Organic Chemistry Some Basic Principles – JEE Main ChemistryPractice Questions & PYQs

The hydride affinity of a carbocation is intrinsically linked to its relative stability, as highly stable carbocations are less reactive and possess a lower drive to accept a hydride ion. According to the principle $\text{Hydride Affinity} \propto \frac{1}{\text{Stability of Carbocation}}$, determining the reactivity requires identifying the most and least stable intermediates based on factors like hyperconjugation and inductive effects. Simply ranking the structures from most to least stable allows you to derive the final sequence by strictly inverting that stability order.

In thin layer chromatography using a non-polar mobile phase like hexane, the stationary phase is polar, meaning compounds with higher polarity will have stronger interactions with the plate and travel a shorter distance from the baseline. By observing the relative movement of the spots, compound C is identified as the most polar substance because it covered the least distance up the plate. The retardation factor, $R_f$, is calculated as the ratio of the distance traveled by the compound to the distance traveled by the solvent front, given by the formula $R_f = \frac{\text{distance travelled by solute}}{\text{distance travelled by solvent}}$. Since compound C traveled 2 units and the solvent front traveled 8 units, the value is $R_f = \frac{2}{8} = 0.25$, which is equivalent to $25 \times 10^{-2}$.

Silica gel acts as a polar stationary phase in column chromatography, where compounds with a higher affinity for the mobile phase move more rapidly and elute earlier, while those with a higher affinity for the stationary phase are retained longer. Since $A$ possesses the highest mobility and the strongest interaction with the mobile phase, followed by $C$, and then $B$, the components will be recovered from the column in the order of their decreasing mobility. Therefore, the substances elute in the sequence $A$, $C$, and $B$.

Column chromatography separates compounds based on their differential affinity for a polar stationary phase, where more polar components are adsorbed more strongly, migrate more slowly, and elute later than less polar components. Since the figure indicates that compound 'a' is the most polar, it travels the slowest and spends the most time in the column, while compound 'c' is the least polar and elutes first. Consequently, statements A, B, and C are factually incorrect regarding the relative polarities and elution sequence, as they misidentify the component with the highest polarity and the corresponding order of elution.

While the inductive effect of methyl groups suggests that basicity should increase with each successive substitution due to enhanced electron density, the basicity of amines in aqueous medium is actually limited by steric hindrance and the solvation energy of the conjugate acid. The secondary amine $\mathrm{Me}_2\mathrm{NH}$ provides the ideal compromise between electronic donation and the stabilization of its conjugate acid through hydrogen bonding, making it the most basic of the group. Conversely, the tertiary amine $\mathrm{Me}_3\mathrm{N}$ faces significant steric crowding that inhibits effective solvation of the cation, which decreases its basicity below that of both secondary and primary amines, leading to the established overall order of $\mathrm{Me}_2\mathrm{NH} > \mathrm{MeNH}_2 > \mathrm{Me}_3\mathrm{N} > \mathrm{NH}_3$.

Separation techniques exploit specific differences in physical properties like boiling points and solubility to isolate components from a mixture. Simple distillation effectively separates mixtures like $\mathrm{CHCl}_3+\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2$ where there is a sufficient difference in boiling points, whereas fractional distillation handles mixtures like $\mathrm{C}_6\mathrm{H}_{14}+\mathrm{C}_5\mathrm{H}_{12}$ containing components with close boiling points. For substances that are steam-volatile and water-insoluble, such as $\mathrm{C}_6\mathrm{H}_5\mathrm{NH}_2+\mathrm{H}_2\mathrm{O}$, steam distillation is the appropriate method, while differential extraction relies on differences in solubility to recover organic compounds from an aqueous solution.

IUPAC priority rules mandate that the principal functional group, such as the carboxylic acid, must be assigned the locant $C_1$ to define the parent carbon chain length. In this six-carbon molecule, numbering from the carboxylic acid places the methyl group at the $C_2$ position and the oxo group at the $C_5$ position, which correctly identifies the compound as 2-methyl-5-oxohexanoic acid by following the systematic protocol for prioritizing functional groups and applying the lowest locant rule for substituents.

