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

The total number of stereoisomers for a molecule depends on the quantity of stereocenters, which is represented by $n$. You can calculate this using the formula $2^n$, where $n$ is the number of chiral centers. Since the molecule possesses two such centers, substituting this value yields $2^2 = 4$ distinct stereoisomers.

Partition chromatography functions by partitioning solutes between a mobile phase and a stationary liquid film that is anchored to an inert support, which makes the first statement correct. While paper chromatography does involve a sheet of paper, the paper acts only as a scaffold for the true stationary phase, which is the layer of water molecules trapped within the fiber structure of the paper. Therefore, identifying the paper material itself as the stationary phase is incorrect, as it is the trapped water that performs this specific role.

The acidity of an organic compound is primarily determined by the stability of the conjugate base formed after the loss of a proton, a factor heavily influenced by the hybridization of the carbon atom involved. An increase in $s$-character within the hybrid orbital enhances the electronegativity of the carbon, which effectively stabilizes the resulting negative charge on the conjugate base and increases the acidity of the parent molecule. Conversely, when comparing these specific structures, the terminal alkyne exhibits the characteristics that make it the least acidic member of the set.

The hybridization state of a carbon atom is determined by the number of pi bonds it forms: carbons involved in a triple bond are $sp$ hybridized, carbons involved in a double bond are $sp^2$ hybridized, and saturated carbons bonded only to single bonds are $sp^3$ hybridized. Examining the structural formula $CH_2=CH-C(CH_3)_2-C\equiv C-CH_3$ shows that the two carbons at the alkene end form a double bond, classifying them as 2 $sp^2$ hybridized atoms, while the two carbons participating in the alkyne bond at positions four and five are both $sp$ hybridized. The remaining 4 carbons-comprising the quaternary carbon at the third position, the terminal methyl group, and the two attached methyl branches-are all bonded exclusively through single bonds, placing them in the $sp^3$ hybridized category.

According to IUPAC priority rules, the principal functional group, which is the alcohol, must be assigned the lowest possible locant, and when both alkene and alkyne functional groups are present, the double bond is assigned the lower number if the locant sets are otherwise identical. For a seven-carbon chain containing a hydroxyl group at position 4, a double bond at position 1, and a triple bond at position 6, this results in the parent name $Hept-1-en-6-yn-4-ol$.

Determining the total number of stereoisomers involves identifying all chiral centers and double bonds capable of geometrical isomerism within the molecular structure. The compound 5-phenylpent-4-en-2-ol contains one chiral carbon atom at the second position and one carbon-carbon double bond at the fourth position that facilitates geometrical isomerism. Since these two stereogenic elements function independently and there is no molecular symmetry, the total count is determined by the expression $2^1 \times 2^1$, which equals 4.

The stability of cyclic conjugated systems is primarily determined by their electronic configuration and aromatic character. Aromatic species like $q$ exhibit exceptional stability due to the cyclic delocalization of $\pi$ electrons, while antiaromatic systems like $p$ are highly unstable because they force unfavorable electron arrangements. Non-aromatic compounds such as $r$ occupy an intermediate position because they lack the significant stabilization of aromatic rings but do not suffer from the destabilizing effects of antiaromaticity, resulting in the stability sequence $q > r > p$.

Carbocation stability is primarily dictated by the magnitude of charge delocalization, where resonance and aromaticity provide significantly more stabilization than hyperconjugation or the inductive effect. Within this set, structure C exhibits the most extensive delocalization of its positive charge, establishing it as the most stable, followed by A, which benefits from greater electronic support than B. Conversely, structure D represents the least stable configuration due to a deficiency in effective stabilizing factors, resulting in the hierarchical stability order $C > A > B > D$.

The carboxylic acid group is the principal functional group, which dictates that the parent name is benzoic acid and that the carbon attached to this group is assigned position 1. Numbering proceeds around the benzene ring to provide the lowest set of locants, placing the hydroxyl group at position 2, the bromine atom at position 3, and the nitro group at position 5. Following the requirement to arrange all substituents in alphabetical order-bromo, hydroxy, and nitro-we combine these prefixes with the parent name to identify the molecule as 3-bromo-2-hydroxy-5-nitrobenzoic acid.

Determining the structural relationship between organic compounds relies on comparing how their atoms are arranged relative to their functional groups and molecular formulas. Compounds are classified as metamers when they feature the same functional group but differ in the alkyl chains attached to it, while functional isomers are identified by having the same molecular formula but containing distinct functional groups. Since the structures in question exhibit these characteristics, statements A and B correctly describe their relationship, whereas the molecules are actually part of a homologous series instead of being position isomers.

Isomerism exists when compounds share the same molecular formula but differ in their structural arrangement or the functional groups present within the molecule. Since ring chain isomerism is a form of structural isomerism where one molecule is cyclic and the other is open-chained, those specific compounds are correctly classified as isomers. Furthermore, primary and secondary amines are distinguished by the difference in the attachment of the nitrogen atom within their respective functional group arrangements, making them classic examples of functional group isomers.

