Haloalkanes And Haloarenes – JEE Main ChemistryPractice Questions & PYQs

The regioselectivity of this reaction is governed by the relative stability of the reactive intermediate, where the formation of a more substituted carbocation is favored due to increased hyperconjugation and inductive stabilization. In electrophilic addition or unimolecular elimination pathways, the reaction proceeds through the pathway that generates the most stable cation, which follows the order $3^\circ > 2^\circ > 1^\circ$, thereby directing the nucleophile or base to the most substituted carbon center. This thermodynamic preference dictates the reaction pathway, yielding the product depicted in option C as the predominant species.

The reaction sequence proceeds in two steps, beginning with an elimination reaction and concluding with an electrophilic addition. First, treating 3-chloro-2-methylbutane with a strong base facilitates an E2 elimination, which yields the more stable, highly substituted alkene, 2-methylbut-2-ene, according to Zaitsev's rule.

In the subsequent step, the addition of hydrogen bromide to this alkene follows Markovnikov's rule. The electrophilic hydrogen atom adds to the carbon atom with the greater number of hydrogen substituents, while the nucleophilic bromide ion bonds to the more substituted carbon atom, resulting in the formation of 2-bromo-2-methylbutane, $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{C}(\mathrm{Br})(\mathrm{CH}_3)-\mathrm{CH}_3$, as the final product Y.

The ability to produce a precipitate with silver nitrate depends on the ease with which a compound dissociates to release a chloride ion and form a stable carbocation intermediate. Tert-butyl chloride, $\mathrm{(CH_3)_3CCl}$, readily ionizes to generate a highly stable tertiary carbocation, allowing the chloride ion to combine with silver to yield $\mathrm{AgCl}$ according to the following reaction:
$\mathrm{(CH_3)_3CCl + AgNO_3 \rightarrow AgCl \downarrow + (CH_3)_3C^+ + NO_3^-}$
In contrast, the carbon-chlorine bonds in the other options are either too strong or involved in bonding structures that prevent the necessary ionization, meaning they fail to liberate the chloride ion required to form the precipitate.

Electrophilic addition reactions to asymmetric alkenes are governed by Markovnikov's rule, which dictates that the electrophile adds to the carbon atom with the greater number of hydrogen substituents to form the most stable carbocation intermediate. The transition state leading to the secondary carbocation, represented by $R-C^+H-CH_3$, is lower in energy compared to the primary carbocation due to increased inductive stabilization and hyperconjugation from the adjacent alkyl groups. This regioselectivity ensures that the nucleophilic species attacks the more substituted carbon, resulting in the more substituted haloalkane as the major product.

The reaction proceeds via an $\mathrm{S}_{\mathrm{N}}1$ mechanism because methanol, a polar protic solvent and weak nucleophile, induces the dissociation of the bromide leaving group to generate a secondary carbocation intermediate. This carbocation undergoes a 1,2-hydride shift to form a more stable tertiary carbocation, which is then attacked by the nucleophilic methanol molecule to form the rearranged ether product.

The nucleophilic substitution of alkyl halides with cyanide reagents is dictated by the ionic or covalent nature of the cyanide source. Potassium cyanide ($KCN$) is an ionic compound that provides free $CN^-$ ions, allowing the carbon atom to act as the nucleophilic center, which leads to the formation of the cyanide product, benzyl cyanide. In contrast, silver cyanide ($AgCN$) is a covalent compound, making the nitrogen atom's lone pair the more accessible nucleophile, resulting in the formation of the isocyanide product, benzyl isocyanide.

The elimination reaction proceeds according to Zaitsev's rule, where the most substituted alkene is formed as the major product because it possesses lower potential energy due to increased hyperconjugation. The formation of the transition state leading to this more stable product involves the removal of a proton from the most hindered $\beta$-carbon, thereby lowering the activation energy for the reaction. Consequently, the more highly substituted structure is thermodynamically preferred over any less substituted isomer produced in competing pathways.

The trifluoromethyl group in $F_{3}C-CH=CH_{2}$ acts as a powerful electron-withdrawing group via its inductive effect, significantly destabilizing the formation of a carbocation on the adjacent alpha-carbon. Consequently, the electrophilic proton preferentially attacks the internal carbon attached to the electron-withdrawing group to avoid a high-energy transition state, which directs the subsequent chloride nucleophile to the terminal carbon, yielding the anti-Markovnikov product. Unlike the other options containing electron-donating groups that facilitate standard electrophilic Markovnikov addition, this alkene is unique in favoring the reverse regiochemistry.

The reaction between silver nitrate and an organic halide proceeds through the precipitation of silver halide whenever a halide ion is effectively released from the parent molecule. This ionization process is heavily dependent on the stability of the resulting carbocation, where molecules capable of forming an aromatic cation experience a significant decrease in activation energy due to resonance stabilization. Consequently, since the compound in option D generates a stable aromatic cation upon ionization, it readily facilitates the release of the halide ion and results in the formation of a precipitate with $AgNO_3$.

