Haloalkanes And Haloarenes – JEE Main ChemistryPractice Questions & PYQs

The reaction between these chlorides and acetate in an acetic acid solvent follows the $S_N1$ mechanism, where the rate-determining step is the formation of a carbocation intermediate. Because the reaction rate is directly proportional to the stability of this intermediate, substrates that generate more stable carbocations will naturally exhibit greater reactivity. This dependency on stability dictates the specific order of reactivity, which matches the sequence presented in option A.

Treating 1-bromo-prop-1-ene with $\mathrm{NaNH}_2$ facilitates the elimination of a molecule of hydrogen bromide. This base-induced dehydrobromination transforms the alkene reactant into the corresponding alkyne, propyne, which is represented by the formula $\mathrm{CH}_3 \mathrm{C} \equiv \mathrm{CH}$.

Subsequently, when this propyne gas flows through a red hot iron tube heated to 873 K, it undergoes a characteristic cyclic trimerization process. The three alkyne units condense to form a six-membered aromatic ring with methyl groups at the 1, 3, and 5 positions, resulting in the formation of mesitylene as the final major product.

The $E_2$ elimination reaction proceeds through a concerted mechanism where two substituents are simultaneously removed from adjacent carbon atoms to generate a stable $\pi$ bond. This pathway converts the reactant into an alkene, which is the structural outcome correctly depicted in option D.

Nucleophilic substitution at a chiral carbon typically results in inversion or racemization, but this changes when the reaction occurs at a carbon atom detached from the stereocenter. In the molecule $\mathrm{CH}_3-\mathrm{CH}(\mathrm{C}_2\mathrm{H}_5)-\mathrm{CH}_2\mathrm{Br}$, the bromine atom is bonded to a primary carbon, while the chiral center is located at the adjacent carbon atom. Because the nucleophilic attack takes place at this primary position, the spatial arrangement at the chiral center remains completely undisturbed, leading to the retention of its configuration.

For isomeric haloalkanes, molecular branching significantly impacts boiling points because it alters the molecular surface area and the strength of intermolecular interactions. Higher degrees of branching create more compact, spherical structures that reduce the contact area between molecules, thereby weakening the van der Waals forces and resulting in lower boiling points. Since the decreasing order of boiling points for these isomers is $A > C > B$, the increasing order is $B < C < A$.

The $E_1$ reaction pathway is governed by the stability of the carbocation intermediate formed in the rate-determining step, meaning that compounds which generate more stable carbocations will undergo dehydrohalogenation more readily. Based on the structures provided, the carbocation formed from D is the most stable, while the carbocation formed from A is the least stable. This stability difference dictates the overall reactivity, establishing the decreasing order as $D > B > C > A$.

The reactivity of substrates in $S_N2$ reactions is predominantly governed by steric hindrance, as the rate of the bimolecular nucleophilic substitution process decreases significantly with increased branching around the electrophilic carbon atom. Additionally, electronic effects such as the resonance stabilization of the transition state or the hybridization of the carbon-halogen bond influence the overall rate, causing primary halides to react much faster than secondary, tertiary, or rigid aryl/vinyl systems. Consequently, the decreasing order of reactivity reflects the transition from less hindered, electron-accessible primary structures to more sterically crowded or electronically deactivated substrates.

The major product is determined by the stability of the intermediate species and the thermodynamic preference for the most substituted alkene, consistent with Zaitsev's rule. During the reaction, the pathway minimizes the activation energy, $E_a$, by facilitating the formation of the product with the highest degree of hyperconjugation and orbital overlap. This thermodynamic control dictates that the isomer with the greatest number of substituents on the $C=C$ double bond is the primary outcome, making it the most stable product.

The $\text{SN}_1$ mechanism is governed by a rate-determining step involving only the substrate, where the leaving group dissociates independently of the nucleophile or base concentration. In contrast, the $\text{E}_2$ mechanism proceeds through a concerted process that necessitates the participation of the base, causing its rate to be dependent on the concentration of that base. Therefore, altering the concentration of the base will not impact the progression of Reaction 1, as its rate-limiting step occurs entirely independently of the base.

$E_2$-elimination follows Zaitsev's rule, which dictates that the formation of the most substituted and stable alkene is favored. Because the bromide ion is a superior leaving group compared to the fluoride ion, the reaction proceeds by removing the bromine atom from the third carbon along with a hydrogen atom from either the second or fourth carbon. Elimination between the second and third carbons results in the more substituted 2-fluoro-2-pentene, making it the major product compared to the less substituted 3-fluoro-2-pentene formed by elimination between the third and fourth carbons.