Hyperconjugation is defined as a permanent electronic effect involving the delocalization of electrons from an adjacent $\sigma$ bond into a vacant p-orbital, meaning the assertion that it is not a permanent effect is incorrect. Since the degree of stabilization depends on the number of available $\alpha\text{-}\mathrm{H}$ atoms, attaching more alkyl groups to a positively charged C atom increases these $\alpha\text{-}\mathrm{H}$ interactions, which consequently leads to greater hyperconjugation and higher stability for the cation.

The basicity of amines is primarily determined by the electron density available on the nitrogen lone pair, which is enhanced by the inductive ($+\mathrm{I}$) effect of alkyl groups but modulated by solvation and steric factors in aqueous solutions. Dimethylamine (E) demonstrates the highest basicity by achieving an optimal balance between inductive stabilization and manageable steric bulk, followed by methylamine (D) which possesses only a single electron-donating methyl group. Ammonia (B), aniline (A), and other less basic variants (C) occupy the remainder of the sequence, as they provide significantly less inductive support or suffer from electron delocalization into an aromatic ring, resulting in the overall order $E > D > B > A > C$.

Hybridization of carbon atoms is determined by the number of sigma bonds and pi bonds surrounding each atom. Carbon atoms involved in a triple bond or two adjacent double bonds utilize $sp$ hybrid orbitals to achieve a linear geometry, and the structure contains 3 such $sp$ hybridized carbon atoms.

Alternatively, a carbon atom participating in a single double bond adopts $sp^2$ hybridization to accommodate a trigonal planar arrangement. Identifying these specific bonding sites within the molecule reveals 5 carbon atoms characterized by $sp^2$ hybridization. The structure thus features 3 $sp$ and 5 $sp^2$ hybridized carbon atoms.

Nucleophiles are chemical species characterized by their capacity to donate electron pairs, often identifiable by the presence of lone pairs or negative charges on their constituent atoms. When evaluating the provided list, we can identify these reactive species by searching for those that have available electron density ready for donation. Counting these specific instances reveals that exactly five of the listed species satisfy the chemical requirements to be classified as nucleophiles.

The degree of unsaturation, or index of hydrogen deficiency, identifies the total number of rings and pi bonds in a hydrocarbon by comparing its structure to that of a saturated alkane. Because a saturated alkane follows the general formula $C_n H_{2n+2}$, any reduction in hydrogen atoms compared to this maximum value indicates the presence of rings or multiple bonds.

The molecular formula of this hydrocarbon is derived from the elemental mass composition. With a total molar mass of 80 g mol⁻¹, the carbon component is 90%, which equals 72 g mol⁻¹ of carbon. Given the atomic mass of carbon is 12, this represents 6 atoms per molecule. The remaining 10% accounts for 8 g mol⁻¹ of hydrogen, equivalent to 8 hydrogen atoms. This results in the molecular formula $C_6H_8$. Applying the standard expression for the degree of unsaturation with $c$ carbon atoms and $h$ hydrogen atoms yields the following calculation:
$DoU = \frac{2c + 2 - h}{2} = \frac{2(6) + 2 - 8}{2} = 3$
This result indicates that the molecule possesses 3 degrees of unsaturation, confirming the presence of a total of three rings and pi bonds.

Ion stability is fundamentally governed by the ability of the molecular structure to disperse charge through resonance, hyperconjugation, and the inductive effect, where the delocalization of charge lowers the potential energy ($E_p$) of the species. In the pairs categorized as (A) and (B), the first ion is more stable because it possesses superior structural features, such as extended conjugation or strong electron-donating substituents, that effectively distribute the charge. In contrast, the second ions in these specific pairs lack these electronic advantages or experience destabilizing effects that render them higher in energy and less stable than their counterparts.

IUPAC nomenclature for branched alkanes requires identifying the longest continuous carbon chain to define the parent alkane and numbering the chain from the end that assigns the lowest possible locants to the substituents. The given hydrocarbon structure, $\mathrm{CH}_3-\mathrm{CH}(\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{CH}(\mathrm{CH}_3)-\mathrm{CH}(\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{CH}_3$, contains a principal chain of eight carbon atoms, establishing it as an octane derivative. Numbering this chain from left to right places the methyl substituents at positions 2, 5, and 6, whereas numbering from right to left assigns the locants 3, 4, and 7; since the set of locants 2, 5, 6 is lower than 3, 4, 7, the correct systematic name is 2,5,6-trimethyloctane.

The acidity of a molecule is fundamentally determined by the stability of its conjugate base, as a more stable species is formed when the parent molecule releases a proton ($H^+$). In the structure corresponding to option D, the conjugate base achieves exceptional stability because the resulting negative charge is spread extensively across the system through resonance. This superior delocalization of charge lowers the potential energy of the conjugate base, effectively facilitating the release of the hydrogen ion.

The dipole moment of an alkene, such as $CH_3CH=CHCH_3$, is determined by the vector sum of individual bond dipoles acting across the double bond. In the cis-isomer, the substituents are positioned on the same side, causing the bond dipoles to reinforce each other and produce a net dipole moment; conversely, in the trans-isomer, the substituents are on opposite sides, leading to the cancellation of these vectors. Therefore, cis-2-butene has a higher dipole moment than trans-2-butene, making the statement that it has a lower dipole moment false.