Haloalkanes And Haloarenes – JEE Main ChemistryPractice Questions & PYQs

A vinylic halide is characterized by a halogen atom attached directly to a carbon atom that forms part of a carbon-carbon double bond. The carbon carrying the halogen in this system is an aliphatic carbon with $\text{sp}^2$ hybridization. By observing the structures, one can identify the correct choice by looking for a halogen positioned on such an unsaturated, non-aromatic carbon.

The Finkelstein reaction proceeds through an $\mathrm{S}_{\mathrm{N}}2$ pathway where the nucleophilic $\mathrm{I}^-$ attacks the alkyl halide. During the formation of the transition state, the negative charge is delocalized across both the incoming iodide and the leaving bromide, which significantly lowers the charge density relative to the concentrated, localized charge on the free $\mathrm{I}^-$ anion. This delocalization renders the transition state less polar than the reactant anion, while acetic acid remains unsuitable because its protic nature solvates the nucleophile and disrupts the reaction mechanism, and the choice of acetone facilitates the precipitation of $\mathrm{Br}^-$ to prevent it from acting as a competing nucleophile.

The rate of an $S_N 1$ reaction is primarily governed by the stability of the carbocation intermediate generated after the leaving group departs. Because a more stable intermediate lowers the energy barrier for the rate-determining step, compounds that effectively stabilize the positive charge through resonance, hyperconjugation, or inductive effects exhibit faster reaction kinetics. Evaluating the stability of the carbocations produced by these specific compounds reveals that the reactivity order is $b > d > c > a$.

Dimensional analysis involves deriving the fundamental units for physical quantities by applying their defining physical laws, where mass ($M$), length ($L$), and time ($T$) serve as the base dimensions. Surface tension corresponds to force per unit length $[M L^0 T^{-2}]$, Young's modulus represents stress $[M L^{-1} T^{-2}]$, the coefficient of viscosity involves the ratio of shear stress to velocity gradient $[M L^{-1} T^{-1}]$, and surface energy per unit area corresponds to $[M T^{-2}]$. Matching these derived dimensions to the respective entries in the lists confirms the correlation A-II, B-I, C-IV, and D-III.

The formation of $\mathrm{AgCl}$ in this reaction is driven by the ionization of chloride ions from the organic substrate when treated with $\mathrm{AgNO}_3$. Because benzylic and tertiary carbocations are stable, chlorine atoms bonded to carbons that generate such intermediates will readily dissociate to release chloride ions. By identifying the number of such susceptible chlorine sites in the structure, the stoichiometry of the precipitation is determined. Since there are two chlorine atoms attached to positions that form these stable carbocations, the reaction proceeds to form 2 moles of $\mathrm{AgCl}$.

The melting point of isomeric dichlorobenzenes is primarily determined by molecular symmetry and the efficiency of packing within the crystal lattice. Para-dichlorobenzene exhibits a highly symmetrical structure, which allows its molecules to pack into a compact and stable crystal lattice that requires significantly more energy to disrupt than the less symmetrical ortho and meta isomers. Consequently, this efficient packing results in stronger intermolecular forces in the solid state, establishing the melting point order as $p\text{-dichlorobenzene} > o\text{-dichlorobenzene} > m\text{-dichlorobenzene}$.

The hydrolysis of alkyl chlorides is characteristically slow because the chloride ion is a poor leaving group, which creates a significant kinetic barrier to substitution. Introducing sodium iodide accelerates this process because the iodide ion acts as a strong nucleophile that rapidly displaces the chloride to form a more reactive alkyl iodide intermediate. This intermediate undergoes hydrolysis much more efficiently because the iodide ion is an excellent leaving group, significantly outperforming the original chloride in this role. Since $I^-$ effectively functions as both a superior nucleophile to initiate the exchange and a superior leaving group to facilitate the final step, its presence catalyzes the overall reaction.

