Alcohols Phenols And Ethers – JEE Main ChemistryPractice Questions & PYQs

Carbocation stability and Zaitsev's rule dictate the path of this transformation. The mechanism begins by generating an intermediate carbocation that rearranges into the most highly substituted and energetically favorable configuration to minimize the system's potential energy. This stable intermediate subsequently loses a proton to form the most substituted alkene, which is thermodynamically favored because the added alkyl groups provide stabilization to the $\text{C=C}$ double bond through hyperconjugation and inductive effects.

Boiling points are determined by the relative strength of intermolecular forces, which follow a hierarchy starting from non-polar interactions and increasing through low and high polarity, up to hydrogen bonding. As a non-polar molecule, the pentane in (B) relies only on weak dispersion forces, resulting in the lowest boiling point, while the ether in (A) and the ketone in (C) exhibit increasing levels of molecular polarity that lead to progressively stronger dipole-dipole attractions. The alcohol in (D) is capable of forming hydrogen bonds, which are significantly stronger than the dipole-dipole forces found in the other molecules, necessitating the highest energy input to overcome and thus yielding the highest boiling point. Consequently, the compounds follow an increasing boiling point order of (B) < (A) < (C) < (D).

The acidity of these organic compounds is primarily governed by the stability of the conjugate base formed after the loss of a proton, where a more stable conjugate base corresponds to a higher acid strength and a correspondingly lower $pK_a$ value. Ethanol acts as the weakest acid in this group because, as an aliphatic alcohol, it cannot stabilize the negative charge on its oxygen atom through resonance, which results in the highest $pK_a$ value of 15.9. In contrast, phenol benefits from the delocalization of the negative charge across the aromatic ring, providing significant resonance stabilization and leading to a lower $pK_a$ value of 10.0.

The presence of a nitro group further enhances acidity by acting as an electron-withdrawing substituent, which helps disperse the negative charge of the phenoxide ion. For m-nitrophenol, the electron-withdrawing capability is limited to the inductive effect because the resonance path does not involve the oxygen atom of the nitro group, resulting in an intermediate $pK_a$ value of 8.3. Finally, p-nitrophenol exhibits the greatest acidity because the nitro group acts through both the inductive and the mesomeric effect, allowing for direct resonance-based delocalization of the negative charge, which yields the lowest $pK_a$ value of 7.1.

The reaction proceeds through the formation of the most stable carbocation intermediate, driven by the thermodynamic favorability of the resulting structure. Following carbocation rearrangement to achieve greater stability, the elimination step occurs in accordance with Zaitsev's rule, which dictates the formation of the most highly substituted and thus thermodynamically stable alkene as the major product. This outcome is determined by the lower activation energy required to form the more substituted transition state relative to less stable isomers.

The rate of aromatic electrophilic substitution depends on the electron density of the benzene ring, as electron-donating groups accelerate the reaction by increasing nucleophilicity while electron-withdrawing groups slow it down. By comparing the electronic effects of the substituents in each compound, the reaction rates can be ordered based on increasing electron density, yielding the progression $c < a < b < d$.

The formation of an aromatic compound significantly lowers the system's energy, acting as a thermodynamic driving force that causes the reaction to proceed rapidly rather than slowly. Within the aromatic product B, the full delocalization of $\pi$ electrons ensures that all $\mathrm{C}-\mathrm{C}$ bonds possess equivalent lengths, displaying a hybrid character rather than distinct single or double bond geometries. Additionally, the reactant A exhibits keto-enol tautomerism, a structural rearrangement involving alpha hydrogens that facilitates the rapid conversion to the stable aromatic system.

Acidity in organic compounds is primarily dictated by how well the resulting conjugate base is stabilized after the removal of a proton. The inclusion of an electron-withdrawing group such as $-\mathrm{NO}_2$ is crucial because it pulls electron density away from the structure, effectively dispersing the negative charge on the conjugate base and lowering the overall energy of the system. This makes the compound containing the $-\mathrm{NO}_2$ group the most acidic compared to the other options, as it inherently favors the deprotonated state.

The acidic strength of carboxylic acids is fundamentally determined by the stability of the resulting carboxylate anion, which is significantly enhanced by the presence of substituents that exert a $-I$ effect. While it is factually correct that a fluorine atom is a more powerful electron-withdrawing group than a chlorine atom due to its higher electronegativity, this intrinsic chemical property does not serve as the primary explanation for the specific acidity order presented in the assertion. The effectiveness of the $-I$ effect is highly sensitive to the spatial distance between the electron-withdrawing substituent and the acidic carboxyl group. Because this effect diminishes rapidly as the distance along the carbon chain increases, the positional arrangement of these substituents is the defining factor in determining the observed order rather than the simple identity of the halogen atoms involved.

The rate at which an alcohol undergoes dehydration is fundamentally determined by the stability of the intermediate carbocation formed, as this step dictates the overall reaction speed. Highly substituted carbocations gain significant stability through inductive and hyperconjugative effects, which directly accelerates the reaction for tertiary alcohols compared to secondary or primary ones. By ranking the alcohols based on the relative stability of their corresponding carbocations, the decreasing order of dehydration is $b > d > c > a$.

When an alkyl halide is subjected to alcoholic $\mathrm{KOH}$, the reagent functions as a strong base, promoting a dehydrohalogenation process that leads to the formation of an alkene via elimination. Because this reaction pathway specifically favors elimination over nucleophilic substitution, any claim suggesting that substitution is the primary reaction occurring under these conditions is fundamentally incorrect. Distinguishing between the basic nature of alcoholic $\mathrm{KOH}$ and the nucleophilic nature of aqueous conditions is essential for predicting the outcome of such transformations.

