Hydrocarbons – NEET UG ChemistryPractice Questions & PYQs

Friedel-Crafts reactions proceed through electrophilic aromatic substitution, where a Lewis acid catalyst such as $AlCl_3$ promotes the formation of a potent electrophile from the organic halide. This electrophile attacks the $\pi$ electron cloud of the aromatic system to form a carbocation intermediate, which then loses a proton to regenerate the aromatic ring and produce the substituted structure seen in option D. The specific regioselectivity of this process is defined by the electronic environment of the ring and the nature of the attacking electrophile.

The planarity of a molecule is primarily dictated by the hybridization of its constituent atoms. When carbon atoms undergo $sp^2$ hybridization, they adopt a trigonal planar geometry that forces all connected atoms to reside within the same flat surface. Because every carbon atom in this molecule is $sp^2$ hybridized, the entire structure remains restricted to a single plane without any three-dimensional distortion.

Ethane molecules undergo constant rotation around the carbon-carbon single bond, shifting between different spatial arrangements known as conformations. In the eclipsed state, the hydrogen atoms on adjacent carbons align directly behind each other, forcing their electron clouds into close proximity and generating significant bond pair-bond pair repulsion, commonly referred to as torsional strain. This repulsive energy destabilizes the eclipsed form, making the staggered conformation-where the hydrogen atoms are positioned to maximize the distance between them-the inherently more stable arrangement.

Terminal alkynes possess acidic protons that can be abstracted by strong bases such as $\mathrm{NaNH}_2$ to generate nucleophilic acetylide anions, which subsequently undergo $\mathrm{S}_N2$ substitution reactions with alkyl halides to extend the carbon chain. In the first step, acetylene reacts with $\mathrm{NaNH}_2$ to form an acetylide ion, $HC\equiv C^-$, which then displaces the bromide from $\mathrm{CH}_3CH_2Br$ to produce 1-butyne ($CH_3CH_2C\equiv CH$) as intermediate X. Since 1-butyne retains a terminal acidic hydrogen, it reacts with the second equivalent of $\mathrm{NaNH}_2$ to form the conjugate base $CH_3CH_2C\equiv C^-$, which undergoes a second $\mathrm{S}_N2$ alkylation upon reaction with $\mathrm{CH}_3CH_2Br$ to yield 3-hexyne ($CH_3CH_2C\equiv CCH_2CH_3$) as the final product Y.

The hybridization of the negatively charged carbon in $CH_3C\equiv C^\Theta$ depends on the sum of its sigma bonds and lone pairs. This carbon atom participates in one sigma bond with the adjacent carbon and holds one lone pair of electrons, which sums to a steric number of 2. Consequently, the carbanion adopts an $sp$ hybridization state to accommodate these two electron domains.

In the nitration of benzene, the active electrophile responsible for the reaction is the nitronium ion ($\mathrm{NO}_2^+$), which is generated through the equilibrium reaction $\mathrm{HONO}_2 + 2\mathrm{H}_2\mathrm{SO}_4 \rightleftharpoons 2\mathrm{HSO}_4^- + \mathrm{H}_3\mathrm{O}^+ + \mathrm{NO}_2^+$. When $\mathrm{KHSO}_4$ is added to the mixture, it dissociates to release an excess of $\mathrm{HSO}_4^-$ ions, creating a common ion effect. According to Le Chatelier's principle, increasing the concentration of these product ions shifts the chemical equilibrium to the left, which reduces the steady-state concentration of the electrophilic $\mathrm{NO}_2^+$ species and ultimately results in a slower rate of nitration.

The synthesis of propene from functionalized three-carbon precursors relies on elimination reactions, where a leaving group is removed to establish a carbon-carbon double bond. Primary alcohols such as $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{OH}$ react with HBr to generate propyl bromide, which subsequently undergoes dehydrohalogenation to yield propene, $\mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}_2$. Similarly, propyl bromide, $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_2-\mathrm{Br}$, is a direct substrate for this elimination process to form the same alkene. In contrast, ketene, $\mathrm{H}_2\mathrm{C}=\mathrm{C}=\mathrm{O}$, is a highly electrophilic species that reacts with HBr to form acetyl bromide rather than undergoing the chemical transformations required to generate a three-carbon alkene.

The acid-catalyzed isomerization of 3,3-dimethyl-1-butene proceeds through a carbocation rearrangement where the alkene is initially protonated to form a secondary carbocation, $\left( \mathrm{CH}_3 \right)_3 \mathrm{C} - \mathrm{CH}^+ - \mathrm{CH}_3$. This intermediate undergoes a 1,2-methyl shift to form a more stable tertiary carbocation, $\left( \mathrm{CH}_3 \right)_2 \mathrm{C}^+ - \mathrm{CH} \left( \mathrm{CH}_3 \right)_2$, which subsequently loses a proton to yield the thermodynamically favored product 2,3-dimethyl-2-butene.

The enthalpy of hydrogenation is inversely proportional to the stability of the alkene, a relationship defined by $\text{Enthalpy of hydrogenation} \propto \frac{1}{\text{stability of alkene}}$. Since less stable alkenes exist at higher energy levels, they liberate a greater amount of heat during the hydrogenation process compared to their more stable, lower-energy counterparts. By determining the relative stability of the provided molecular structures, the hydrogenation enthalpies can be ranked by simply reversing the stability order of the compounds.

