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

Geometrical isomerism requires that each carbon atom participating in a double bond must be attached to two distinct substituents to allow for different spatial arrangements. If either carbon atom is bonded to two identical groups, such as two methyl substituents ($-CH_3$), rotation about the double bond fails to produce unique geometric isomers. Because the double bond designated as I features a carbon atom substituted with two identical groups, it violates this necessary condition and thus does not exhibit geometrical isomerism.

Partition chromatography operates on the principle of differential distribution of solutes between a stationary liquid phase and a mobile phase, whereas the use of a finely divided solid adsorbent as the stationary phase is the fundamental mechanism of adsorption chromatography. Consequently, the assertion that the stationary phase is a finely divided solid adsorbent is false, representing the incorrect statement among the options provided. The remaining statements are valid, as partition chromatography can utilize gaseous mobile phases, relies on the continuous equilibration of solutes between phases, and includes techniques such as paper chromatography where the stationary liquid phase is held on a solid support.

Resonance stabilization arises from the delocalization of $\pi$ electrons or lone pairs across a conjugated system, which reduces the electronic energy of the molecule. Species such as the acetate ion ($CH_3COO^-$), formamide ($HCONH_2$), and the allyl cation ($CH_2=CH-CH_2^+$) possess conjugated systems that allow for effective electron delocalization, leading to significant stabilization. In contrast, the molecule in option D, $CH_3CH_2NH_2$, contains only localized sigma bonds and lacks the necessary conjugated system or adjacent p-orbitals for resonance, rendering it the least resonance stabilized.

Acid-base extraction relies on the principle that basic organic compounds, such as amines, undergo protonation by strong acids like $1M\ HCl$ to form ionic ammonium salts. These resulting salts are highly water-soluble due to their polar, ionic nature, which facilitates their migration from the non-polar organic solvent into the aqueous layer. Consequently, the compound corresponding to option (II) possesses the necessary basic character to react with the acid and partition into the aqueous phase.

The reactivity of an organic compound toward an aqueous solution of sodium carbonate is determined by its acidity, where a lower $pK_a$ value signifies a stronger acid capable of reacting more readily. 2,4,6-trinitrophenol contains three strongly electron-withdrawing nitro groups that stabilize the phenoxide ion through resonance and inductive effects, resulting in a very low $pK_a$ value of approximately 0.38. This enhanced acidity makes it significantly more reactive than other candidates, such as phenol or acetic acid, in the acid-base neutralization reaction with carbonate ions.

The net dipole moment of a disubstituted benzene derivative is determined by the vector sum of individual bond dipoles, expressed as $\mu_{net} = \sqrt{\mu_1^2 + \mu_2^2 + 2\mu_1\mu_2 \cos \theta}$, where $\theta$ is the angle between substituents. Because $\cos 60^\circ$ is greater than $\cos 120^\circ$ or $\cos 180^\circ$, the ortho-substituted isomer, identified as structure (I), yields the maximum resultant vector magnitude compared to the meta and para isomers.

The numbering of cyclic substituents follows the lowest locant rule, which dictates that substituents must be assigned numbers that result in the lowest possible locant set, and ties are broken by alphabetical priority. Numbering the carbon bearing the two methyl groups as position 1 and the carbon with the ethyl group as position 2 results in the locant set 1, 1, 2, whereas starting the numbering at the ethyl group leads to a higher locant set of 1, 2, 2. Consequently, the priority to achieve the lowest locant set combined with the alphabetical requirement to list ethyl before dimethyl confirms that 2-ethyl-1,1-dimethylcyclohexane is the valid IUPAC name.

Because glycerol thermally decomposes before it reaches its boiling point at normal atmospheric pressure, it cannot be purified using standard heating methods. The soap industry utilizes distillation under reduced pressure to overcome this limitation, as lowering the external pressure significantly reduces the temperature at which the liquid boils. This physical adjustment allows the glycerol to vaporize safely below its decomposition threshold, effectively separating it from the spent-lye while preserving its chemical integrity.

The absolute configuration is assigned using the Cahn-Ingold-Prelog priority rules, where atoms attached to a chiral center are ranked by atomic number, with the highest atomic number receiving the highest priority. After assigning priorities from 1 to 4, the molecule is oriented such that the lowest priority group is pointing away from the viewer, where a clockwise sequence of priorities 1-2-3 denotes the ($R$) configuration and a counter-clockwise sequence denotes the ($S$) configuration. In this specific stereoisomer, the priorities at the second carbon are arranged in a counter-clockwise direction, yielding the ($2S$) configuration, while the priorities at the third carbon are arranged in a clockwise direction, resulting in the ($3R$) configuration.

A neopentyl group is characterized by a quaternary carbon atom bonded to three methyl groups and a methylene bridge, represented as $\mathrm{(CH_3)_3C-CH_2-}$, whereas a vinyl group is the ethenyl moiety, $\mathrm{-CH=CH_2}$. Combining these two structural units creates the hydrocarbon $\mathrm{(CH_3)_3C-CH_2-CH=CH_2}$, which features a five-carbon main chain containing a double bond at the first position and two methyl substituents at the fourth position. This structure, known as 4,4-dimethyl-1-pentene, accounts for the seven total carbon atoms and includes both the neopentyl and vinyl functional groups required by the conditions.

Geometrical isomerism occurs when both carbon atoms involved in a carbon-carbon double bond are connected to two distinct groups, creating the potential for different spatial arrangements like cis and trans configurations. In 1-phenyl-2-butene, with the structure $C_6H_5-CH_2-CH=CH-CH_3$, each double-bonded carbon atom is attached to a hydrogen atom and a unique alkyl or aryl-substituted group, which satisfies this condition. Conversely, the other options contain terminal methylene groups ($CH_2=$) or a carbon atom bonded to two identical substituents, which prevents the existence of geometrical isomers.

