Hydrocarbons – NEET UG ChemistryPractice Questions & PYQs

Conjugation in organic molecules occurs strictly when two double bonds are separated by exactly one single bond, enabling continuous orbital overlap. The structures of the isomers reveal that hepta-1,6-diene, hepta-1,4-diene, and hepta-1,5-diene have double bonds separated by four, two, and three single bonds respectively, which fails the requirement for conjugation. Only in hepta-1,3-diene do the double bonds occupy the first and third positions with exactly one single bond between them, identifying it as the only conjugated compound in the set.

The stability of different conformations of $n$-butane, represented by ${CH}_{3} {CH}_{2} {CH}_{2} {CH}_{3}$, is determined by the degree of steric repulsion between substituents as the molecule rotates around its central carbon-carbon bond. Among the possible orientations, the staggered conformation positions the hydrogen atoms attached to the two central carbon atoms at the greatest possible distance from each other. Because this specific geometry minimizes the repulsive forces between these tetrahedrally arranged atoms, it results in the lowest potential energy, making it the most stable conformation.

The hydration of acetylene catalyzed by $HgSO_4$ and dilute $H_2SO_4$ proceeds via the electrophilic addition of water across the triple bond to form an unstable enol intermediate known as vinyl alcohol ($CH_2=CHOH$). This species rapidly undergoes keto-enol tautomerization to rearrange into the more stable carbonyl isomer, yielding acetaldehyde ($CH_3CHO$) as the final product.

Chlorination under sunlight follows specific pathways where methane undergoes free radical substitution to form chloroform as shown by the reaction $CH_4 + 3Cl_2 \xrightarrow{\text{sunlight}} CHCl_3 + 3HCl$, resulting \in product X ($CHCl_3$) with 3 chlorine atoms. Similarly, benzene undergoes a photochemical addition reaction, represented by the equation $C_6H_6 + 3Cl_2 \xrightarrow{\text{sunlight}} C_6H_6Cl_6$, which produces the organic product Y ($C_6H_6Cl_6$) containing 6 chlorine atoms.

The rate of electrophilic aromatic substitution is governed by the electron density of the ring, where electron-donating substituents accelerate the reaction and electron-withdrawing groups retard it. Compound I reacts fastest as the $- {OCH}_{3}$ group increases electron density through its releasing effect, whereas compound III is the slowest because the $- {NO}_{2}$ group withdraws electrons and reduces reactivity. This places the unsubstituted compound II in the middle, resulting in the sequence I > II > III.

The inherent stability of arenes stems from the resonance provided by the continuous cyclic delocalization of $\pi$-electrons throughout the ring. This special electronic configuration makes arenes remarkably resistant to addition reactions, as such processes would disrupt the aromatic system, favoring alternative reaction pathways to maintain their stability. Consequently, electrophilic addition reactions are not compatible with arenes, distinguishing them from simple alkenes where such additions are commonplace.

Chlorination of benzene is a classic example of electrophilic aromatic substitution, which requires the generation of a powerful electrophile to overcome the stability of the aromatic ring. This activation occurs when molecular chlorine interacts with the Lewis acid catalyst $FeCl_3$, leading to the heterolytic cleavage of the chlorine bond. The process is defined by the following reaction:

$Cl_2 + FeCl_3 \rightarrow FeCl_4^- + Cl^+$

The resulting $Cl^+$ species acts as the reactive electrophile that attacks the electron-rich benzene ring to initiate the substitution.

The quality of commercial gasoline is fundamentally determined by its octane number, where a higher value indicates superior performance. Because introducing branching into a hydrocarbon chain effectively increases this octane number, branched structures are specifically favored to create higher-quality fuel. These branched configurations allow for a smoother combustion process in the engine compared to their straight-chain counterparts.

The Friedel-Crafts alkylation reaction serves as a standard method for introducing an alkyl substituent onto a benzene ring via electrophilic aromatic substitution. In this process, benzene ({C}{6}{H}{6}$) reacts with methyl chloride ({CH}_{3}{Cl}$) in the presence of an anhydrous ${AlCl}_{3}$ catalyst, which facilitates the attachment of a methyl group to the benzene ring. This reaction successfully yields methylbenzene, which is the chemical structure of toluene.

The reaction between 2-bromopentane and potassium ethoxide proceeds via an E2 elimination mechanism, where the strong base extracts a hydrogen atom from the carbon chain, resulting in the loss of a bromide ion and the formation of a double bond. Following Zaitsev's rule, the pathway that yields the most highly substituted alkene is thermodynamically favored because these products possess greater stability. Consequently, trans-2-pentene ($CH_{3}-CH=CH-CH_{2}-CH_{3}$) forms as the major product, as it is more substituted than 1-pentene and avoids the steric strain present in the cis-2-pentene isomer.

Electrophilic aromatic substitution proceeds most rapidly when a substituent increases the electron density within the benzene ring. While both $- {CH}_{3}$ and $- {OH}$ groups serve as activators, the $- {OH}$ group exerts a strong activating influence, significantly surpassing the weak activation provided by the $- {CH}_{3}$ group. Since the $- {Cl}$ group acts as a deactivator, phenol remains the most electron-rich and therefore the most easily attacked species among these compounds.

The stereochemical outcome of addition reactions is governed by the specific mechanism-whether the addition is syn or anti-and the geometry of the alkene. When bromine adds to trans-but-2-ene in the presence of $\mathrm{CCl}_4$, the process proceeds via an anti-addition pathway, which requires the bromine atoms to attack from opposite faces of the planar alkene. This specific stereochemistry results in the formation of meso-2,3-dibromobutane; therefore, any representation of the product that deviates from this meso structure is chemically inaccurate, rendering Statement I incorrect.