The readiness of a molecule to undergo deprotonation in a basic medium is determined by the acidity of the hydrogen atom, which is governed by the stability of the resulting conjugate base. When a molecule loses a proton to form an anion, structural factors such as resonance stabilization, inductive effects, or electronegativity drastically lower the energy of the transition state, making the removal of $H^+$ significantly easier. Structure a facilitates the most effective delocalization or stabilization of the developing negative charge, thereby allowing it to donate its proton more readily than the other structures in the series.

Resonance in the carbonate ion ($\mathrm{CO}_3^{2-}$) involves drawing multiple Lewis structures to describe electron distribution, but these individual drawings are purely hypothetical tools rather than actual snapshots of the molecule. The true chemical entity is a resonance hybrid, which functions as a weighted average of all these theoretical possibilities and represents the stable, real structure of the ion. Because the electron density is delocalized throughout this hybrid form, the individual resonating structures cannot be isolated, nor do they exist as independent, interchangeable states.

A molecular conformation achieves maximum stability when its potential energy $U$ is minimized, as this occurs when the repulsive forces between atoms and rotation-related energy barriers are kept as low as possible. In this case, the arrangement depicted in option A represents the structure that effectively minimizes both van der Waals strain, caused by steric crowding, and torsional strain, resulting from eclipsing interactions. By reducing these destabilizing forces to their absolute minimum, the molecule settles into its most energetically favorable state.

The exceptional stability of benzene arises from resonance, which is fundamentally tied to the delocalization of the $\pi$ electron cloud across the planar carbon ring. Because these electrons occupy bonding molecular orbitals rather than being restricted to localized double bonds, they experience a stronger electrostatic attraction to the carbon nuclei, which effectively lowers the potential energy of the system compared to a hypothetical cyclohexatriene structure. This enhanced interaction within the delocalized framework accounts for the significant stability observed in benzene compared to its non-aromatic counterpart.

Column chromatography operates on the principle of differential affinity, where components in a mixture are separated based on the strength of their interaction with the stationary phase versus their solubility in the mobile phase. Because compound A emerges from the column first, it demonstrates a weaker affinity for the stationary adsorbent, allowing it to migrate more rapidly with the solvent flow. Conversely, compound B remains in the column longer, which indicates that it forms stronger bonds with the adsorbent, significantly retarding its progress. This stronger adsorption limits how far B moves from the starting position compared to the solvent front. According to the definition $R_f = \frac{\text{distance covered by substance from base line}}{\text{total distance covered by solvent form base line}}$, this reduced migration distance directly results in a lower $R_f$ value for compound B.

Dipole moments in cyclic compounds are dramatically increased when the separation of charge allows the molecule to attain a more stable electronic configuration. In the case of the compound in option B, the molecule readily exists in a polar form because both the resulting positive and negative regions achieve aromatic stability. This strong tendency to gain aromatic character at both ends drives significant charge separation, resulting in the highest measured dipole moment among the choices provided.

Paper chromatography functions based on the principles of partition chromatography, where the separation of substances depends on their differential distribution between two phases. While the cellulose paper acts as the physical support medium, the actual stationary phase is not the paper itself, but rather the layer of water molecules adsorbed within the microscopic pores of the paper fibers. Consequently, as the mobile phase moves through the paper, the components are partitioned between this stationary aqueous layer and the moving solvent.

Aromaticity is determined by a molecule's adherence to Hückel's rule, which requires a planar, cyclic, and fully conjugated system with a total of $(4n+2)\pi$ electrons. Tropolone contains a seven-membered ring that accounts for $6\pi$ electrons within the cycle, providing the necessary conditions for aromaticity. When the $2\pi$ electrons from the exocyclic carbonyl group are added to these $6\pi$ electrons, the molecule possesses a total of $8\pi$ electrons, confirming the validity of the first statement.