Water gas is a reactive mixture of carbon monoxide and hydrogen. In the presence of a cobalt catalyst, this mixture undergoes hydrogenation to produce methanol, a process chemically expressed as $\mathrm{CO} + 2\mathrm{H}_2 \xrightarrow{\text{Cobalt}} \mathrm{CH}_3\mathrm{OH}$.

The oxidation of cumene involves the insertion of atmospheric oxygen into the benzylic $C-H$ bond via a free radical mechanism to form cumene hydroperoxide. This intermediate, structurally represented as $C_6H_5-C(CH_3)_2-OOH$, is subsequently subjected to acid-catalyzed rearrangement to yield phenol and acetone as the final products.

The haloform reaction proceeds by the treatment of a methyl ketone with iodine in the presence of a base, resulting in the cleavage of the methyl group to yield a haloform precipitate ($CHI_3$) and the sodium salt of a carboxylic acid, which constitutes product A. Subsequent treatment with $H_3O^+$ protonates the intermediate carboxylate salt to generate the final carboxylic acid product, identified as B.

The formation of a tetraacetate upon treating $\mathrm{C_5H_{10}O_5}$ with $\mathrm{Ac_2O}$ identifies four hydroxyl groups within the molecule, indicating the remaining oxygen functions as an aldehyde. This functional group assignment is confirmed by the oxidation of the compound with $\mathrm{Br_2-H_2O}$, which produces an aldonic acid with the molecular formula $\mathrm{C_5H_{10}O_6}$.

Reduction of the compound using $\mathrm{HI}$ removes all oxygen atoms to reveal the parent carbon skeleton. Because this reduction yields isopentane, the backbone must be a branched five-carbon chain, which identifies the molecule as an aldopentose derivative containing an isopentane structural framework.

To synthesize methyl phenyl ether, we rely on the Williamson ether synthesis, which proceeds through an $S_N2$ pathway where a phenoxide ion attacks an alkyl halide. In this reaction, sodium phenoxide (\text{PhO}^- \text{Na}^+$) serves as the nucleophile, targeting the electrophilic carbon \in methyl bromide (\text{MeBr}$) to displace the bromide ion and form the ether linkage. Alternative reactant pairs are unsuitable because aryl halides are inherently resistant to nucleophilic substitution, and combining phenoxide with methanol results only in an acid-base equilibrium rather than the desired ether product.

Ozonolysis of an alkene involves the oxidative cleavage of a carbon-carbon double bond to produce aldehydes or ketones rather than alcohols. In contrast, hydroboration-oxidation adds water across the double bond following an anti-Markovnikov mechanism to yield alcohols, while the nucleophilic substitution of an alkyl halide with aqueous $\text{NaOH}$ produces an alcohol via an $\text{S}_\text{N}2$ or $\text{S}_\text{N}1$ pathway. Similarly, the reaction of a Grignard reagent, such as $\text{RMgBr}$, with a ketone followed by acidic hydrolysis serves as a standard method for the synthesis of alcohols.

When hydrogen peroxide reacts with an acidified solution of dichromate, it forms chromium pentoxide, denoted as $\mathrm{CrO}_5$, which possesses a unique butterfly-shaped molecular structure. Because this peroxo species is unstable in aqueous environments, carrying out the reaction in the presence of amyl alcohol allows the compound to be extracted into the organic layer, where its characteristic intense blue coloration becomes clearly visible.

The acidity of substituted phenols is determined by the electronic effects of the substituents, where electron-withdrawing groups ($-M$ and $-I$) stabilize the phenoxide ion and increase acidity, thereby lowering the $\mathrm{pK}_a$, while electron-donating groups ($+I$ and hyperconjugation) destabilize the ion and decrease acidity, thereby raising the $\mathrm{pK}_a$. Among the given compounds, 2,4-Dinitrophenol is the most acidic due to the strong $-M$ and $-I$ effects of two nitro groups, followed in decreasing order of acidity by 4-nitrophenol, 3-chlorophenol, and phenol. The least acidic compound is 2,4,5-trimethylphenol because the three methyl groups exert a significant $+I$ effect that destabilizes the phenoxide ion, so the increasing order of $\mathrm{pK}_a$ values is 2,4-Dinitrophenol < 4-nitrophenol < 3-chlorophenol < Phenol < 2,4,5-trimethylphenol.

Acidity is fundamentally determined by the stability of the conjugate base formed after the deprotonation of the hydroxyl group, where a more stable conjugate base corresponds to a stronger acid. By assessing the electronic and structural characteristics of these compounds, we determine that the stability of their respective conjugate bases follows the sequence $E > C > D > A > B$. Because the strength of the acid is directly proportional to this stability, the resulting acidity order is $E > C > D > A > B$.

The synthesis of compound 'B' proceeds via the diazotization of aniline followed by a nucleophilic substitution reaction. Aniline reacts with nitrous acid, generated in situ from $NaNO_2$ and $HCl$ at low temperatures ($273-278\ K$), to yield benzene diazonium chloride as the intermediate 'A'. Subsequent treatment of this diazonium salt with $CuCN/KCN$ facilitates the Sandmeyer reaction, displacing the diazonium group with a cyano group to produce benzonitrile ($C_6H_5CN$), which is compound 'B'.