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

Applying the principles of stoichiometry to the reaction sequence allows us to deduce the molecular formula of compound A. The conversion of alcohol B into 2-methyl-2-butene using copper at 573 K characterizes a dehydration process of a five-carbon tertiary alcohol. Since compound A reacts with methylmagnesium bromide to generate this five-carbon alcohol, it must be a four-carbon carbonyl compound represented by the molecular formula $C_4H_8O$.

Calculating the mass percentage requires determining the total molar mass of the compound and the specific contribution from its carbon atoms. The total mass is derived from the equation $12 \times 4 + 8 \times 1 + 16 \times 1 = 72$. Consequently, the mass percentage of carbon is calculated as $\frac{48}{72} \times 100$, which results in 66.67%.

The determination of the major product in a chemical reaction depends fundamentally on the relative stability of the intermediate species formed during the mechanism, such as carbocations or transition states, governed by electronic and steric effects. Because the specific reactant structures and reagents were not provided, it is impossible to identify the precise chemical pathway or the resulting major product. Please provide the chemical structures or reaction equation to receive a complete, step-by-step analysis of the reaction mechanism.

The distinction between these isomers is rooted in their specific modes of hydrogen bonding, as the ortho-isomer (A) primarily features intramolecular hydrogen bonding, whereas the para-isomer (B) engages in extensive intermolecular hydrogen bonding. This network of intermolecular interactions in (B) facilitates more efficient molecular packing, which stabilizes the crystal lattice and renders (B) more crystalline than (A). Furthermore, these robust intermolecular forces require greater thermal energy to overcome during phase changes, which directly results in a higher boiling point for (B) compared to (A).

Ozonolysis of unsaturated hydrocarbons results in the oxidative cleavage of carbon-carbon double bonds to yield aldehydes or ketones, while Tollens' reagent selectively oxidizes aldehyde groups to carboxylic acids without affecting existing ketone functionalities. The formation of 3-oxo-hexanedicarboxylic acid ($HOOC\text{-}CH_2\text{-}C(=O)\text{-}CH_2\text{-}CH_2\text{-}COOH$) from intermediate A implies that A was 3-oxo-hexanedial ($OHC\text{-}CH_2\text{-}C(=O)\text{-}CH_2\text{-}CH_2\text{-}CHO$), which is the product of oxidative cleavage of a cyclic hydrocarbon X containing two double bonds. Consequently, X must possess a six-carbon cyclic skeleton with specific positions of unsaturation that, upon ozonolysis, generate the required dialdehyde-ketone intermediate, ultimately leading to the identified dicarboxylic acid.

The outcome of this reaction is determined by the unique electrophilic nature of $\mathrm{B_2H_6}$, which functions as a highly selective reducing agent. Unlike nucleophilic alternatives that may react indiscriminately, this reagent possesses a distinct mechanism that allows it to precisely target the necessary functional groups. Because of this specialized reactivity, $\mathrm{B_2H_6}$ acts as the most suitable choice to successfully complete the transformation.

The electrophilic aromatic substitution reaction proceeds through the generation of a nitronium ion ($NO_2^+$) as the active electrophile, which is formed by the reaction of concentrated nitric acid with concentrated sulfuric acid. This electrophile subsequently undergoes an electrophilic attack on the electron-rich benzene ring to form a non-aromatic carbocation intermediate, followed by deprotonation to restore aromaticity and produce nitrobenzene.

The compound (A) is an ester with the molecular formula ${C}_{6} {H}_{12} {O}_{2}$, which undergoes acid-catalyzed hydrolysis to yield a carboxylic acid (B) and an alcohol (C). The immediate formation of white turbidity upon treatment with Lucas reagent (anhydrous ${ZnCl}_{2}$ and conc. ${HCl}$) acts as a specific diagnostic test identifying the alcohol as a tertiary alcohol, as this reagent facilitates rapid nucleophilic substitution with tertiary substrates to produce insoluble alkyl chlorides. Therefore, the structure (A) must be an ester containing a tertiary alkyl group, which produces the required tertiary alcohol upon hydrolysis.

The reaction of unsymmetrical alkenes with hydrogen bromide in the absence of peroxides follows Markovnikov's rule, where the electrophilic addition of the proton to the carbon with the greater number of hydrogen atoms forms the more stable carbocation intermediate, leading to the formation of the secondary bromide. Conversely, when the reaction occurs in the presence of organic peroxides, it proceeds via a free-radical mechanism known as the peroxide effect, which favors the addition of the bromine atom to the terminal carbon to produce the primary bromide as the major product.

The reaction proceeds via an $S_{N}2$ mechanism, where the nucleophile performs a backside attack on the electrophilic carbon center simultaneously as the leaving group departs. This concerted process results in a complete inversion of stereochemical configuration, leading to the formation of the substitution product which matches the specific molecular structure depicted.

Decarboxylation of a carboxylic acid with sodalime yields an arene, indicating that $[P]$ is a methylbenzoic acid isomer ($\mathrm{C}_8\mathrm{H}_8\mathrm{O}_2$) because it produces toluene ($\mathrm{C}_7\mathrm{H}_8$) upon heating. Electrophilic aromatic substitution of $p$-toluic acid occurs at the positions ortho to the methyl group, which are equivalent due to the symmetric para-substitution pattern, thus resulting in a single monobrominated isomer $\mathrm{C}_8\mathrm{H}_7\mathrm{O}_2\mathrm{Br}$. Other isomers, such as $o$-toluic acid and $m$-toluic acid, lack this symmetry and would generate a mixture of products upon reaction with $\mathrm{Br}_2/\mathrm{FeBr}_3$.

