Alcohols Phenols And Ethers – JEE Main ChemistryPractice Questions & PYQs

Hyperconjugative stabilization of a carbocation occurs via the delocalization of electrons from adjacent $\sigma$ C-H bonds into the empty vacant $p$-orbital of the positively charged carbon atom. The total number of hyperconjugative structures is calculated using the formula $n + 1$, where $n$ represents the number of $\alpha$-hydrogen atoms attached to the carbon atoms directly bonded to the carbocationic center. Since the resulting value is 7, the specific carbocation formed in the reaction must contain 6 $\alpha$-hydrogen atoms, which generate 6 distinct no-bond resonance contributors in addition to the original carbocation structure.

Acetals and ketals are structurally classified as ethers, and because ethers are chemically unreactive toward basic reagents, these compounds remain stable in basic media, confirming that the assertion is true. However, the provided reason is false because the alkoxide ion, $RO^-$, is chemically classified as a poor leaving group, rather than one with a high leaving tendency. Therefore, the stability of these functional groups in basic conditions is attributed to the lack of reactivity of the ether linkages, not to any mechanism involving the departure of an alkoxide species.

The conversion of triphenylmethyl chloride to triphenylmethyl alcohol in the presence of pyridine occurs via an $S_N1$ mechanism, which dictates the kinetic behavior of the system. Since the rate-determining step involves the slow, unimolecular dissociation of the alkyl halide substrate to form a stable carbocation, the reaction rate is strictly dependent on the substrate concentration alone. Therefore, the rate of the reaction does not change with varying concentrations of the nucleophile, $OH^-$, which is represented graphically as a straight horizontal line parallel to the concentration axis.

The major product is determined by the relative stability of the possible intermediates, where the reaction pathway with the lowest activation energy dominates. In electrophilic additions, the formation of the more stable carbocation intermediate, influenced by hyperconjugation and inductive effects from surrounding alkyl groups, dictates the regioselectivity, whereas in stereospecific concerted mechanisms, steric hindrance prevents the reagent from approaching the more crowded site to ensure the formation of the thermodynamically favored product.

The empirical formula $\mathrm{C}_6 \mathrm{H}_6 \mathrm{O}$ and the presence of a sooty flame characterize compound 'A' as phenol, which contains an aromatic ring substituted with a hydroxyl group. Because the hydroxyl group is a strong activating group, performing an electrophilic substitution with bromine in a low-polarity solvent like $\mathrm{CS}_2$ restricts the reaction, preventing the rapid multi-substitution typically observed in polar media. This selective process yields mono-brominated products, with para-bromophenol typically dominating the mixture as product 'B' due to minimized steric hindrance compared to the ortho-isomer.

The transformation follows a nucleophilic addition-elimination mechanism where the lone pair of the reactant attacks the electrophilic center of the substrate, generating a tetrahedral intermediate. This intermediate subsequently undergoes elimination of the leaving group to restore the pi-bond system, yielding the final product A which is the most thermodynamically stable isomer under the specified reaction conditions.

Rancidity in food products like butter is primarily caused by the oxidative degradation of fats, a process that antioxidants effectively mitigate. Butylated hydroxyanisole acts as a sacrificial antioxidant, extending the shelf life of butter by intercepting atmospheric oxygen before it can compromise the food's quality. This mechanism functions because the antioxidant is chemically more reactive toward oxygen than the organic compounds within the butter, meaning the oxygen preferentially consumes the antioxidant rather than degrading the food, which confirms that the reason provided is a scientifically accurate explanation for the assertion.

Fehling's reagent utilizes the tartrate ion ($C_4H_4O_6^{2-}$) as a complexing agent to maintain copper(II) ions in solution under alkaline conditions. The tartrate ion acts as a tetradentate ligand by binding to the central $Cu^{2+}$ ion through four oxygen donor atoms, specifically two from the carboxylate groups and two from the hydroxyl groups, resulting in a denticity of 4.

Acid dissociation constants, represented as $pK_a$, are inversely related to acidic strength, meaning the weakest acid corresponds to the highest $pK_a$ value. Electronic effects dictate this strength: the $-M$ effect of the $\mathrm{NO}_2$ group stabilizes the conjugate base to enhance acidity, while the $+M$ effect of $\mathrm{N}(\mathrm{CH}_3)_2$ and the hyperconjugation of an isopropyl group destabilize the conjugate base to reduce acidity. Because the acidity follows the order $(c) > (a) > (d) > (b)$, the resulting $pK_a$ order is the inverse sequence of $(b) > (d) > (a) > (c)$.

The strength of the nucleophile dictates the reaction pathway when 2-methyl propyl bromide reacts with ethoxide versus ethanol. Because $\mathrm{C}_2\mathrm{H}_5\mathrm{O}^-$ is a strong nucleophile, it promotes an $\mathrm{S}_{\mathrm{N}} 2$ mechanism, which proceeds without carbocation rearrangement, yielding iso-butyl ethyl ether as product 'A'. Conversely, $\mathrm{C}_2\mathrm{H}_5\mathrm{OH}$ acts as a weak nucleophile, facilitating a process where the leaving group departs to form a primary carbocation; this unstable intermediate undergoes a 1,2-hydride shift to produce a more stable tertiary carbocation, which leads to the formation of tert-butyl ethyl ether as the $\mathrm{S}_{\mathrm{N}} 1$ product 'B'.

