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

Monochlorination of 2-methylbutane, $CH_3CH(CH_3)CH_2CH_3$, occurs at four non-equivalent sets of hydrogen atoms, resulting in structural isomers with distinct stereochemical properties. Substitution at either of the equivalent terminal methyl groups attached to C2 yields 1-chloro-2-methylbutane, which possesses a chiral center at C2, generating a pair of enantiomers. Substitution at the C2 carbon creates 2-chloro-2-methylbutane, which is achiral and exists as a single isomer. Substitution at the C3 methylene group produces 2-chloro-3-methylbutane, which contains a chiral center at C3 and thus yields a pair of enantiomers. Finally, substitution at the terminal methyl group attached to C3 results in 1-chloro-3-methylbutane, which is achiral and exists as one isomer, leading to a total of $2 + 1 + 2 + 1 = 6$ isomeric products.

The reaction of 3-methylhex-2-ene with $\mathrm{HBr}$ in the presence of peroxide follows a free-radical anti-Markovnikov mechanism. The bromine radical selectively attacks the alkene to form the most stable radical intermediate, which results in the bromine atom bonding to the second carbon and a hydrogen atom bonding to the third carbon. The resulting product is 2-bromo-3-methylhexane, which contains two distinct stereogenic centers. Since the molecule lacks any internal plane of symmetry, the total number of stereoisomers is calculated using the formula $2^n$, where $n$ is the number of chiral centers. Substituting $n=2$ gives a total of 4 stereoisomers.

Ethane molecules experience continuous rotation around the carbon-carbon single bond, creating an infinite series of spatial arrangements known as conformations that are constantly interconverting. The staggered conformation features a dihedral angle of $60^{\circ}$ and is the most stable arrangement because it minimizes torsional strain between hydrogen atoms. Conversely, the eclipsed conformation forces these hydrogen atoms into the closest possible proximity, resulting in significant repulsion that makes it the least stable form of the molecule.

The acidity of a molecule is fundamentally determined by the stability of its conjugate base, as a more stable conjugate base indicates a stronger acid because it facilitates the easier release of the proton. By evaluating the electronic and structural factors affecting these conjugate bases, we can rank the specific hydrogen atoms according to how readily they dissociate. Consequently, the ascending order of acidity for these atoms is $C < D < B < A$.

The Inductive effect operates through $\sigma$-bonds due to electronegativity differences, the Mesomeric effect involves electron delocalization through conjugated systems, and Hyperconjugation requires the interaction of $\sigma_{C-H}$ bonds with an adjacent unsaturated system. Compounds featuring an alkyl group attached to an aromatic ring or a double bond satisfy these criteria, as the alkyl group exerts an inductive effect, the unsaturated moiety facilitates mesomeric delocalization, and the $\alpha$-hydrogen atoms provide the necessary overlap for hyperconjugation. Consequently, the four compounds possessing this specific structural arrangement are the ones that exhibit all three electronic effects simultaneously.

The stability of carbanions is primarily determined by electronic effects, such as aromaticity, and structural constraints like angle strain. Compound (d) is aromatic, which makes it the most stable species in the group. In compounds (b) and (c), where the carbon atom is $\mathrm{sp}^3$ hybridized, relative stability is dictated by angle strain theory, confirming that (c) is more stable than (b). When considering these factors alongside the classification of (a) as anti-aromatic, the overall stability order is $d > a > c > b$.

The retention factor, denoted as $R_f$, describes the migration of a solute relative to the movement of the solvent front, calculated as the ratio of the distance traveled by the solute to the distance traveled by the solvent front. Since both Sample A and Sample C share the same solvent front distance of 12.5, the ratio of their $R_f$ values simplifies to the ratio of their respective migration distances. Specifically, the $R_f$ for A is $\frac{5}{12.5}$ and for C is $\frac{10}{12.5}$, which results in the quotient $\frac{5}{10}$, equaling 0.5. To express 0.5 in the format of $x \times 10^{-2}$, we recognize that 0.5 is equivalent to $50 \times 10^{-2}$, identifying that $x$ is 50.

Groups such as $-\mathrm{NH}_2$, $-\mathrm{OH}$, and $-\mathrm{OR}$ possess lone pairs of electrons on the atom directly attached to the conjugated system, allowing them to donate electron density into the ring and exhibit a $+R$ effect. In contrast, the $-\mathrm{COOH}$ group contains a carbon atom double-bonded to an electronegative oxygen, which acts as an electron sink that pulls pi-electron density away from the conjugated system, resulting in a $-R$ effect.

The relative strength of conjugate bases is governed by an inverse relationship with the acidity of their corresponding parent acids. Since the acidity of these compounds follows the order $\mathrm{HCl} > \mathrm{CH}_3\mathrm{COOH} > \mathrm{H}_2\mathrm{O} > ROH$, the basicity of their conjugate bases must follow the exact opposite trend. Consequently, because $\mathrm{HCl}$ is the strongest acid, its conjugate base $\mathrm{Cl}^{-}$ acts as the weakest, while the conjugate base of the weakest acid, $ROH$, is the strongest. This relationship establishes the decreasing order of basic strength as $\mathrm{RO}^{-} > \mathrm{OH}^{-} > \mathrm{CH}_3\mathrm{COO}^{-} > \mathrm{Cl}^{-}$.

Organic compounds containing closed ring structures are broadly classified into aromatic and alicyclic categories, where alicyclic compounds specifically include cyclic molecules that do not possess aromatic character. Aromaticity requires a planar cyclic system with a delocalized $\pi$ electron cloud following Hückel's rule of $4n+2$ $\pi$ electrons, a condition that cyclohexene does not satisfy. As cyclohexene is a six-membered carbocyclic ring with a single double bond and no aromatic delocalization, it is categorized as an alicyclic compound.