The reaction of organic halides with silver nitrate proceeds via an $S_N1$ mechanism, where the rate-determining step is the formation of a carbocation intermediate. Because the ease of this ionization process is directly proportional to the stability of the cation produced, molecules that form more stable carbocations react significantly faster. Comparing the stability of the resulting intermediates, the decreasing order of reactivity is $B > A > C > D$.

The fundamental principle governing these reaction pathways is that increased steric hindrance around the reaction center effectively blocks the nucleophilic attack required for substitution while simultaneously promoting the elimination mechanism. System B exhibits more steric crowding, which elevates the rate of elimination relative to system A, establishing that $k_e(B) > k_e(A)$. Given that the ratio is defined as $\mu = \frac{k_s}{k_e}$, this substantial increase in the elimination rate constant for system B ensures that its ratio $\mu_B$ is smaller than the corresponding ratio $\mu_A$, resulting in $\mu_A > \mu_B$.

The Friedel-Crafts reaction is an electrophilic aromatic substitution that proceeds effectively only when the aromatic ring is sufficiently activated by electron-donating groups to stabilize the reaction intermediate. Strong electron-withdrawing groups deactivate the ring toward electrophilic attack, while basic substituents such as amino groups coordinate irreversibly with the Lewis acid catalyst ($AlCl_3$), preventing the formation of the necessary electrophile. Option C features an electron-donating substituent that increases the ring's nucleophilicity without deactivating it or binding the catalyst, thereby facilitating the highest reaction yield.

The $S_N^1$ mechanism is governed by the formation of a planar carbocation, $R^{\oplus}$, during the rate-determining step, a process that is significantly promoted by bulky substituents that relieve steric crowding upon transition to the $sp^2$ hybridized state. Because the intermediate is planar, the incoming nucleophile can attack from either face, inevitably resulting in a racemic product mixture. Finally, because the rate-determining step is the dissociation of the leaving group rather than a direct displacement by a nucleophile, the reaction proceeds readily with weak nucleophiles.

A racemic mixture consists of an equal ratio of two enantiomers, which are non-superimposable mirror images that can only exist when a molecule possesses at least one chiral center. Because chirality is a necessary condition for the phenomenon of racemization, any reaction yielding a product that lacks a chiral center will inherently be unable to generate a racemic product. Since the reaction in option A produces a compound devoid of any chiral centers, it is the only case where racemization is impossible.

Metal displacement reactions occur spontaneously only when a more reactive metal replaces a less reactive metal from its aqueous salt solution, as determined by standard electrode potential values in the electrochemical series. Reactions (b) and (d) are feasible because zinc and iron are more electropositive than copper, allowing for the spontaneous processes represented by $Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$ and $Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$. Conversely, reactions where a less reactive metal attempts to displace a more reactive one fail to proceed because they are thermodynamically unfavorable under standard conditions.

The bond length of a $C-Cl$ bond is primarily dictated by its bond order, where the introduction of pi-character significantly shortens the distance between the two atoms. In the case of the compound in option C, the lone pair on the chlorine atom undergoes delocalization into the neighboring conjugated system, which imparts a partial double bond character to the $C-Cl$ bond. Because this resonance increases the bond order beyond that of a standard single bond, the resulting distance is shorter than in the other compounds where such delocalization is absent.

The reaction proceeds via an initial acid-catalyzed ring opening of the epoxide followed by the substitution of the resulting hydroxyl group. Protonation of the epoxide oxygen increases the carbocation character of the more substituted carbon, facilitating the nucleophilic attack by the bromide ion to form a bromohydrin intermediate. With excess $HBr$, this intermediate undergoes a second substitution where the hydroxyl group is protonated and displaced by another bromide ion, ultimately yielding 1,2-dibromopropane.

The regioselective addition of hydrogen bromide to the alkene $\mathrm{CH_3CH=CHCH(CH_3)_2}$ follows Markovnikov's rule, where the electrophilic hydrogen atom preferentially adds to the unsaturated carbon atom bearing the greater number of hydrogen atoms to facilitate the formation of the more stable carbocation intermediate. Following this initial protonation step, the nucleophilic bromide ion attacks the carbocation to complete the addition across the double bond. This chemical sequence dictates the specific orientation of the atoms, resulting in $\mathrm{CH_3CH(Br)CH_2CH(CH_3)_2}$ as the major product.

When reacting organic halides with aqueous $\mathrm{AgNO}_3$, the process follows an $S_N1$ mechanism where the rate-determining step is the ionization of the carbon-halogen bond to generate a carbocation intermediate. Because the speed of precipitation is governed by the ease of forming this intermediate, the compound capable of yielding the most stable carbocation will react most rapidly. The structure in B is the most reactive in this set because it leads to the formation of the most stable carbocation, thereby resulting in the fastest precipitation of the corresponding silver halide.