Hydrocarbons – JEE Main ChemistryPractice Questions & PYQs

Reductive ozonolysis ($O_3, H_2O/Zn$) and oxidative cleavage using acidic potassium permanganate ($KMnO_4/H^+$) are two distinct chemical processes that cleave alkenes, though they yield different byproducts based on the oxidation state of the reagents. When a compound like 2-methylpropene ($CH_3-C(CH_3)=CH_2$) reacts with $O_3, H_2O/Zn$, the double bond is cleaved to form acetone as the major product and formaldehyde ($HCHO$) as the byproduct ' $a$ '. Formaldehyde is readily oxidized to formic acid, which is the well-known substance produced by ants.

Conversely, when the same compound, 2-methylpropene, reacts with the strong oxidizing agent $\mathrm{KMnO}_4/H^+$, it undergoes oxidative cleavage to produce the same major product, acetone, but instead of formaldehyde, it yields carbon dioxide as the byproduct ' $b$ ' due to the complete oxidation of the terminal carbon atom. Thus, ' $X$ ' must be the reagent for reductive ozonolysis, $\mathrm{O}_3, \mathrm{H}_2\mathrm{O}/\mathrm{Zn}$, and ' $Y$ ' must be the reagent for oxidative cleavage, $\mathrm{KMnO}_4/\mathrm{H}^{+}$, making the correct option D.

The reaction proceeds via an acid-catalyzed cyclization mechanism, wherein the protonation of one alkene group generates a carbocation that intramolecularly attacks the second alkene double bond to form a cyclic terpene derivative. During this transformation, the original diene structure undergoes ring closure, which restricts the remaining unsaturation to a single double bond within the ring. This resulting cyclic structure contains only one carbon-carbon double bond, thereby accounting for exactly 2 $sp^2$ hybridized carbon atoms.

The formation of the major product in this electrophilic addition reaction is governed by the relative stability of the intermediate carbocation species formed during the mechanism. Upon protonation of the alkene double bond, a secondary carbocation is initially generated, which rapidly undergoes a 1,2-hydride shift to transform into a more stable tertiary carbocation, as tertiary carbocations are significantly stabilized by hyperconjugation and inductive effects. The final major product is obtained through the nucleophilic attack of the bromide ion on this rearranged, more stable tertiary carbocation intermediate, rather than the initial secondary one.

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The oxidative cleavage of alkenes by acidic $\mathrm{KMnO}_4$ involves the breaking of the carbon-carbon double bond, where a terminal methylene group ($\mathrm{R}-\mathrm{CH}=\mathrm{CH}_2$) produces carbon dioxide and a carboxylic acid, whereas a disubstituted carbon ($\mathrm{R}_2\mathrm{C}=$) yields a ketone. 2-methyl propene, having the structure $(\mathrm{CH}_3)_2\mathrm{C}=\mathrm{CH}_2$, reacts to form acetone $(\mathrm{CH}_3\mathrm{COCH}_3)$ and carbon dioxide, the latter of which generates effervescence. In contrast, isomers like but-1-ene produce propanoic acid with gas evolution but no ketone, while but-2-ene isomers produce only acetic acid.

Oxymercuration-Demercuration acts as an effective method for the hydration of alkenes, resulting in the net addition of $\mathrm{H}_2\mathrm{O}$ across the double bond. This transformation proceeds according to Markovnikov's rule, ensuring the hydroxyl group attaches to the more substituted carbon atom. Because the mechanism involves a bridged cyclic mercurinium ion intermediate rather than a free carbocation, it avoids unwanted skeletal rearrangements that might otherwise complicate the final product.

When a chemical reaction generates a single stereocenter within a molecule, the resulting product exists in two non-superimposable mirror image forms known as enantiomers. Applying the general stereochemical rule $N = 2^n$, where $n$ represents the number of chiral centers, substituting $n = 1$ confirms that exactly two distinct spatial arrangements are possible. This pair of configurations accounts for all potential stereoisomeric variations for that specific site, resulting in a total of 2 stereoisomers for Product 'P'.