The aromatic behavior of tropolone is specifically linked to the delocalization of the endocyclic electrons rather than the entire molecular pi-electron framework. Since the $\pi$ electrons of the carbonyl group are located outside the cyclic conjugation pathway, they remain isolated from the aromatic system and do not participate in Hückel's rule requirements. Consequently, the second statement incorrectly identifies these exocyclic electrons as being involved in the aromaticity of the compound.

Selecting the appropriate separation technique requires an analysis of the distinct physical and chemical characteristics of the substances in a mixture. $\mathrm{H}_2\mathrm{O}$ and $\mathrm{CH}_2\mathrm{Cl}_2$ form two immiscible layers with different densities, making them suitable for separation via differential solvent extraction, while column chromatography is ideal for mixtures where components exhibit varying affinities for a stationary phase, often driven by hydrogen bonding. Fractional distillation is the preferred method for separating substances like kerosene and naphthalene, as it leverages the significant difference in their boiling points to achieve purification. Finally, crystallization is an effective strategy for isolating components such as $\mathrm{NaCl}$ from $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6$, since the ionic nature of the salt allows it to crystallize out of a saturated solution.

The retardation factor, or $R_f$ value, is determined by the ratio of the distance a solute travels to the total distance the solvent front advances from the baseline, which is calculated using the formula $R_f = \frac{\text{Distance travelled by substance}}{\text{Distance travelled by solvent}}$. Given that the compound travelled 3.0 cm while the solvent moved 5.0 cm, we divide 3.0 by 5.0 to obtain 0.6. Expressing this result in the requested format gives $0.6 = 6 \times 10^{-1}$, identifying the missing value as 6.

The acidity of carboxylic acids is fundamentally governed by the stability of the corresponding conjugate base, the carboxylate anion, formed after the release of a proton. Electron-withdrawing substituents stabilize this negative charge through the inductive effect, and the strength of this stabilization is determined by the electronegativity and the number of substituents attached to the alpha carbon. As shown by the stability order of the conjugate bases, the trifluoromethyl group ($\mathrm{F}_3\mathrm{C}-$) exerts the strongest electron-withdrawing influence, followed by the monohalogenated groups where the influence decreases with electronegativity in the order of $\mathrm{F}- > \mathrm{Cl}- > \mathrm{Br}-$. Consequently, the acidity decreases as the electron-withdrawing ability of the substituent diminishes, resulting in the overall descending order of $\mathrm{F}_3\mathrm{C}-\mathrm{COOH} > \mathrm{F}-\mathrm{CH}_2-\mathrm{COOH} > \mathrm{Cl}-\mathrm{CH}_2-\mathrm{COOH} > \mathrm{Br}-\mathrm{CH}_2-\mathrm{COOH} > \mathrm{CH}_3\mathrm{COOH}$.

The stability of a carbocation is primarily determined by its electronic environment, with resonance providing the most significant stabilization. Carbocation (B) possesses the highest stability due to resonance, while carbocations (A) and (C) are both stabilized by hyperconjugation from their $4\alpha$ hydrogen atoms. Because carbocation (C) is further destabilized by the $-I$ effect of the oxygen atom compared to (A), the overall decreasing order of stability is $B > A > C$.

The retention factor, denoted by $R_f$, describes how a substance migrates during paper chromatography and is defined as the distance travelled by the substance divided by the distance travelled by the solvent front. When comparing two different substances analyzed within the same chromatographic run, both are referenced against the same solvent front distance, which simplifies the comparison of their relative migration rates.

In this experiment, the solvent front moved 3.25 cm, while substance A moved 2.08 cm and substance B moved 1.05 cm. Because both distances share the common denominator of the solvent front, the ratio of the $R_f$ values for substance A to substance B simplifies to the ratio of their respective migration distances. The calculation is as follows:

$\frac{(R_f)_A}{(R_f)_B} = \frac{2.08}{1.05} \approx 2$