Hydrocarbons – NEET UG ChemistryPractice Questions & PYQs

Cracking is the chemical process responsible for breaking down large, heavy hydrocarbon molecules into smaller, more valuable fragments. During this reaction, complex higher alkanes are subjected to high temperatures or catalysts, which force them to split into a mixture of smaller alkanes and alkenes. Because these resulting fragments possess significantly lower molecular weights compared to the original liquid hydrocarbons, they exist in the gaseous state, making this process a standard industrial method for the production of natural gas.

The relative stability of different conformers for ethylene glycol, $HO-CH_2-CH_2-OH$, is determined by the competition between steric hindrance and stabilizing attractive forces. Among the available choices, conformer (d) represents the most stable state because its unique geometry facilitates the formation of intermolecular H-bonding. This critical interaction significantly lowers the overall potential energy of the molecule, rendering this specific orientation more favorable than the others.

The reaction of toluene with $Cl_2$ proceeds through two distinct mechanisms depending on whether an electrophilic catalyst or a radical initiator is employed. In the presence of the Lewis acid $FeCl_3$, an electrophilic aromatic substitution occurs on the benzene ring, where the methyl group acts as an ortho and para director, yielding a mixture of o- and p-chlorotoluene as X. When the reaction is instead exposed to sunlight, homolytic cleavage of the chlorine molecules generates radicals that initiate a free-radical chain reaction on the side-chain, leading to the successive substitution of hydrogen atoms on the methyl group until it is fully converted into trichloromethyl benzene, identified as Y.

This transformation represents a classic Friedel-Crafts alkylation, a fundamental method used to introduce an alkyl group onto an aromatic ring via an electrophilic substitution reaction. The anhydrous $\mathrm{AlCl}_3$ functions as a Lewis acid catalyst by extracting a chloride ion from $\mathrm{CH}_3\mathrm{Cl}$, which generates a highly reactive methyl carbocation ($\mathrm{CH}_3^+$) electrophile. Once formed, this electrophile attacks the benzene ring to replace a hydrogen atom, directly yielding toluene as the product.

To determine the hybridization of a carbon atom, identify the total number of sigma bonds present, where four sigma bonds denote $sp^3$ hybridization, three correspond to $sp^2$, and two signify $sp$ hybridization. Examining the structure, carbon 2 forms a triple bond with carbon 1, resulting in two sigma bonds and two pi bonds, which confirms its $sp$ state. Carbon 3 is connected to four single bonds, while carbon 5 participates in one double bond and two single bonds, classifying them as $sp^3$ and $sp^2$ hybridized respectively. Carbon 6 is bonded to four distinct carbon groups via single bonds, making it $sp^3$ hybridized and completing the required sequence.

Geometrical or cis-trans isomerism occurs when rotation is restricted around a double bond found in functional groups like ${C}={C}$, ${C}={N}$, or ${N}={N}$, preventing the attached atoms from moving freely. Because of this rigid structure, 2-butene permits distinct spatial orientations where the substituents are either on the same side or opposite sides of the double bond. In contrast, the other listed compounds do not possess the necessary restricted rotation within a double bond to accommodate these configurations.

The electrophilic addition of hydrogen bromide to an alkene proceeds via the formation of a carbocation intermediate, which often undergoes a rearrangement to generate a more stable carbocation before the bromide ion attacks. In the reaction of 3-methylbut-1-ene with $\mathrm{HBr}$, initial protonation of the double bond creates a secondary carbocation, $\mathrm{CH}_3-\mathrm{CH}(\mathrm{CH}_3)-\mathrm{CH}^+-\mathrm{CH}_3$, which rapidly undergoes a 1,2-hydride shift to form a more stable tertiary carbocation, $\mathrm{CH}_3-\mathrm{C}^+(\mathrm{CH}_3)-\mathrm{CH}_2-\mathrm{CH}_3$. Subsequent nucleophilic attack by $\mathrm{Br}^-$ on this tertiary carbocation yields $\mathrm{CH}_3-\underset{\underset{\mathrm{CH}_3}{\vert}}{\overset{\underset{\mathrm{Br}}{\vert}}{\mathrm{C}}}-\mathrm{CH}_2\mathrm{CH}_3$ as the major product.