During the nitration of benzene, $H_2SO_4$ functions as a stronger Bronsted-Lowry acid by donating a proton to $HNO_3$, which acts as a Bronsted-Lowry base by accepting that proton. This interaction facilitates the formation of the electrophilic nitronium ion ($NO_2^+$), which is necessary for the subsequent substitution on the benzene ring. Consequently, because nitric acid accepts the proton, it behaves as the base, while sulfuric acid behaves as the acid in this chemical mixture.

2-methylbutane contains a tertiary hydrogen atom, which makes it reactive enough to undergo oxidation by $\mathrm{KMnO}_4$ to form 2-methylbutan-2-ol. Conversely, straight-chain $n$-alkanes are chemically inert toward such oxidizing agents because they lack sufficiently reactive carbon-hydrogen bonds to participate in this reaction. Consequently, the first statement holds true, while the second statement is false due to the fundamental difference in reactivity between branched and linear alkanes.

Thermodynamic and kinetic stability dictate the outcome of this reaction, as the mechanism consistently favors the pathway that proceeds through the lowest energy transition state. By analyzing the electronic and steric environment of the reactants, it becomes clear that the pathway leading to the structure in option B involves the formation of the most stable intermediate. This preference minimizes the activation energy, ensuring that the specific configuration found in option B predominates over all other potential chemical arrangements.

The stability of a carbocation intermediate directly dictates the energy of the transition state, which in turn determines the rate of its formation. Carbocation $B$ is more stable than $A$ because it is a secondary carbocation, which benefits from a stronger $+I$ effect and a greater number of $\alpha$-hydrogens available for hyperconjugation. This increased stability lowers the activation energy required for its generation, allowing carbocation $B$ to be formed at a relatively faster rate.

The synthesis of 4-bromo-2-nitroethyl benzene from benzene requires a careful sequence of electrophilic aromatic substitutions and functional group transformations, prioritizing the establishment of the ortho/para-directing ethyl substituent before introducing the bromine and nitro groups. The process begins with the Friedel-Crafts acylation of benzene using $\mathrm{CH_3COCl} / \mathrm{AlCl}_3$ to generate acetophenone. This intermediate is subsequently treated with a $\mathrm{Zn}-\mathrm{Hg} / \mathrm{HCl}$ mixture via Clemmensen reduction to convert the acetyl group into an ethyl group, resulting in the formation of ethyl benzene.

Once ethyl benzene is prepared, it undergoes bromination using $\mathrm{Br}_2 / \mathrm{AlBr}_3$ to yield 4-bromoethylbenzene, as the ethyl group serves as an activating, ortho/para-directing substituent that directs the bromine atom to the para position. The final step is the nitration of this intermediate using $\mathrm{HNO}_3 / \mathrm{H_2SO_4}$. Because the bromine atom exerts an electron-withdrawing inductive effect that directs the incoming nitro group to its meta position, the nitro group is guided to the carbon atom ortho to the ethyl group, successfully completing the synthesis of 4-bromo-2-nitroethyl benzene.

Nucleophilic aromatic substitution reactions proceed more readily when the benzene ring is electron-deficient, as the presence of $- {NO}_{2}$ groups at ortho and para positions effectively withdraws electron density through resonance and induction. This depletion of electron density on the aromatic ring stabilizes the anionic transition state formed during the reaction, thereby facilitating the attack of nucleophiles on the carbon atom attached to the halogen. As the number of electron-withdrawing nitro groups increases, the electrophilic nature of the carbon center is further enhanced, causing the reaction rate to increase systematically from the unsubstituted compound to those containing multiple nitro substituents.

