Haloalkanes And Haloarenes – JEE Main ChemistryPractice Questions & PYQs

When an alkyl halide reacts with sodium cyanide ($\mathrm{NaCN}$), the ionic nature of the reagent leaves the carbon atom as the nucleophilic center, leading to the formation of an alkyl cyanide, represented here as compound $A$. In contrast, silver cyanide ($\mathrm{AgCN}$) possesses a covalent $\mathrm{Ag}-\mathrm{C}$ bond, which prevents the carbon atom from acting as the nucleophile and forces the nucleophilic attack to occur through the nitrogen atom. This fundamental difference in bonding dictates that the nitrogen atom must bond to the alkyl group, resulting in the formation of an alkyl isocyanide, which corresponds to compound $B$.

When a chemical reaction involves a bulky base, steric hindrance becomes the deciding factor that pushes the reaction toward elimination rather than substitution. Because the base is too sterically demanding to effectively approach the electrophilic carbon for a substitution process, it acts by abstracting a proton to initiate an $E2$ elimination pathway. This kinetic preference for eliminating a proton leads to the formation of the double bond, ultimately yielding 2-Methyl prop-1-ene as the major product.

The stability of the reaction product is primarily dictated by the electronic configuration of the starting molecule, where an extensive resonance network involving the oxygen lone pair and most of the carbon skeleton discourages any substitution that would disrupt this conjugation. Because attacking the resonance-participating sites would destabilize the compound, the bromine reagent is restricted to the only two carbons that do not participate in this delocalization: carbon 12 and carbon 13. Since carbon 12 specifically functions as an $\alpha$-carbon, it facilitates an energetically favorable process involving the donation of an $\text{H}^+$ ion and the addition of a $\text{Br}^-$ ion, ultimately limiting the reaction to the incorporation of exactly one bromine atom.

The reaction involves the electrophilic aromatic substitution known as nitrosation, where the mixture of $NaNO_2$ and $HCl$ generates the active electrophile, the nitrosonium ion ($NO^+$), in situ at temperatures between $0^\circ C$ and $5^\circ C$. The dialkylamino group ($-N(CH_3)(C_2H_5)$) acts as a strongly activating group that directs the incoming electrophile to the ortho and para positions; however, the para-substituted product is exclusively formed as the major product because significant steric hindrance at the ortho positions prevents attack at those sites.

The reaction proceeds via the electrophilic addition of the proton to the double bond, adhering to Markovnikov's rule to form the most stable secondary carbocation intermediate. This intermediate subsequently undergoes a 1,2-hydride shift to generate a more stable tertiary carbocation, which is then attacked by the $Br^-$ nucleophile to yield the final product. The thermodynamic stability of the resulting tertiary carbocation intermediate drives this rearrangement, making the tertiary alkyl bromide the major product of the reaction.

The chemical formula $C_8H_{10}$ corresponds to ethylbenzene, which possesses a degree of unsaturation of $4$, indicating one benzene ring and one alkyl side chain. Subjecting ethylbenzene to a mixture of concentrated nitric and sulfuric acids facilitates electrophilic aromatic substitution, where the nitro group is predominantly directed to the para position, yielding $p$-nitroethylbenzene. The subsequent reaction with bromine under heating conditions promotes free-radical substitution specifically at the benzylic position, replacing a hydrogen atom with a bromine atom to form $1$-bromo-1-(4-nitrophenyl)ethane. Finally, treating this intermediate with alcoholic potassium hydroxide triggers a dehydrohalogenation reaction, which removes the bromine and an adjacent hydrogen atom to form a carbon-carbon double bond, resulting in the formation of $p$-nitrostyrene as the final product.

The reaction of benzene with chlorine in the presence of sunlight is a free radical addition process where three molecules of chlorine add across the double bonds of the aromatic ring to produce benzene hexachloride. The balanced chemical equation for this transformation is $C_6H_6 + 3Cl_2 \xrightarrow{h\nu} C_6H_6Cl_6$ As the molecular formula of the product $X$ is $C_6H_6Cl_6$, the total number of hydrogen atoms present in the molecule is 6.

The identification of the intermediate product 'A' relies on the selective reactivity of the functional groups present in the reactant towards the given reagents, specifically following nucleophilic attack or electrophilic substitution mechanisms. In such reactions, the reagent acts as the reactive species, creating a transition state that facilitates the formation of a specific stable intermediate, which corresponds to the structure depicted in option C. Recognizing the electronic and steric factors governing the site of attack allows for the predictable determination of the reaction pathway and the final product structure.

The reaction proceeds via an $S_N2$ mechanism at room temperature, a concerted substitution where the nucleophile attacks the electrophilic center while simultaneously displacing the leaving group. Because this process occurs without affecting the stable $\pi$ bonds present elsewhere in the substrate, the unsaturation characteristics of the original molecule are retained in the structure of the final product. Specifically, the product 'P' contains one double bond which constitutes a single $\pi$ bond, effectively resulting in 2 $\pi$ electrons.

The empirical formula is calculated by dividing the elemental mass percentages by their respective atomic masses, which yields a ratio of approximately $8:9:1$ for Carbon, Hydrogen, and Bromine, corresponding to the molecular formula $\mathrm{C}_8\mathrm{H}_9\mathrm{Br}$.