The Carius method for estimating halogens functions on the stoichiometric principle that all bromine atoms in an organic sample are precipitated as silver bromide. Because the mole ratio between silver bromide and bromine is one-to-one, identifying the mass of the precipitate allows for the deduction of the exact mass of bromine originally present in the compound.

The molar mass of silver bromide is $108 + 80 = 188$ g/mol. Dividing the 141 mg of silver bromide obtained by this molar mass results in 0.00075 moles of bromine. Multiplying this molar quantity by the atomic weight of bromine, 80 g/mol, yields 60 mg of bromine. Expressing this mass as a percentage of the original 250 mg organic sample involves the calculation $(60 / 250) \times 100$, which results in 24%.

Molecular dipole moments are determined by the weighted average of all accessible conformations, rather than a single static arrangement. While a molecule may pass through specific configurations where individual bond dipoles cancel out to yield $\mu = 0$, the continuous rotation around single bonds allows for an infinite variety of asymmetrical geometries. Because these dominant non-zero conformations persist in the overall ensemble, the effective observed $\mu$ remains significantly non-zero for these specific structures.

Kjeldahl estimation relies on the principle that organic nitrogen is fully converted into ammonia during digestion, which is then neutralized by an excess of standard sulfuric acid. By determining how much acid remains unreacted using a back-titration with sodium hydroxide, we can pinpoint the exact amount of ammonia evolved from the organic compound.

Calculate the initial milliequivalents of the sulfuric acid solution using $Molarity \times Volume \times n\text{-factor}$, where the n-factor for sulfuric acid is 2. This gives $60 \times \frac{1}{10} \times 2 = 12$ milliequivalents. Then, determine the milliequivalents of sodium hydroxide used to neutralize the excess acid, which is $20 \times \frac{1}{10} \times 1 = 2$ milliequivalents. The difference between these two values represents the milliequivalents of acid actually consumed by the ammonia, which is $12 - 2 = 10$ milliequivalents. Finally, determine the percentage of nitrogen by applying the standard formula where the mass of the organic compound is 1.4 g:

$\% \text{ of } N = \frac{1.4 \times 10}{1.4}$

This calculation leads to 10%.

Carbocation stability is primarily governed by the extent of charge delocalization, where the dispersal of positive charge across the molecular framework significantly lowers its internal energy. Greater delocalization, typically achieved through extensive resonance or conjugation, directly correlates with increased stability of the cation. Consequently, structure III provides the most effective pathway for this charge distribution, followed by structure I, while structure II demonstrates the least, establishing the overall trend as $III > I > II$.

Tautomerism refers to a specific type of structural isomerism where a molecule exists in a dynamic equilibrium between two distinct forms, typically involving the migration of a hydrogen atom alongside the movement of a double bond. This phenomenon is a hallmark of carbonyl compounds, such as ketones, provided they contain at least one $\alpha$-hydrogen attached to a carbon atom directly adjacent to the carbonyl group.

2-Pentanone satisfies this requirement, as its structure features an $\alpha$-hydrogen that can be removed to facilitate the shift into an isomeric enol form. This interaction establishes a reversible equilibrium between the keto and enol structures:

$\text{CH}_3-\text{C}(=\text{O})-\text{CH}_2-\text{CH}_2-\text{CH}_3 \rightleftharpoons \text{CH}_3-\text{C}(\text{OH})=\text{CH}-\text{CH}_2-\text{CH}_3$

The basicity of a conjugate base is inversely related to the acidity of its corresponding conjugate acid, as the most stable conjugate bases originate from the strongest acids. Because the negative charge on the carboxylate ion is delocalized through resonance, $\mathrm{RCOO}^-$ is the weakest base, whereas the carbanion $\mathrm{R}^-$ is derived from an extremely weak alkane acid and thus possesses the highest electron density. This trend results in an increasing basicity order of $\mathrm{RCOO}^- < \mathrm{HC} \equiv \mathrm{C}^- < \mathrm{NH}_2^- < \mathrm{R}^-$.

Optical isomerism arises in molecules containing a chiral center, which is a carbon atom bonded to four completely unique chemical groups. In 3-methyl-1-pentene, represented by the formula $\mathrm{H}_2\mathrm{C}=\mathrm{CH}-\mathrm{CH}(\mathrm{CH}_3)-\mathrm{CH}_2\mathrm{CH}_3$, the carbon atom at the third position satisfies this requirement because it is attached to a hydrogen atom, a methyl group, an ethyl group, and a vinyl group. This asymmetry around the chiral carbon allows the molecule to exist in different spatial arrangements that are non-superimposable mirror images of each other.

Geometrical isomerism in alkenes occurs when both carbon atoms of the double bond ($C=C$) are bonded to two distinct substituent groups. In 2-butene ($CH_3-CH=CH-CH_3$), each carbon of the double bond is attached to a hydrogen atom and a methyl group, which allows for the existence of different spatial arrangements known as cis and trans isomers. Conversely, in propene, 2-methyl propene, and 2-methyl-2-butene, at least one of the double-bonded carbons is attached to two identical groups, preventing the formation of geometrical isomers.

IUPAC nomenclature rules dictate that the longest continuous carbon chain defines the parent alkane name, while substituents are assigned the lowest possible locants. Neopentane, represented by the formula $\mathrm{C(CH_3)_4}$, contains a central carbon atom attached to four methyl groups, which identifies a three-carbon chain as the propane parent. Because this central carbon supports two methyl substituents, the systematic name is 2,2-dimethylpropane.