Vigorous oxidative cleavage of a cyclic alkene using hot alkaline potassium permanganate ($KMnO_4$) results in the rupture of the carbon-carbon double bond to produce a dicarboxylic acid, identifying compound 'A'. In contrast, mild syn-dihydroxylation of the same cyclic alkene using cold dilute alkaline $KMnO_4$ or osmium tetroxide ($OsO_4$) preserves the carbon skeleton to form a vicinal diol, identifying compound 'B'.

Metallic sodium reacts by displacing acidic hydrogen atoms to evolve hydrogen gas, a property characteristic of substances containing protons attached to highly electronegative atoms or $sp$-hybridized carbons. In the case of But-2-yne ($CH_3-C \equiv C-CH_3$), all hydrogen atoms are bound to $sp^3$-hybridized carbons rather than the triply bonded carbon atoms, meaning there are no acidic protons available for the sodium to replace. Consequently, unlike terminal alkynes such as ethyne or compounds with hydroxyl groups like tert-butyl alcohol, But-2-yne remains chemically inert toward metallic sodium.

The reaction sequence first converts acetylene $\mathrm{CH} \equiv \mathrm{CH}$ into benzene via cyclotrimerization, followed by the Gattermann-Koch formylation to produce benzaldehyde. Examining the structure of benzaldehyde reveals that all 6 carbon atoms within the benzene ring are $sp^2$ hybridized, as they each participate in a conjugated system involving a double bond. The additional carbon atom found in the aldehyde group also features a double bond to oxygen, which characterizes it as $sp^2$ hybridized. Consequently, the sum of these 6 ring carbons and 1 aldehyde carbon identifies 7 $sp^2$ hybridized carbon atoms in the final product.

The transformation of nitrobenzene to m-chlorophenol proceeds through strategic functional group manipulation, leveraging the meta-directing nature of the nitro group and the reactivity of diazonium salts. Initially, the nitro group directs the incoming chlorine substituent to the meta position via electrophilic aromatic substitution with $\mathrm{Cl}_2$ and $\mathrm{FeCl}_3$, yielding m-chloronitrobenzene.

Subsequently, the reduction of the nitro group to an amino group using $\mathrm{Fe}$ and $\mathrm{HCl}$ produces m-chloroaniline. This amine is converted to a diazonium salt intermediate using $\mathrm{NaNO}_2$ and $\mathrm{HCl}$ at $0^{\circ}\mathrm{C}$. Finally, the diazonium group is replaced by a hydroxyl group upon hydrolysis with $\mathrm{H}_2\mathrm{O}$ in an acidic environment, successfully generating the m-chlorophenol product.

Hybridization is determined by the number of sigma bonds connected to an atom. In $CH_2 = C = CH - CH_3$, carbon 1 is part of a terminal double bond and connects to two hydrogen atoms and one carbon, placing it in an $sp^2$ hybridized state. Carbon 2 resides in the center of an allene structure, bonded to two other carbons via two double bonds, which forces a linear geometry and defines it as $sp$ hybridized. Carbon 3 is attached to one hydrogen atom and two carbons through sigma bonds associated with one double and one single bond, characterizing it as $sp^2$ hybridized. Lastly, carbon 4 is a terminal methyl group involved only in single sigma bonds with four atoms, confirming it as $sp^3$ hybridized.

The formation of the major product P is governed by Zaitsev's rule, which dictates that the most substituted alkene is the thermodynamically most stable and thus the preferred product in elimination reactions. This stability arises because the alkyl groups adjacent to the $C=C$ double bond provide greater hyperconjugative stabilization, effectively lowering the energy of the transition state and the final product relative to less substituted isomers. Consequently, the reaction pathway favors the formation of the most highly substituted alkene, which represents the minimum Gibbs free energy state accessible under the reaction conditions.

Alcoholic $\mathrm{KOH}$ is widely employed to facilitate $\beta$-elimination reactions, whereas the $\mathrm{Pd}/\mathrm{BaSO}_4$ mixture functions as the Lindlar catalyst. Benzene hexachloride results from an addition reaction of chlorine with benzene, and polyacetylene is specifically utilized in the manufacturing of electrodes for battery technology.