Haloalkanes And Haloarenes – NEET UG ChemistryPractice Questions & PYQs

The electrophilic addition of hydrogen bromide to an unsymmetrical alkene proceeds via the formation of the most stable carbocation intermediate according to Markovnikov's rule. When $\mathrm{C}_6\mathrm{H}_5\text{CH}=\text{CHCH}_3$ reacts with $\text{HBr}$, protonation yields a benzylic carbocation at the carbon adjacent to the phenyl ring, which is exceptionally stable due to resonance delocalization. Subsequent nucleophilic attack of the bromide ion on this highly stable intermediate yields $\mathrm{C}_6\mathrm{H}_5 \underset{\mathrm{Br}}{\underset{|}{\mathrm{C}}}\text {HCH}_2\text {CH}_3$ as the major product.

Racemisation during a hydrolysis reaction depends on the presence of a chiral carbon atom that allows for the formation of a racemic mixture of enantiomers. Since only compound (iv) features a chiral center among the given choices, it is the sole structure capable of yielding this equal mixture upon nucleophilic attack. Therefore, compound (iv) is the only one that undergoes racemisation when treated with a solution of KOH.

Conformational isomerism involves the generation of different spatial arrangements of atoms through the free rotation around single carbon-carbon bonds. Since structures I and II specifically represent the staggered and eclipsed forms of a molecule, they are directly interconvertible via this bond rotation. These distinct rotational states classify them as a pair of conformers.

Tertiary halides undergo an $S_N1$ mechanism that proceeds through a carbocation intermediate and requires the attack of a dissociated negative ion. The success of this ionic pathway depends on the ease of ionization, which increases with the ionic character of the $M-F$ bond. Because rubidium has the largest ionic radius among the given alkali metals, its bond with fluorine exhibits the highest ionic character, thereby making the reaction most favorable.

Optical isomerism requires a molecule to lack a plane of symmetry and possess a chiral center, which allows for non-superimposable mirror images. Maleic acid, or *cis*-butenedioic acid, contains a rigid carbon-carbon double bond and a plane of symmetry passing through the molecule, preventing the formation of chiral centers. Instead of optical activity, it exhibits geometrical isomerism due to restricted rotation about the double bond.

Nucleophilic substitution via the unimolecular pathway relies fundamentally on the stability of the carbocation intermediate formed in the rate-determining step. Among the given choices, $\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{C}(\mathrm{CH}_{3})(\mathrm{C}_{6}\mathrm{H}_{5})\mathrm{Br}$ ionizes to form a tertiary benzylic carbocation. This intermediate is exceptionally stable because the positive charge is effectively dispersed through resonance with two phenyl rings alongside the electron-donating inductive effect of the methyl group. Consequently, the significantly lowered activation energy makes this tertiary halide the most reactive in the series.

The reactivity of a $\text{C-X}$ bond toward nucleophilic attack is fundamentally governed by carbocation stability and electronic effects within the molecular structure. Tertiary alkyl halides readily generate stable $3^\circ$ carbocations, making compound III significantly more reactive than the secondary alkyl halide represented by compound IV. In contrast, unsubstituted aryl halides are notoriously unreactive due to resonance imparting partial double bond character to the carbon-halogen linkage, placing compound I near the beginning of the sequence. However, the introduction of an electron-withdrawing $-\text{NO}_2$ group significantly depletes the ring electron density, boosting the reactivity of compound II just above the plain aryl system while remaining below the aliphatic carbocation pathways.

Treating an alkyl halide with magnesium in dry ether generates an organometallic Grignard reagent, which converts benzyl bromide into benzylmagnesium bromide. Subsequent aqueous acid hydrolysis of this intermediate replaces the metal halide moiety with a proton. This sequence successfully reduces the benzyl halide to toluene represented by $\mathrm{C_6H_5CH_3}$.

Nucleophilic substitution reactions occur when an electron-rich species bearing a lone pair or negative charge displaces a leaving group attached to an aliphatic carbon atom. Aqueous potassium hydroxide dissociates to provide hydroxide ions, which act as strong ambient nucleophiles capable of attacking alkyl halides. In the reaction $\mathrm{RX}+\mathrm{KOH}\rightarrow\mathrm{ROH}+\mathrm{KX}$, the halide atom is replaced by the hydroxyl group to yield an alcohol.

Stereoisomerism in organic molecules arises from different spatial arrangements that can encompass both geometrical and optical centers. The given compound contains one carbon-carbon double bond capable of cis-trans isomerism and one chiral carbon atom responsible for optical activity. Combining the two possible geometrical configurations with the two optical arrangements calculated as $2^n$ yields a total of four distinct stereoisomers.

Optical inactivity in a molecule that possesses stereogenic centers typically arises from internal compensation where opposite configurations within the same structure cancel each other's rotations. Such molecules contain multiple chiral centers but maintain an internal plane or center of symmetry, rendering the overall structure achiral. Therefore, a substance that features stereocenters yet fails to rotate plane-polarized light is classified as a meso compound.