Conversely, the hydroxylation of alkenes using alkaline $\mathrm{KMnO}_4$ occurs via a syn-addition mechanism, where both hydroxyl groups are delivered simultaneously to the same face of the carbon-carbon double bond. When cis-but-2-ene is subjected to this reaction, the syn-delivery of hydroxyl groups creates the symmetric meso-2,3-butanediol. Since this matches the standard stereochemical result for this reaction, Statement II is factually correct.

The Diels-Alder reaction involves a $[4+2]$ cycloaddition between a conjugated diene and a dienophile to produce a cyclic adduct, where endo and exo isomers are typically formed as stereoisomers. When analyzing the relationship between compounds $K$ and $L$ in these sequences, the presence of restricted rotation around a carbon-carbon double bond or within a rigid framework prevents free rotation of the attached substituents. Because these configurations differ in their relative spatial positioning while maintaining identical connectivity, they are classified as geometrical isomers, which is a specific type of isomerism distinct from the endo-exo stereochemical outcome of the cycloaddition.

Friedel-Crafts alkylation is an electrophilic aromatic substitution process characterized by the attachment of an alkyl group to an aromatic ring through the use of an alkyl halide and a Lewis acid catalyst such as $AlCl_3$. To successfully perform this specific reaction, benzene $C_6H_6$ must be paired with an alkylating agent such as methyl chloride $CH_3Cl$, which allows the catalyst to facilitate the formation of the required carbocation electrophile. Other reagents like acetyl chloride $CH_3COCl$ would lead to acylation instead, making the combination of benzene and methyl chloride the necessary reactant pair for achieving alkylation.

The cylindrical geometry of an alkyne molecule arises because the two bonding carbon atoms are $sp$-hybridized, forcing the triple bond to consist of one $\sigma$ bond and two $\pi$ bonds. This structure forms as the two carbon atoms establish a single $\sigma$ bond through the head-on overlap of their hybrid orbitals, while their two unhybridized p-orbitals overlap laterally to produce two $\pi$ bonds. Because these two $\pi$ bonds are arranged perpendicularly to each other along the bond axis, they create a symmetrical, cylindrical cloud of electron density around the center of the molecule.

The production of benzene is achieved through established pathways such as the soda-lime decarboxylation of sodium benzoate ($C_6H_5COONa + NaOH \xrightarrow{\Delta} C_6H_6 + Na_2CO_3$), the reduction of phenol using zinc dust ($C_6H_5OH + Zn \rightarrow C_6H_6 + ZnO$), or the catalytic trimerization of ethyne ($3C_2H_2 \xrightarrow{\text{Red hot Fe tube}} C_6H_6$). Option B describes a process, typically the Friedel-Crafts alkylation, where the reaction mechanism facilitates the substitution of a hydrogen atom on the benzene ring with an alkyl group. Because this electrophilic substitution alters the molecular structure by adding a side chain, the resulting product is an alkylbenzene rather than the unsubstituted benzene hydrocarbon.

The bromine water test serves as a reliable method to identify unsaturation in organic molecules, utilizing the characteristic reddish-orange color of bromine dissolved in $\mathrm{CCl}_4$. In the presence of carbon-carbon double or triple bonds, the bromine participates in an addition reaction, effectively stripping the color from the solution. Since compound C fails to trigger this reaction, it must be a saturated hydrocarbon lacking the reactive unsaturation sites required to discharge the bromine solution.

The major product in a reaction sequence is primarily determined by the principle of regioselectivity, where the electronic and steric environment of the reaction center dictates the kinetically and thermodynamically favored pathway. In this specific transformation, the reagents provided selectively favor the transition state that minimizes steric hindrance and maximizes the stability of the intermediate, ensuring that the reactive species is directed toward the most electron-rich or accessible site. Consequently, the reaction mechanism proceeds through this lower-energy pathway to yield the structural configuration represented in option C, which is inherently more stable than the alternatives presented in the other options.

Alkanes generally resist oxidation, yet those possessing a tertiary hydrogen atom-where the hydrogen is attached to a carbon bonded to three other carbon groups-can be oxidized into corresponding alcohols using $\mathrm{KMnO}_4$. The reaction proceeds by replacing this highly reactive tertiary hydrogen with a hydroxyl group to form the alcohol. To satisfy this structural requirement in the expression $R_2-\overset{\vert}{\mathrm{C}}\underset{R_3}{\overset{R_1}{\vert}}-H$, all three substituents attached to the central carbon must be alkyl groups, meaning $R_1=\mathrm{CH}_3$, $R_2=\mathrm{CH}_3$, and $R_3=\mathrm{CH}_3$.

Hydroboration-oxidation is a quintessential reaction in organic chemistry that follows anti-Markovnikov regioselectivity, meaning the boron atom preferentially bonds to the less substituted carbon of the alkene. When propene undergoes this reaction with diborane, it forms tri-n-propylborane, formulated as $(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2)_3\mathrm{B}$, not the branched tri-isopropylborane described in the first statement. Therefore, the first statement incorrectly predicts the regiochemistry of the hydroboration product.

Conversely, the second statement correctly describes the subsequent oxidation step. In the presence of hydrogen peroxide and a base, organoboranes are oxidized to alcohols with retention of configuration at the carbon center. Treating the branched intermediate $((\mathrm{CH}_3)_2\mathrm{CH})_3\mathrm{B}$ under these conditions replaces the boron attachment points with hydroxyl groups, effectively yielding propan-2-ol, which confirms that the second statement is chemically accurate.