The base-mediated elimination of 2-chloro-1-phenylbutane proceeds via an $E2$ mechanism where the stability of the resulting alkene dictates the major product. Removing a proton from the adjacent carbon atoms leads to either a conjugated or non-conjugated double bond, and the formation of 1-phenylbut-1-ene is favored because the phenyl ring provides extended conjugation, significantly lowering the transition state energy. Subsequent treatment of 1-phenylbut-1-ene with $\mathrm{Hg}(\mathrm{OAc})_2 / \mathrm{H}_2\mathrm{O}$ followed by $\mathrm{NaBH}_4$ triggers an oxymercuration-demercuration sequence. This process results in the Markovnikov addition of water across the double bond, where the hydroxyl group selectively attaches to the benzylic carbon due to its enhanced ability to stabilize partial positive charge, ultimately yielding 1-phenylbutan-1-ol as the major product $Y$.

In electrophilic aromatic substitution, the $Br$ atom acts as an ortho/para directing group due to its specific electronic effects. While both positions are activated, the ortho sites are subject to significant steric hindrance, which inhibits attack by incoming electrophiles. Consequently, the major product is predominantly formed at the para position, where steric crowding is minimized and the reaction proceeds most efficiently.

Friedel-Crafts alkylation reactions are governed by the existing substituents on the aromatic ring, which determine the regioselectivity of the incoming alkyl group. When an electron-donating substituent such as an $-\text{OR}$ group is present, it enhances the electron density at the ortho and para positions through resonance, thereby directing the incoming electrophile to these locations and establishing the structure shown in option A as the major product.

Vinyl halides exhibit significant resistance to nucleophilic substitution primarily because the carbon-halogen bond possesses partial double bond character, which makes the bond shorter, stronger, and significantly harder to break than in typical alkyl halides. Although the assertion correctly identifies this lack of reactivity, the reasoning provided is flawed because it claims the intermediate carbocation is stabilized by $\pi$-electrons. In reality, vinyl carbocations are highly unstable species that do not form readily, which invalidates the proposed explanation regarding their stability.

The rate of an $S_N1$ reaction is fundamentally governed by the stability of the carbocation intermediate formed after the leaving group departs, as this step constitutes the rate-determining process. Assessing the reactivity of these compounds involves comparing the carbocation each would form; those where the positive charge is effectively stabilized by resonance, hyperconjugation, or inductive effects will react significantly faster. By evaluating how effectively each structure stabilizes its resulting carbocation, you can rank the compounds from the least stable intermediate to the most stable one to determine the correct increasing order of reaction rates.

Free-radical substitution proceeds via the abstraction of a hydrogen atom to generate the most stable radical intermediate. Abstraction of a $\gamma$-hydrogen from the allylic position results in the formation of an allylic radical, $\text{CH}_3-\dot{\text{CH}}-\text{CH}=\text{CH}_2$, which is significantly stabilized by resonance with the adjacent $\pi$-bond. This resonance stabilization lowers the activation energy required for the formation of this intermediate, making the $\gamma$-hydrogens the most reactive site for bromination compared to the less stable vinyl or alkyl radicals.

Electrophilic aromatic substitution is governed by the electronic and steric directing effects of existing substituents on the benzene ring, where the acetamido group ($-NHCOCH_3$) functions as an activating, ortho-para directing substituent. In this reactant, the para position is sterically blocked by the $-CH_3$ group, which forces the incoming bromine electrophile to substitute at the ortho position relative to the acetamido group. This regioselectivity leads to the formation of the ortho-substituted product as the major outcome.

Dehydrohalogenation of alkyl halides in the presence of alcoholic $KOH$ proceeds via an E2 elimination mechanism that follows Zaitsev's rule, which states that the most substituted alkene is the major product due to its higher thermodynamic stability. The reaction involves the removal of a proton from an adjacent $\beta$-carbon to form a double bond, and the pathway that results in a more highly substituted alkene is favored because it is stabilized by greater hyperconjugation. Consequently, the reaction preferentially yields the internal alkene over the terminal alkene, as the internal double bond is more substituted and energetically favorable.

The reaction of the allylic alcohol $CH_3-CH=CH-CH_2OH$ with $PBr_3$ proceeds via an $S_N2'$ mechanism or involves a resonance-stabilized allylic intermediate, where the bromide ion nucleophile attacks the secondary allylic carbon. This leads to the formation of the rearranged product, 3-bromo-1-butene, as shown in the following reaction:

$CH_3-CH=CH-CH_2OH \xrightarrow{PBr_3} CH_3-CH(Br)-CH=CH_2$

This outcome is favored because the pathway allows for the formation of the more substituted allylic halide, which is thermodynamically preferred in this rearrangement.

The reaction of a tertiary alkyl halide with a strong base such as sodium ethoxide in ethanol proceeds via an E2 elimination mechanism, where the formation of the most stable alkene is favored according to Saytzeff's rule. In this specific substrate, the removal of a proton from the benzylic carbon adjacent to the tertiary center allows for the formation of a double bond that is in conjugation with the phenyl ring, providing significant resonance stabilization. Consequently, the highly stable conjugated product, $\mathrm{C}_6\mathrm{H}_5\mathrm{CH}=\mathrm{C}(\mathrm{CH}_3)\mathrm{CH}_2\mathrm{CH}_3$, is formed as the major product.

The Friedel-Crafts alkylation process relies on the generation of an electrophilic carbocation, which typically necessitates the cleavage of a carbon-halogen bond. In the case of vinyl chloride, $CH_2=CHCl$, the chlorine atom is bonded to an $sp^2$ hybridized carbon that exhibits partial double bond character due to resonance. Because this electronic structure prevents the formation of a carbocation, it is impossible for this compound to undergo a Friedel-Crafts reaction with benzene.