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

The retention factor, $R_f$, represents the ratio between the distance a solute has migrated and the total distance covered by the solvent front. This relationship is defined by the formula $R_f = \frac{\text{distance travelled by solute}}{\text{distance travelled by solvent front}}$. Using the provided measurements of 2 cm for the solute and 5 cm for the solvent, the ratio becomes 2/5, which equals 0.4. This decimal value, when expressed in scientific notation, is $4 \times 10^{-1}$.

Chromatography operates by exploiting the differing rates at which various components of a mixture travel through a porous stationary phase when propelled by a flowing mobile phase, such as a liquid or gas. This separation process relies fundamentally on the principle of adsorption, where substances are sequestered based on their unique affinities for the medium rather than their structural form. Because the technique is driven by these specific interaction rates with the stationary phase, the actual physical state of the pure compound being isolated does not influence the separation.

Carbocation stability is primarily governed by the ability of substituents to donate electron density to the electron-deficient positive carbon center through the mesomeric ($+\text{M}$), hyperconjugative ($+\text{H}$), and inductive ($+\text{I}$) effects. Groups that can donate electrons via resonance or hyperconjugation significantly decrease the positive charge density, while electron-withdrawing substituents exhibiting $-\text{I}$ or $-\text{M}$ effects increase the charge density and destabilize the structure. Consequently, the most stable carbocation possesses the strongest electron-donating groups, and stability decreases as electron-donating ability diminishes and electron-withdrawing character increases, resulting in the sequence $A > C > B > D$.

To classify a cyclic compound as aromatic, it must satisfy Hückel's rule by being planar, featuring complete delocalization of $\pi$ electrons, and containing $(4n+2)\pi$ electrons, where $n$ is an integer. The structure in option A fulfills all these requirements, most notably possessing 6 $\pi$ electrons, which aligns with the Hückel criteria when $n=1$.

The Carius method for estimating halogens functions on the principle of stoichiometric conservation, assuming that every atom of bromine originally present in the organic compound is fully recovered within the precipitate of $\text{AgBr}$. Since the chemical formula of $\text{AgBr}$ indicates a one-to-one molar ratio between bromine and the total compound, the mass of bromine in the precipitate is directly proportional to its atomic mass relative to the molecular mass of the precipitate.

Calculate the molar mass of $\text{AgBr}$ by adding the atomic mass of silver, 108, to that of bromine, 80, which equals 188 g/mol. To find the amount of bromine recovered from the 0.2397 g of precipitate, multiply this mass by the fraction $\frac{80}{188}$. This value represents the actual mass of bromine obtained from the original organic sample. To determine the percentage, divide this mass by the initial sample mass of 0.15 g and multiply the result by 100. Evaluating the expression $\frac{0.2397 \times 80}{188 \times 0.15} \times 100$ results in 68.

Resonance requires an uninterrupted system of overlapping p-orbitals, which typically occurs through alternating single and double bonds or adjacent lone pairs. In the compound ${CH}_{3} {CH}_{2} {CH}={CHCH}_{2} {NH}_{2}$, the pi electrons of the double bond and the lone pair on the nitrogen atom are separated by a saturated methylene group. Because this $sp^3$ hybridized carbon disrupts the continuity of the p-orbital system, electron delocalization is blocked, and the molecule cannot exhibit resonance.

Simple distillation relies on a substantial difference in boiling points between two liquids to enable their effective separation, specifically requiring a gap greater than 20°C. When evaluating a mixture of propanol and propanone, we observe that propanol boils at 97°C and propanone at 56°C. Given the calculation
$97 - 56 = 41$
which results in a difference of 41°C, this exceeds the standard threshold and confirms that the method is valid because the Reason provides the exact scientific criterion necessary for the Assertion to hold true.

Acid strength is fundamentally determined by the stability of the conjugate base formed after deprotonation, which is enhanced by factors such as the electronegativity of the atom bonded to the hydrogen, resonance stabilization, or the inductive effect of electron-withdrawing groups that disperse negative charge. While the alkane in option A is essentially non-acidic because it cannot stabilize a resulting carbanion, the compound in option D possesses specific structural features that provide significant stabilization to the conjugate base, thereby facilitating proton loss and rendering it the strongest acid.

When you rotate the carbon-carbon single bond in an ethane molecule, the spatial arrangement of the hydrogen atoms shifts, creating different conformational states identified as staggered and eclipsed. Because these distinct shapes are produced purely through rotation about a single bond, they are formally categorized as rotational isomers, more commonly referred to as rotamers.

Functional group isomerism arises when isomers possess the same molecular formula but contain different functional groups, which inherently gives them different chemical behaviors. For the molecular formula $\mathrm{C}_3\mathrm{H}_6\mathrm{O}$, the atoms can be arranged into an aldehyde structure, $\mathrm{CH_3-CH_2-CH=O}$, or a ketone structure, $\mathrm{CH_3-CO-CH_3}$. Because these arrangements differ by their specific functional groups despite having the same molecular formula, they serve as a clear example of functional group isomerism.

The hybridization state of a carbon atom is determined by the number of sigma bonds surrounding it, where four sigma bonds indicate $sp^3$ hybridization and three sigma bonds indicate $sp^2$ hybridization. Carbon a is $sp^3$ hybridized as it forms four single bonds including those with 3 H atoms, while both carbon b and carbon c are $sp^2$ hybridized because they participate in double bonds. This variation in bonding geometry, resulting from the presence or absence of pi bonds, defines the specific orbital arrangement for each carbon atom.