The $S_N2$ mechanism proceeds most efficiently with primary substrates due to minimal steric hindrance, following the reactivity order $1^{\circ} > 2^{\circ} > 3^{\circ}$, while $S_N1$ reactivity is governed directly by the stability of the carbocation intermediate formed in the rate-determining step. When considering aromatic systems, electrophilic substitution rates are inversely related to the presence of electron-withdrawing groups, such that $\text{rate} \propto \frac{1}{\text{EWG}}$, because these substituents decrease the electron density available for the electrophile. Conversely, the rate of nucleophilic aromatic substitution is enhanced by the presence of electron-withdrawing groups, where the $\text{rate} \propto \text{number of EWG}$, as these groups effectively stabilize the anionic intermediate formed during the reaction.

Chlorobenzene undergoes electrophilic aromatic substitution where the chlorine atom directs the incoming acyl group to the ortho and para positions due to its lone pair resonance contribution. Although both positions are activated, the para-substituted product is formed as the major isomer to minimize the steric repulsion between the bulky $-COR$ group and the chlorine atom. This steric preference ensures that the reaction proceeds to produce the para isomer over the more hindered ortho product.

Nucleophilic substitution on haloarenes is facilitated by the presence of electron-withdrawing groups (EWGs) on the benzene ring, which stabilize the negatively charged intermediate formed during the reaction via their strong negative mesomeric ($-M$) effect. Conversely, electron-donating groups (EDGs) like methoxy groups destabilize this transition state through their positive mesomeric ($+M$) effect, significantly retarding the reaction rate. Consequently, the reactivity increases proportionally with the number and strength of electron-withdrawing nitro groups, creating an order where compounds with the highest density of $-M$ substituents react most rapidly compared to those possessing electron-donating substituents.

The reactivity of alkyl halides in an $S_{\mathrm{N}}1$ mechanism is governed by the relative stability of the carbocation intermediate formed after the leaving group departs. In structure A, the loss of $\mathrm{Br}_{(a)}$ produces a carbocation stabilized by resonance with the adjacent phenyl ring, whereas in structure B, the departure of $\mathrm{I}_{(a)}$ results in a cation stabilized through conjugation. Structure C restricts the reaction to the $\mathrm{Br}_{(b)}$ position because Bredt's rule prohibits the formation of a carbocation at the strained bridgehead $\mathrm{Br}_{(a)}$ site, and in structure D, the removal of $\mathrm{Br}_{(a)}$ generates a highly stable $3^\circ$ carbocation.

A molecule exhibits chirality when a carbon atom is bonded to four distinct groups, and the presence of two such centers in $\mathrm{C}_4\mathrm{H}_8\mathrm{DBr}$ is satisfied by 2-bromo-3-deuterobutane ($\mathrm{CH}_3-\mathrm{CHBr}-\mathrm{CHD}-\mathrm{CH}_3$). In this structure, the carbon at position 2 is bonded to $-\mathrm{CH}_3$, $-\mathrm{H}$, $-\mathrm{Br}$, and the $-\mathrm{CHDCH}_3$ group, while the carbon at position 3 is bonded to $-\mathrm{CH}_3$, $-\mathrm{H}$, $-\mathrm{D}$, and the $-\mathrm{CHBrCH}_3$ group, making both centers chiral. Other isomeric structures listed contain only one chiral center or no chiral centers at all because they lack four distinct substituents on the respective carbon atoms.

The precipitation reaction with $\mathrm{AgNO}_3$ depends on the generation of free halide ions through the heterolytic cleavage of the carbon-halogen bond to form a carbocation intermediate. This process is hindered in compounds such as vinyl halides, aryl halides, or specific bridgehead halides because the C-X bond possesses significant partial double-bond character due to resonance, or because geometric constraints prevent the formation of a stable, planar carbocation. Consequently, the bond remains intact, preventing the release of halide ions and the subsequent formation of an insoluble silver halide precipitate.