The absolute configuration of a chiral centre is assigned using the Cahn-Ingold-Prelog priority rules, ranking the attached groups as $-\text{Cl}$ ($1$), $-\text{CH}_2\text{CH}_3$ ($2$), $-\text{CH}_3$ ($3$), and $-\text{H}$ ($4$). Tracing from priority $1$ to $2$ to $3$ results in a clockwise direction when the lowest priority group points away from the viewer, which defines the $R$-configuration. This specific spatial arrangement correctly orients the substituents so that the clockwise progression of priorities matches the required stereochemical descriptor.

Chirality in organic molecules generally requires the presence of an asymmetric carbon atom bonded to four entirely different groups. In 3-bromopentane, the central carbon bearing the bromine atom is attached to two identical ethyl groups on either side, creating a plane of symmetry. This structural symmetry eliminates any possibility of optical isomerism, making the molecule completely achiral.

Unimolecular nucleophilic substitution reactions proceed through a carbocation intermediate, making their feasibility entirely dependent on the stability of that resulting carbocation. Benzyl chloride readily generates a carbocation that is exceptionally stabilized by resonance delocalization across the adjacent aromatic ring. This profound stabilization lowers the activation energy so significantly that the reaction exclusively favors the pathway involving carbocation formation.

Determining the absolute configuration of a stereocenter relies on assigning priorities to the attached groups based on decreasing atomic number and mass, ensuring the lowest priority substituent is oriented away from the viewer. In the case of 1-bromo-1-chloroethane, bromine receives the highest priority, followed by chlorine, the carbon chain, and finally hydrogen. Tracing a path from the highest to the lowest priority groups moves in a clockwise direction, which establishes the stereochemical designation as $R$.

Vinyl halides exhibit extreme resistance toward nucleophilic substitution because the lone pair of electrons on the chlorine atom participates in resonance with the adjacent carbon-carbon double bond ($\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{Cl}$). This delocalization imparts a partial double bond character to the $\mathrm{C}-\mathrm{Cl}$ bond, significantly strengthening it compared to a standard single bond. Consequently, breaking this bond requires a prohibitively high amount of energy, making this species the least reactive in nucleophilic substitution.

Enantiomers are stereoisomers that relate to each other as non-superimposable mirror images. To spot them among various structural choices, you need to look for molecules that reflect one another across an axis yet fail to coincide when you attempt to overlap them in three dimensions. The compounds in the first option uniquely share this exact spatial relationship, making them true enantiomers.

The strength of the carbon-halogen ($\mathrm{C}-\mathrm{X}$) bond dictates the ease with which elimination occurs, as weaker bonds require less energy to break during the reaction. Moving down the halogen group, atomic size increases, resulting in longer and progressively weaker bonds in the order $\mathrm{C}-\mathrm{I} > \mathrm{C}-\mathrm{Br} > \mathrm{C}-\mathrm{Cl} > \mathrm{C}-\mathrm{F}$. Because the carbon-iodine bond has the lowest bond dissociation energy, it breaks most easily, establishing a reactivity sequence where alkyl iodides react the fastest and alkyl fluorides react the slowest.

Converting an alkyl halide into an amide derivative relies on a sequential progression of nucleophilic substitution, catalytic reduction, and acylation reactions. Treating ethyl chloride with sodium cyanide brings about the replacement of the chlorine atom by a cyano group, extending the carbon chain to form propanenitrile as intermediate $X$ represented by $\mathrm{CH}_3\mathrm{CH}_2\mathrm{Cl} \xrightarrow{\mathrm{NaCN}} \mathrm{CH}_3\mathrm{CH}_2\mathrm{CN}$. Hydrogenating this nitrile in the presence of a nickel catalyst reduces the cyano functionality down to a primary amine, yielding propan-1-amine as intermediate $Y$ through $\mathrm{CH}_3\mathrm{CH}_2\mathrm{CN} \xrightarrow{\mathrm{Ni}/\mathrm{H}_2} \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{NH}_2$. Subjecting this resulting primary amine to acetic anhydride causes an acetylation reaction, wherein the amino group attacks the anhydride to generate $N$-propylacetamide as the final product $Z$ according to $\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{NH}_2 \xrightarrow{\text{acetic anhydride}} \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{NHCOCH}_3$.

Coupling an alkyl chloride ($\mathrm{C}_{4}\mathrm{H}_{9}\mathrm{Cl}$) with sodium in ether forms a symmetrical higher alkane via the Wurtz reaction, where the structural symmetry determines the number of possible monochloro derivatives. Tertiary butyl chloride undergoes this coupling to form 2,2,3,3-tetramethylbutane, a highly symmetrical hydrocarbon where all hydrogen atoms are equivalent primary hydrogens. Consequently, monochlorination of this resulting hydrocarbon yields exclusively one distinct chloro derivative.