The total number of stereoisomers for a molecule is determined by the number of its stereocenters, denoted by $n$. When a structure contains 3 stereocenters, the possible spatial arrangements are calculated using the expression $2^n$. Substituting the value $n = 3$ into this calculation results in $2^3 = 8$, which defines the total count of distinct stereoisomers for the given structure.

Chain isomerism in alkanes arises from the variation in the arrangement of the carbon skeleton, which is systematically determined by considering decreasing main chain lengths and placing the remaining carbon atoms as alkyl substituents. For the molecular formula $\mathrm{C}_7\mathrm{H}_{16}$, the possibilities begin with the straight-chain n-heptane ($1$ isomer). Reducing the main chain to six carbons yields 2-methylhexane and 3-methylhexane ($2$ isomers). With a main chain of five carbons, the dimethyl substituted isomers are 2,2-dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane, and 3,3-dimethylpentane, along with the ethyl substituted isomer 3-ethylpentane ($5$ isomers). Finally, reducing the main chain to four carbons results in the trimethyl substituted isomer 2,2,3-trimethylbutane ($1$ isomer). Summing these arrangements gives $1 + 2 + 5 + 1 = 9$ distinct chain isomers.

According to IUPAC nomenclature rules, the principal functional group dictates the suffix of the parent chain and receives the lowest possible locant. When both ketone and alcohol groups are present, the ketone group holds higher priority, requiring the suffix -one and designating the hydroxyl group as the substituent hydroxy. In the molecule $\mathrm{HO}-\mathrm{CH}_2-(\mathrm{CH}_2)_3-\mathrm{CH}_2-\mathrm{COCH}_3$, numbering the seven-carbon chain starting from the end closest to the carbonyl group assigns the ketone to position 2 and the hydroxyl group to position 7, establishing 7-hydroxyheptan-2-one as the correct IUPAC name. Consequently, 2-oxoheptan-7-ol is incorrect because it treats the lower-priority alcohol as the senior functional group and uses an improper suffix.

The rate of electrophilic aromatic substitution is governed by the electron density of the aromatic ring, which is significantly enhanced by electron-donating groups and reduced by electron-withdrawing groups. Consequently, the reactivity follows a hierarchy where strong activating substituents (III) show the highest rate, followed by unsubstituted benzene (I), then weakly deactivating halogen substituents (II), and finally, systems with strongly deactivating nitro groups (IV) where the incoming electrophile ($E^+$) encounters the lowest electron density.

According to IUPAC priority rules, the carboxylic acid group takes precedence over aldehyde, alcohol, and alkene groups, designating the parent structure as an alkanoic acid. The main carbon chain is numbered starting from the carboxyl carbon, which necessitates treating the aldehyde group as a formyl substituent at position 2 and the hydroxyl group at position 4. Because the seven-carbon chain contains a double bond starting at position 6, the unsaturated parent acid is named hept-6-enoic acid, resulting in 2-formyl-4-hydroxyhept-6-enoic acid.

The rate of electrophilic aromatic substitution is fundamentally governed by the electron density within the benzene ring, as electrophiles are inherently drawn to regions rich in electrons. Substituents that donate electrons via resonance effectively activate the ring, thereby facilitating this attack. Because the hydroxyl group in phenol ($C_6H_5OH$) provides significant resonance-based electron donation, this molecule maintains the highest electron density among the available choices, rendering it the most reactive towards incoming electrophiles.

Aromaticity is governed by Hückel's rule, which mandates that a molecule must be cyclic, planar, fully conjugated, and contain $4n + 2$ $\pi$-electrons, where $n$ is an integer. Compounds B and D satisfy these structural and electronic requirements, allowing for complete delocalization of $\pi$-electron density and maximal resonance stabilization. Conversely, the other options fail to meet these conditions due to violations of planarity, incomplete conjugation pathways, or an electron count that does not conform to the $4n + 2$ formula.

The total number of geometrical isomers in a chain with multiple stereocenters is fundamentally governed by the relationship between the number of available double bonds and the inherent symmetry of the molecule. When a structure possesses three stereocenters and exhibits symmetry, the redundancy created by the identical terminal groups restricts the total count of distinct configurations because certain arrangements become equivalent upon spatial rotation or reflection.

For this specific symmetrical framework, the constraints prevent the full $2^n$ theoretical combinations, leading instead to a restricted set of valid forms. By accounting for these symmetrical limitations, we identify the distinct arrangements, which include the fully symmetrical (EE) and (ZZ) forms alongside the intermediate (EZ) configurations, ultimately resulting in a total of 4 unique geometrical isomers.

Brønsted basicity in amines is primarily governed by the availability of the lone pair on the nitrogen atom, which is enhanced by the positive inductive ($+I$) effect of attached alkyl groups. Secondary amines generally exhibit the highest basicity in aqueous media because they strike an optimal balance between electronic stabilization of the conjugate acid by alkyl groups and effective solvation of the protonated species. The structure in option D possesses the most favorable combination of these factors, maximizing the electron density on the nitrogen and providing the greatest stability upon protonation compared to the other options.

The structure $\mathrm{HOCH}(\mathrm{CN})_2$ is represented in bond line notation by a central vertex connected to one hydroxyl group and two cyano groups. In this arrangement, the oxygen atom of the hydroxyl group is bonded to the central carbon, while the two nitrile groups, expressed as $\mathrm{C}\equiv\mathrm{N}$, are drawn branching out to satisfy the valency of the central carbon. This schematic eliminates the explicit carbon and hydrogen symbols of the main chain, emphasizing the connectivity of the functional groups to the central methine carbon.