Metamerism arises when isomers share the same molecular formula but exhibit a different distribution of alkyl groups bonded to either side of a polyvalent functional group such as an ether, ketone, or secondary amine. In the correct pair, such as diethyl ether ($\mathrm{CH}_3\mathrm{CH}_2-\mathrm{O}-\mathrm{CH}_2\mathrm{CH}_3$) and methyl propyl ether ($\mathrm{CH}_3-\mathrm{O}-\mathrm{CH}_2\mathrm{CH}_2\mathrm{CH}_3$), the oxygen atom is flanked by two ethyl groups in the first molecule and by a methyl and a propyl group in the second. This change in the alkyl group arrangement flanking the central heteroatom creates distinct isomers, which is the defining characteristic of metamerism.

The molecular structure $C_5H_{10}$, specifically 1,2-dimethylcyclopropane, allows for different stereoisomeric arrangements based on the positions of the methyl groups relative to the plane of the cyclopropane ring. The cis-isomer possesses an internal plane of symmetry that makes it an optically inactive meso compound. Meanwhile, the trans-isomer lacks such symmetry and exists as a pair of non-superimposable mirror images known as enantiomers, giving a combined total of three stereoisomers.

The degree of unsaturation for the molecular formula $C_5H_{10}$ is calculated as $DU = 5 - \frac{10}{2} + 1 = 1$, confirming the presence of one double bond in an acyclic chain. The straight-chain isomer pent-1-ene does not exhibit geometrical isomerism, whereas pent-2-ene exists as distinct cis and trans geometrical isomers due to the restricted rotation around the double bond.

The branched-chain structures derived from the 2-methylbutane skeleton result in three additional isomers: 2-methylbut-1-ene, 3-methylbut-1-ene, and 2-methylbut-2-ene. Among these branched alkenes, none exhibit geometrical isomerism because at least one of the doubly bonded carbons is attached to two identical groups. Summing these contributions yields one isomer for pent-1-ene, two isomers for pent-2-ene, and one isomer each for the three branched alkenes, totaling 6 acyclic isomers.

The stability of n-butane conformations is fundamentally governed by steric and torsional interactions, where lower repulsion corresponds to higher stability and lower potential energy. The anti conformation sits at the lowest potential energy level because the bulky $-\mathrm{CH}_3$ groups are spaced furthest apart, while the fully eclipsed form reaches the maximum potential energy due to significant repulsion between these groups at the front and rear carbon atoms. Consequently, the energy increases as the molecule transitions through the gauche and partially eclipsed states, establishing an order that proceeds from the stable anti arrangement to the highest-energy fully eclipsed structure.

Many high boiling organic liquids are inherently heat-sensitive, frequently decomposing at temperatures equal to or below their normal boiling point, which makes conventional distillation ineffective. To address this, reduced pressure distillation is employed to lower the external pressure on the system, thereby significantly decreasing the boiling point of the liquid to a range where it can vaporize without reaching its decomposition threshold. This technique ensures that these sensitive compounds can be purified efficiently while maintaining their structural integrity.

The retardation factor, $R_{f}$, is a unitless ratio calculated by dividing the distance a substance migrates from the baseline, $x$, by the distance the solvent front migrates from that same baseline, $y$, as shown by the formula $R_{f} = \frac{x}{y}$. Because this operation divides a length by a length, the result is dimensionless, confirming that it cannot be measured in units of distance like meters or centimeters. Furthermore, since the migration of a compound depends on its specific adsorption interactions within a given mobile phase, the $R_{f}$ value is not a fixed property and will change whenever the solvent is altered, which invalidates the claim that it remains constant.

Numbering a carbon chain containing both alkenes and alkynes prioritizes the double bond when locants are equivalent, identifying a six-carbon parent chain with a double bond starting at position 2 and a triple bond at position 4. The bromine substituent attached at the second carbon completes the name structure as 2-bromohex-2-en-4-yne. Applying the Cahn-Ingold-Prelog priority rules to the groups attached to the double bond reveals that the higher-priority groups lie on opposite sides of the alkene linkage, which necessitates the $E$ stereochemical descriptor. Combining these components yields the systematic name (2E)-2-bromohex-2-en-4-yne.

The acidic strength of these compounds is dictated by the stability of the conjugate base, which is enhanced by electron-withdrawing groups and reduced by electron-donating groups. The strong $-R$ and $-I$ effects of the $-{NO}_{2}$ group make the substituted phenol (II) the most acidic, followed by the carboxylic acid (III), which is inherently more acidic than the parent phenol (I). Although the $+I$ effect of the $-{CH}_{3}$ group in (IV) further decreases its acidity, the final observed order of acid character is II > III > IV > I.

The separation of organic mixtures relies on distinct physical properties, such as boiling points and volatility in the presence of steam, to isolate components efficiently. Simple distillation is highly effective when two substances exhibit a large difference in boiling points, as is the case with chloroform ($CHCl_3$) (334 K) and aniline (457 K), allowing the component with the lower boiling point to be distilled first. Regarding steam distillation, this technique is suitable for purifying compounds that are immiscible with water and volatile in steam, such as aniline. Because the mixture of aniline and water boils when the sum of their individual vapor pressures equals atmospheric pressure, the process occurs at a temperature below both the boiling point of aniline and 373 K, verifying that aniline boils below its normal boiling point during this procedure.