Markovnikov's rule dictates that the proton from $HCl$ adds to the carbon atom of the double bond that already possesses more hydrogen atoms, resulting in the most stable carbocation intermediate. 1-methylcyclohexene undergoes protonation at the C2 position to form a tertiary carbocation at C1, while methylenecyclohexane undergoes protonation at its exocyclic methylene group to generate an identical tertiary carbocation at the ring C1. Since both alkenes produce the same tertiary carbocation upon reaction with $HCl$, they both successfully result in the formation of 1-chloro-1-methylcyclohexane.

Ozonolysis of a cyclic alkene involves the oxidative cleavage of the carbon-carbon double bond, resulting in an acyclic dicarbonyl compound with the same number of carbon atoms as the original ring. The provided structure corresponds to pentanedial ($OHC-(CH_2)_3-CHO$), which is a five-carbon chain containing two aldehyde groups. Because the resulting dicarbonyl compound retains the five carbons of the cyclic precursor, it is formed by the ozonolysis of cyclopentene.

When an aromatic ring substituted with an alkyl group like the $-\mathrm{CH}_3$ group undergoes electrophilic aromatic substitution, that substituent acts as an ortho-para director because it increases the electron density at these specific positions. Treating this compound with $\mathrm{Br}_2$ in the presence of $\mathrm{FeBr}_3$ generates the electrophilic bromine species, which is then directed to attack the ring at the positions activated by the methyl group. This reaction mechanism results in the formation of both ortho-bromotoluene and para-bromotoluene as the primary products.

The combustion product carbon dioxide, represented as $CO_2$, features a central carbon atom involved in two double bonds, resulting in linear geometry and sp hybridization. Among the organic compounds provided, ethane ($C_2H_6$) and ethanol ($C_2H_5OH$) consist of carbon atoms linked only by single bonds, which characterizes sp3 hybridization, while ethene ($C_2H_4$) contains a double bond corresponding to sp2 hybridization. Ethyne, denoted as $C_2H_2$, contains a carbon-carbon triple bond, forcing its carbon atoms to adopt the sp hybridization required for linear geometry, which perfectly matches the state found in $CO_2$.

This transformation proceeds through an anti-Markovnikov addition followed by a Williamson ether synthesis. Treating 1-butene with hydrogen bromide and peroxide induces a radical mechanism that positions the bromine at the less substituted carbon, forming 1-bromobutane. In the subsequent stage, the ethoxide nucleophile from sodium ethoxide displaces the bromide, resulting in the ether $\mathrm{CH}_3-(\mathrm{CH}_2)_3-\mathrm{O}-\mathrm{CH}_2\mathrm{CH}_3$.

The reaction of $\mathrm{HC} \equiv \mathrm{CH}$ with $\mathrm{H}_2\mathrm{SO}_4$ and $\mathrm{Hg}^{2+}$ proceeds through a vinyl alcohol intermediate to form acetaldehyde, $\mathrm{CH}_3\mathrm{CHO}$. The Victor Meyer test is a chemical procedure specifically designed to distinguish between primary, secondary, and tertiary alcohols by observing characteristic color changes. Because acetaldehyde contains a carbonyl group rather than the hydroxyl group required for this alcohol classification assay, it does not give the Victor Meyer test.

A compound exhibits aromaticity only if it is a planar, cyclic, fully conjugated system possessing $(4n+2)$ $\pi$ electrons, where $n$ is an integer. In contrast to benzene, the cyclopentadienyl anion, and the cycloheptatrienyl cation, all of which satisfy this condition with 6 $\pi$ electrons, the cyclopentadienyl cation contains only 4 $\pi$ electrons. This electron count follows the $4n$ rule rather than the $(4n+2)$ rule, rendering the cyclopentadienyl cation anti-aromatic and therefore non-aromatic.

Friedel-Crafts alkylation and acylation reactions are electrophilic substitutions that require an electron-rich aromatic ring to interact effectively with the Lewis acid-coordinated electrophile. Nitrobenzene ($C_6H_5NO_2$) contains a strong electron-withdrawing nitro group that significantly reduces the electron density of the benzene ring, rendering it highly deactivated and inert toward these reaction conditions. Conversely, the alkyl groups present in toluene, cumene, and xylene act as electron-donating groups, which increase the ring's nucleophilicity and facilitate the electrophilic attack.

The ability to distinguish between these isomers relies on the acidity of the terminal hydrogen atom present in 1-alkynes, which is entirely absent in internal alkynes like 2-butyne. Because the hydrogen attached to the $sp$ hybridized carbon in 1-butyne is sufficiently acidic, it readily reacts with the strong base $NaNH_2$ to form a sodium acetylide salt while releasing hydrogen gas ($H_2$). In contrast, 2-butyne lacks such an acidic proton because its triple bond is positioned between two carbons, meaning it does not react with $NaNH_2$ and allows for a clear chemical differentiation.

The stability of conformational isomers is governed by the minimization of steric hindrance, where bulky substituents seek to maximize their separation to avoid repulsive interactions. Within the various rotations of $n$-butane, the anti-conformation positions the two large methyl groups at a dihedral angle of $180^\circ$, keeping them as far apart as possible. This arrangement significantly reduces steric repulsion, establishing it as the most energetically favorable and stable conformer.

Strong oxidizing agents like $KMnO_4$ in an alkaline medium oxidize the alkyl side chain of ethylbenzene exclusively at the benzylic carbon atom, converting the entire side chain into a carboxylic acid group. This reaction proceeds because the presence of at least one benzylic hydrogen allows for the formation of intermediate species that facilitate the cleavage of the C-C bond in the alkyl chain, resulting in the formation of benzoic acid.