reactant into the indicated product. In contrast, the remaining options accurately depict valid organic transformations, including the hydrolysis of acid chlorides like $\mathrm{CH}_3\mathrm{COCl}$ to $\mathrm{CH}_3\mathrm{COOH}$ and the oxidation of primary alcohols to carboxylic acids.

Hydroboration-oxidation of a terminal alkene using $\mathrm{BH}_3$ followed by $\mathrm{H}_2\mathrm{O}_2/\mathrm{OH}^-$ installs a hydroxyl group at the less substituted carbon via anti-Markovnikov addition, yielding a primary alcohol. Subsequent treatment with pyridinium chlorochromate ($\mathrm{PCC}$) selectively oxidizes this primary alcohol to an aldehyde without over-oxidizing it to a carboxylic acid. This specific two-stage sequence uniquely provides the required regioselectivity and controlled oxidation state necessary for the transformation.

Ozonolysis using $(i)\,\mathrm{O_3}$ followed by $(ii)\,\mathrm{Zn-H_2O}$ cleaves carbon-carbon multiple bonds to yield specific carbonyl fragments. Chromium trioxide ($\mathrm{CrO_3}$) selectively oxidizes alcohols to their corresponding carbonyl compounds or carboxylic acids. Alkaline potassium permanganate ($\mathrm{KMnO_4}/\mathrm{KOH},\,\Delta$) brings about vigorous oxidative cleavage of alkenes or alkyl side chains to form carboxylic acids.

The formation of product 'P' proceeds via a stable carbocation intermediate where the more substituted carbon accommodates the positive charge due to hyperconjugation and inductive effects. Subsequent nucleophilic attack on this thermodynamically favored intermediate yields the corresponding substituted derivative. This mechanistic pathway strictly adheres to regioselective principles, ensuring maximum stability of the resulting molecular structure.

Diborane ($\mathrm{B}_2\mathrm{H}_6$) serves as a uniquely selective hydroborating and reducing agent that efficiently transforms carboxylic acids into primary alcohols while leaving other functional groups like reducible halides or isolated double bonds intact through specialized coordination pathways. In a separate mechanistic pathway, it adds across terminal alkene double bonds via a concerted, syn-addition process that places boron at the less substituted carbon, which upon alkaline hydrogen peroxide oxidation yields anti-Markovnikov primary alcohols ($\mathrm{R}-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{OH}$). These dual capabilities uniquely identify diborane as the reagent capable of accomplishing both specific chemical transformations.

The conversion of an acyl chloride into an aldehyde via catalytic hydrogenation relies on a specially deactivated palladium catalyst. During this reaction, acid chlorides (\text{RCOCl}$) are treated with hydrogen gas over palladium supported on barium sulfate (\text{Pd/BaSO}_4$) to selectively yield aldehydes (\text{RCHO}$) without letting the reduction proceed further to alcohols. This classic reduction process is known as the Rosenmund reaction.

The addition of an unsymmetrical reagent to an unsymmetrical alkene follows Markovnikov's rule, where the electrophilic portion adds to the $sp^2$ carbon possessing a greater number of hydrogen atoms to form the more stable carbocation intermediate. Subsequent nucleophilic attack yields the highly substituted alkyl derivative as the major product. This regioselective pathway is thermodynamically favored due to hyperconjugation and inductive stabilization provided by adjacent alkyl groups on the intermediate carbocation.

The sequential chemical transformation involves the reduction of nitrobenzene (\text{C}_6\text{H}_5\text{NO}_2$) to aniline (\text{C}_6\text{H}_5\text{NH}_2$), followed by diazotization to form a diazonium salt, which subsequently undergoes nucleophilic substitution to yield the final substituted aromatic product $[C]$. The high reactivity of the intermediate diazonium ion facilitates efficient replacement of the diazo group under mild conditions.

The Clemmensen reduction is a powerful synthetic method used to convert aldehydes and ketones into their corresponding hydrocarbons by completely removing the carbonyl oxygen. During this process, the $\text{C}=\text{O}$ group reacts with zinc amalgam and concentrated hydrochloric acid, replacing the oxygen atom with two hydrogen atoms to form a $-\text{CH}_2-$ linkage. Consequently, product $(A)$ represents the fully reduced alkane derivative derived from the starting substrate.

Tollens' reagent, composed of an ammoniacal silver nitrate solution, is uniquely suited to distinguish aldehydes from ketones because only aldehydes undergo oxidation when gently warmed in an alkaline medium. During this transformation, the aldehyde is converted into its corresponding carboxylate anion, denoted as $\text{RCOO}^-$, while silver ions are reduced to metallic silver. This distinct redox reaction deposits a characteristic silver mirror along the inner walls of the container, visually signaling the presence of the aldehyde group.

The exhaustive conversion of primary alcohols into carboxylic acids requires powerful oxidizing agents capable of transforming the intermediate aldehyde into a carboxyl group without getting consumed prematurely. Chromium trioxide in aqueous sulfuric acid ($\mathrm{CrO}_3 - \mathrm{H}_2\mathrm{SO}_4$) and acidified potassium dichromate ($\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7 + \mathrm{H}_2\mathrm{SO}_4$) function through chromium-based oxidation pathways that rapidly push primary substrates to carboxylic acids. Similarly, alkaline potassium permanganate followed by acidification ($\mathrm{KMnO}_4 + \mathrm{KOH}/\mathrm{H}_3\mathrm{O}^{+}$) provides vigorous oxidative cleavage and conversion capabilities, whereas copper at elevated temperatures only achieves dehydrogenation to aldehydes or ketones, and modified chromium reagents stop short at the aldehyde stage.

Ketones possessing alpha-hydrogen atoms undergo base-catalyzed aldol addition reactions where enolate ions attack the carbonyl carbon of another molecule to build carbon-carbon bonds. This self-condensation of acetone ($\mathrm{CH}_3\mathrm{COCH}_3$) yields a product containing both a secondary alcoholic hydroxyl group resulting from the protonated alkoxide intermediate and an unreacted carbonyl group belonging to the ketone family. Consequently, the resulting adduct structure features dual functional characteristics, specifically a hydroxyl group alongside a ketonic carbonyl moiety.