Isomer A is identified as 1-bromo-1-phenylethane because the benzylic bromine is reactive toward nucleophilic substitution, allowing for the formation of a pale yellow $\mathrm{AgBr}$ precipitate upon treatment with alcoholic $\mathrm{AgNO}_3$, and the molecule contains a chiral center that confers optical activity. Furthermore, its oxidation with $\mathrm{KMnO}_4$ selectively removes the benzylic side chain to produce benzoic acid. Isomer B is identified as 1-ethyl-4-bromobenzene, where the bromine atom is directly bonded to the aromatic ring and thus remains inert to alcoholic $\mathrm{AgNO}_3$, and oxidation of the ethyl group at the para position results in p-bromobenzoic acid.

Carbocation stability is driven by electron-donating effects from adjacent alkyl groups, making secondary carbocations inherently more stable than primary ones through hyperconjugation and inductive stabilization. Although the reaction begins by generating the primary $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CH}_2^+$ species, it quickly undergoes a 1,2-hydride shift to reach a lower energy state. This migration results in the formation of the more stable secondary carbocation, $\mathrm{CH}_3-\overset{\oplus}{\mathrm{C}}\mathrm{H}-\mathrm{CH}_3$.

Electrophilic aromatic substitution is primarily governed by the strongest electron-releasing group attached to the benzene ring, which dictates the site of electrophilic attack. In a compound featuring both a hydroxyl group ($-\mathrm{OH}$) and a methyl group ($-\mathrm{CH}_3$), the hydroxyl group acts as the superior activator due to its strong $+R$ effect, directing the electrophile to the ortho and para positions relative to itself. Consequently, when the methyl group is positioned para to the hydroxyl group, the para position relative to the $-\mathrm{OH}$ group is equivalent to the meta position with respect to the $-\mathrm{CH}_3$ group, thereby producing the desired meta-substituted product.

The ozonolysis of an aromatic ring is determined by the specific localization of double bonds within the two possible Kekule resonance structures, which directly dictates the cleavage products. For molecules such as toluene ($C_7H_8$), m-xylene, and p-xylene, the resonance structures are essentially symmetric or equivalent, ensuring that these compounds consistently yield identical product mixtures. O-xylene differs from these isomers because it possesses two non-equivalent resonance contributors where the double bonds are positioned differently relative to the methyl substituents, leading to the formation of unique ozonolysis products compared to the others.

The $S_{N}1$ reaction proceeds through a unimolecular mechanism where the rate-determining step involves the dissociation of the leaving group to form a planar carbocation intermediate. When a halogen is directly attached to a bridgehead carbon atom in a bicyclic system, the rigid, constrained geometry of the molecule prevents the carbon from adopting the required $sp^2$ planar configuration. This geometric restriction effectively inhibits the formation of the carbocation, rendering such bridgehead halides completely inactive toward this specific reaction pathway.

Elimination reactions, such as dehydrohalogenation or dehydration, proceed according to Saytzeff's rule, which states that the most highly substituted alkene is the major product due to its increased thermodynamic stability. The reaction mechanism involves the removal of a leaving group and a proton from an adjacent carbon to form a $\pi$ bond, favoring the product with the greatest number of alkyl substituents. This regioselectivity confirms that the compound A is the most substituted alkene isomer, which corresponds to the structure provided in the correct option.

The reaction proceeds via a bimolecular nucleophilic substitution mechanism, represented as $S_N2$, which specifically targets the reactive benzylic carbon. Because this mechanism requires a backside attack by the nucleophile, the process results in an inversion of configuration at the stereocenter. Direct substitution on the benzene ring is prevented by the inherent stability and electronic nature of the aromatic system, ensuring the transformation is confined entirely to the benzylic position.

The reaction sequence proceeds via an initial free-radical addition of $HBr$ to 1-butene in the presence of peroxide, which follows anti-Markovnikov's rule to generate 1-bromobutane. This primary alkyl halide subsequently undergoes an $S_N2$ reaction with sodium ethoxide ($CH_3CH_2ONa$), where the ethoxide ion acts as a strong nucleophile to displace the bromide ion. Given the primary nature of the substrate, the substitution pathway is kinetically favored over the elimination pathway, leading to the formation of 1-ethoxybutane as the major product.

Hydroboration-oxidation of an alkene involves the syn-addition of water across the double bond, proceeding with anti-Markovnikov regioselectivity and complete retention of configuration at any existing chiral centers. During the reaction, the borane reagent coordinates to the less hindered carbon of the alkene, and the subsequent oxidation step replaces the boron atom with a hydroxyl group without affecting the stereochemistry of the chiral carbon at the 3-position. Consequently, the product maintains the original $(S)$ configuration at the C3 position, resulting in the formation of $(3S)$-3-methylpentan-1-ol as the final structure.

When a carboxylic acid is treated with thionyl chloride ($SOCl_2$) in the presence of methanol ($CH_3OH$), the carboxylic acid functional group undergoes a transformation into a methyl ester. Thionyl chloride acts as an activating agent to convert the $-\mathrm{COOH}$ group into an acid chloride intermediate, which is highly reactive toward nucleophilic substitution. Subsequent reaction with methanol replaces the chlorine atom with a methoxy group, effectively resulting in the formation of the methyl ester derivative of the original acid.

The ability of a halogenated compound to produce a yellow precipitate upon treatment with silver nitrate depends on the ease of carbon-halogen bond cleavage, which is determined by the stability of the carbocation intermediate formed upon ionization. Compound IV is unique among the choices because it generates a benzyl carbocation, which is highly stabilized by resonance conjugation with the aromatic ring, thereby facilitating the release of the halide ion. In contrast, compounds I, II, and III cannot undergo this reaction because their corresponding carbocations are highly unstable, meaning the halide ions are not liberated to react with $Ag^+$ ions to form the characteristic precipitate.