The reaction proceeds via an electrophilic addition mechanism where the protonation of the alkene double bond generates the most stable carbocation intermediate, following Markovnikov's rule. This carbocation intermediate may undergo rapid rearrangement via hydride or alkyl shifts to reach a more stable tertiary or resonance-stabilized state, which is subsequently attacked by the nucleophile to yield the thermodynamically favored major product. The chemical structure represented in the second option is the only one that accounts for this carbocation stability and the necessary regiospecificity required for the reaction's major product.

Stoichiometric coefficients define the molar relationships between reactants and products within a balanced chemical reaction. By examining the reaction stoichiometry, it becomes clear that $x$ corresponds to 2 moles and $y$ corresponds to 1 mole. Dividing these molar amounts establishes the ratio as $\frac{x}{y} = \frac{2}{1} = 2$ providing the final relationship between these quantities.

The iodoform test is positive for compounds containing either a methyl ketone group ($CH_3CO-$) or a secondary alcohol group ($CH_3CH(OH)-$) that can be oxidized to a methyl ketone. 3-Methylbutan-2-ol ($CH_3CH(OH)CH(CH_3)_2$), 1-phenylethanol ($C_6H_5CH(OH)CH_3$), and 1-phenylpropan-2-ol ($C_6H_5CH_2CH(OH)CH_3$) all possess the required $CH_3CH(OH)-$ moiety, while 3,3-dimethylbutan-2-one ($CH_3COC(CH_3)_3$) contains the necessary $CH_3CO-$ group, resulting in four positive reactions. Conversely, 1-phenylbutan-2-one lacks a methyl group adjacent to the carbonyl carbon, and 2-methylbutan-2-ol is a tertiary alcohol that cannot be oxidized to the requisite methyl ketone, causing both to yield negative results.

Boiling points are primarily dictated by the strength of intermolecular forces, and for compounds of comparable molecular mass, the presence of extensive hydrogen bonding significantly raises the boiling point. Butan-1-ol, which is $C_4H_9OH$, possesses a hydroxyl group that allows for intermolecular hydrogen bonding, creating a stronger association between its molecules compared to ethoxyethane, $C_2H_5OC_2H_5$, which cannot participate in such bonding. This strong intermolecular attraction in the alcohol requires more energy to overcome, directly causing it to have a higher boiling point than the ether. Consequently, both the assertion regarding the higher boiling point and the reason identifying extensive hydrogen bonding as the cause are scientifically accurate and causally linked.

The reaction is driven by the formation of a stable carbocation intermediate and the subsequent generation of the most highly substituted alkene. After protonation, the oxygen atom bears a positive charge in the unstable $\mathrm{O}^{\oplus} \mathrm{H}_2$ group, which departs to form an initial carbocation that rapidly undergoes a 1,2-methyl shift to a more stable tertiary carbocation, increasing the number of $\alpha$-H from 2 to 6. This stabilization ensures that the final elimination step yields the most thermodynamically favored alkene, characterized by the highest number of $\alpha$-H, resulting in the major product C.

Electrophilic addition of $HBr$ to an alkene proceeds through the formation of a carbocation intermediate, where the stability of the carbocation governs the reaction pathway. The initial protonation of the alkene double bond creates a secondary carbocation, which promptly undergoes a 1,2-hydride shift to form a significantly more stable tertiary carbocation due to increased hyperconjugation and inductive stabilization from the adjacent alkyl groups. The bromide nucleophile subsequently attacks this tertiary carbocation to yield 2-bromo-2-methylbutane as the major product, as this pathway minimizes the transition state energy and maximizes the thermodynamic stability of the final species.

A molecule exhibits chirality when it contains a stereocenter, typically a carbon atom bonded to four unique chemical groups, which prevents the molecule from being superimposable on its mirror image. Among the structural isomers of $\mathrm{C}_4\mathrm{H}_{10}\mathrm{O}$, only butan-2-ol possesses a chiral center at the second carbon, as it is attached to a hydrogen atom, a hydroxyl group, a methyl group, and an ethyl group. Because this chiral carbon allows for two non-superimposable mirror-image configurations, specifically the ($R$)- and ($S$)-enantiomers, there are two distinct chiral alcohol molecules possible for this molecular formula.

Acrolein is a significant byproduct in the analytical detection of glycerol, yet the provided statements fail to accurately describe its nature and the necessary reaction conditions. Statement II is incorrect because acrolein possesses a distinct, pungent, and suffocating odour, which contradicts the claim that it is fruity. Additionally, Statement I is incorrect because characterizing the process simply as heating with $KHSO_4$ does not sufficiently account for the controlled environmental factors and rigorous conditions needed to reliably produce acrolein. Consequently, both of these statements are inaccurate.

The solubility of a compound in aqueous $\mathrm{NaOH}$ but not in aqueous $\mathrm{NaHCO}_3$ indicates that the substance is a weak acid with a $pK_a$ greater than that of carbonic acid ($\approx 6.4$), which is a defining characteristic of phenols. The production of a violet color upon treatment with neutral $\mathrm{FeCl}_3$ serves as a specific diagnostic test for phenolic compounds due to the formation of iron(III) phenoxide complexes. These properties confirm that the structure must be a phenol.

The Sandmeyer reaction is a chemical process used to synthesize aryl halides from primary aromatic amines by treating the diazonium salt intermediate with copper(I) bromide. In this specific pathway, aniline is first diazotized using sodium nitrite and hydrochloric acid at low temperatures to form benzenediazonium chloride, which then undergoes a nucleophilic substitution reaction to replace the diazonium group with a bromine atom. This sequence follows the transformation $C_6H_5NH_2 \xrightarrow{NaNO_2/HCl} C_6H_5N_2^+Cl^- \xrightarrow{CuBr/HBr} C_6H_5Br$, resulting in the formation of bromobenzene.