Determining the identity of intermediate (A) requires a systematic application of the reaction conditions to the starting material to establish the primary functional group modification. By assessing the regioselectivity and stereochemical outcomes of the mechanism, the structure of (A) is confirmed as the necessary precursor that enables the subsequent transformation into the final product (D). This deductive approach ensures that the electronic stability and valency of the proposed intermediate remain consistent with the overall reaction sequence.

Acidity is determined by the stability of the conjugate base formed upon deprotonation, as a more stable conjugate base corresponds to a stronger acid. Carboxylic acids ($b$) are the most acidic in this set because their conjugate base is effectively stabilized by equivalent resonance structures. Alcohols ($c$) follow as the next most acidic group, while terminal alkynes ($d$) rank below them since the negative charge on an $sp$ hybridized carbon is less stabilized than on an oxygen atom. Aliphatic hydrogens ($a$) exhibit the lowest acidity, which establishes the decreasing order of $b > c > d > a$.

The hydrolysis of an ester produces a carboxylic acid [B] and an alcohol [C], and the subsequent oxidation of [C] with $\mathrm{CrO}_{3}-\mathrm{H}_{2}\mathrm{SO}_{4}$ (Jones reagent) to yield [B] indicates that [C] must be a primary alcohol, as primary alcohols are oxidized to carboxylic acids while secondary alcohols form ketones. In the structure provided in option C, the alcohol segment is a secondary alcohol, $\mathrm{CH_3CH_2CH(CH_3)OH}$, which would form a ketone upon oxidation rather than the required carboxylic acid. Therefore, this structure is not possible because it cannot satisfy the chemical requirements of the given reaction sequence.

The major product of a chemical reaction is determined by the stability of the transition state or intermediate species, which dictates the pathway with the lowest activation energy, often represented as $E_a$. Organic synthesis typically favors the formation of the most substituted and thermodynamically stable product, which minimizes steric strain and maximizes electronic stabilization through effects such as hyperconjugation. This regioselective preference ensures the conversion of reactants into the specific isomer presented in option D.

The alkaline hydrolysis of esters proceeds through a nucleophilic substitution mechanism where the rate is primarily determined by the electrophilic strength of the carbonyl carbon atom. Substituents attached to the acyl group influence this rate significantly, as electron-withdrawing groups increase the partial positive charge on the carbonyl carbon to facilitate faster nucleophilic attack by the $OH^-$ ion, while electron-donating groups decrease this charge and hinder the reaction. By evaluating the electronic environment surrounding the carbonyl group in each structure, the reactivity is dictated by the magnitude of the positive charge on the carbonyl carbon, resulting in the observed sequence.

The determination of a major product in a chemical reaction is governed by the relative stability of the intermediate states, where the pathway proceeding through the transition state with the lowest activation energy ($E_a$) dominates. In transformations such as electrophilic additions, this involves the formation of the most stable carbocation intermediate according to Markovnikov's rule, or in the case of substitution or elimination reactions, the optimization of electronic and steric factors to produce the most thermodynamically stable product. This mechanistic preference ensures the reaction yields the isomer that minimizes the potential energy of the system, confirming the structure presented in the correct option as the primary outcome.

The efficiency of acetal formation depends on the nucleophilic attack of an alcohol on a carbonyl carbon, where the reaction $\text{rate} \propto \frac{1}{\text{steric crowding of aldehyde}}$. Consequently, formaldehyde (\text{HCHO}$) acts as the most reactive aldehyde due to the absence of steric bulk, and methanol (\text{MeOH}$) serves as the optimal alcohol partner for this process. Using $^{t}\text{BuOH}$ is disadvantageous because its significant steric hindrance and tendency to form a carbocation in an acidic medium lead to unwanted side reactions rather than successful acetal synthesis.

The reaction sequence begins with the haloform oxidation of acetophenone using $\mathrm{NaOCl}$, which generates a benzoate intermediate that yields benzoic acid upon treatment with acid. This carboxylic acid is converted into an active acyl species, such as an acid chloride, which then undergoes a nucleophilic substitution reaction with aniline, $\mathrm{PhNH_2}$. The final product $Y$ is the amide $\mathrm{PhCONHPh}$, produced through the condensation of the activated benzoate derivative with the amino group of aniline.

The iodoform reaction utilizing $\mathrm{I}_2/\mathrm{NaOH}$ is a specific chemical transformation used to identify compounds containing a methyl ketone ($\mathrm{CH}_3\mathrm{CO}-$) or a methyl carbinol ($\mathrm{CH}_3\mathrm{CH(OH)}-$) moiety, which are converted into a carboxylate salt and a characteristic yellow precipitate of iodoform ($\mathrm{CHI}_3$). This selective oxidative cleavage occurs specifically at the methyl-bearing carbonyl or alcohol functionality, distinguishing these compounds from other substrates that do not undergo the haloform reaction under these basic conditions. Given the requirement to isolate or identify this specific structural feature, the $\mathrm{I}_2/\mathrm{NaOH}$ reagent provides the necessary specificity that general oxidizing agents like $\mathrm{KMnO}_4$ or Tollen's reagent lack.

The reaction proceeds through an electrophilic attack on the double bond to form the most stable carbocation intermediate, which is primarily stabilized by hyperconjugation and inductive effects. The subsequent nucleophilic attack occurs preferentially at the more substituted carbon atom because the transition state leading to this pathway possesses the lowest activation energy, $E_{a}$, thereby favoring the formation of the structure in option B.