Chemical substitution pathways are governed by the electronic effects of substituents, which dictate whether a reaction proceeds via a unimolecular or bimolecular mechanism. Electron-donating groups stabilize the carbocation intermediate formed in the rate-determining step, characterizing an $S_N1$ reaction that follows first-order kinetics. Conversely, electron-withdrawing groups increase the electrophilic nature of the target site, facilitating an $S_N2$ mechanism where the rate depends on both the substrate and the nucleophile, leading to second-order kinetics.

The boiling point of alkyl halides, which generally correlates with molecular size and mass ($BP \propto \text{Size/Mass}$), rises as the halogen or alkyl group grows, while branching in isomeric halides decreases the boiling point by reducing intermolecular surface area. Simultaneously, the density of these compounds exhibits a positive correlation with the atomic mass of the halogen, causing density to increase as heavier halogens replace lighter ones in the structure.

When straight-chain alkanes, such as $n$-hexane, are heated in the presence of anhydrous $\mathrm{AlCl}_3$ and hydrogen chloride gas, they undergo structural isomerization to form branched-chain alkanes. This catalytic process involves the rearrangement of the carbon skeleton, which occurs because the resulting branched-chain structures are generally more thermodynamically stable than their linear counterparts.

In the case of $\mathrm{CH}_3(\mathrm{CH}_2)_4\mathrm{CH}_3$, the molecule undergoes this rearrangement to yield 2-methylpentane, which is the major product of the isomerization reaction. This branched isomer features a methyl group attached to the second carbon atom of the pentane chain, specifically represented as $\mathrm{CH}_3\underset{\mathrm{CH}_3}{\underset{\vert}{\mathrm{CH}}}-(\mathrm{CH}_2)_2\mathrm{CH}_3$. By converting the linear alkane into this branched configuration, the system achieves a more stable molecular arrangement under the influence of the acidic catalyst and heat.

The rate of $S_NAr$ reactions is primarily determined by the stability of the negatively charged Meisenheimer intermediate, which is enhanced when electron-withdrawing groups occupy the ortho or para positions relative to the leaving group. These positions allow for the resonance delocalization of the negative charge onto the substituent, significantly lowering the activation energy. Conversely, an electron-withdrawing group located at the meta position cannot stabilize this intermediate through resonance, rendering it highly ineffective at promoting the reaction. Consequently, the compound featuring the meta-substituted group will display the lowest rate towards substitution.

The reaction follows Markovnikov's rule, where the electrophilic proton ($H^+$) adds to the alkene's double bond to generate the most stable carbocation intermediate. In the case of 3-methyl-1-butene, the initial protonation creates a secondary carbocation, which immediately undergoes a 1,2-hydride shift to form a more stable tertiary carbocation. Subsequently, the nucleophilic bromide ion ($Br^-$) attacks this tertiary carbocation to yield 2-bromo-2-methylbutane as the major product.

The identification of halogenated compounds relies on their molecular composition, specifically the count of halogen atoms present within each structure. Chloral ($CCl_3CHO$) and Chloropicrin ($CCl_3NO_2$) contain 3 chlorine atoms each, while Freon-12 ($CCl_2F_2$) contains only 2 chlorine atoms. In contrast, Gammaxene, also known as benzene hexachloride ($C_6H_6Cl_6$), contains 6 chlorine atoms, which is the highest number of chlorine atoms among the compounds provided.

A net molecular dipole moment ($\mu \neq 0$) arises when the vector sum of individual bond dipoles is non-zero, a condition determined by molecular asymmetry and the electronegativity difference between constituent atoms. The compounds in the correct pair, such as $\text{CH}_2\text{Cl}_2$, lack the required symmetry to cancel bond dipoles, ensuring that the vector sum of their polar bonds remains non-zero. Conversely, molecules in the incorrect pairs, such as trans-butene or 1,4-dichlorobenzene, exhibit geometric symmetry that leads to the complete cancellation of bond dipoles, resulting in a net dipole moment of zero ($\mu = 0$).