Determining the hybridization of carbon atoms relies on the nature of the bonds surrounding each carbon center, where the presence of a triple bond corresponds to $\mathrm{sp}$ hybridization, a double bond to $\mathrm{sp}^2$ hybridization, and only single bonds to $\mathrm{sp}^3$ hybridization. In the structure provided, carbon 1 is part of a terminal triple bond, characterizing it as $\mathrm{sp}$ hybridized. Carbon 3 is bonded to carbon 2, carbon 4, and two hydrogen atoms entirely through single bonds, which results in $\mathrm{sp}^3$ hybridization. Meanwhile, carbon 5 is engaged in a double bond with carbon 4 while having single bonds with carbon 6 and its attached hydrogen, identifying it as $\mathrm{sp}^2$ hybridized.

Markovnikov's rule governs these electrophilic addition reactions, determining that the nucleophilic component of an addendum attaches to the more substituted carbon atom of the unsymmetrical double or triple bond. During the first step, the hydrogen atom from $\mathrm{HCl}$ binds to the terminal carbon of $\mathrm{CH}_3\mathrm{CH}_2-\mathrm{C}\equiv\mathrm{CH}$ while the chlorine atom bonds to the internal carbon, producing the intermediate $\mathrm{CH}_3-\mathrm{CH}_2-\mathrm{CCl}=\mathrm{CH}_2$. In the subsequent reaction with $\mathrm{HI}$, the hydrogen atom from the acid again attaches to the terminal methylene group, and the iodine atom bonds to the already substituted internal carbon to yield the final geminal dihalo product $\mathrm{CH}_3\mathrm{CH}_2-\underset{\underset{\mathrm{Cl}}{\vert}}{\overset{\overset{\mathrm{I}}{\vert}}{\mathrm{C}}}-\mathrm{CH}_3$.

The process of ozonolysis specifically cleaves carbon-carbon double bonds to form carbonyl compounds, and the structure of the reactant determines the resulting products. Because acetone consists of a central carbonyl group bonded to two methyl groups, its formation requires the presence of a $(\mathrm{CH}_3)_2\mathrm{C}=$ moiety within the alkene. 2-Methyl-2-butene, $\mathrm{H}_3\mathrm{C}-\mathrm{C}(\mathrm{CH}_3)=\mathrm{CH}-\mathrm{CH}_3$, contains this exact structural arrangement, resulting in the production of acetone and acetaldehyde upon cleavage, unlike the other isomers which yield different carbonyl compounds.

Ozonolysis of internal alkynes facilitates the cleavage of the carbon-carbon triple bond, which initially yields a 1,2-diketone intermediate, such as $\mathrm{CH}_3\mathrm{COCOCH}_2\mathrm{CH}_3$ when starting from $\mathrm{CH}_3\mathrm{C}\equiv\mathrm{CCH}_2\mathrm{CH}_3$. During the subsequent hydrolysis step, this diketone undergoes further oxidative cleavage at the bond connecting the two carbonyl groups. This reaction ultimately produces two distinct carboxylic acids, specifically acetic acid ($\mathrm{CH}_3\mathrm{COOH}$) and propanoic acid ($\mathrm{HOOCCH}_2\mathrm{CH}_3$).

The rate of catalytic hydrogenation with $H_2$ is fundamentally linked to the stability of the alkene, where more stable compounds sit at a lower energy level and require a higher activation energy to proceed through the reaction pathway. Alkyl substituents enhance alkene stability through electronic effects while simultaneously creating steric bulk that obstructs the accessibility of the catalyst surface to the double bond. Consequently, the alkene with the fewest substituents will be the most unstable and least hindered, allowing it to react significantly faster than its more crowded counterparts.

The reaction between $\mathrm{HBr}$ and propene in the presence of peroxide follows a free radical mechanism known as the Kharasch effect, which leads to anti-Markovnikov addition rather than the standard electrophilic pathway. In this process, the peroxide initiates the formation of a bromine free radical that attacks the alkene to generate a more stable secondary carbon radical intermediate. This intermediate subsequently abstracts a hydrogen atom from a $\mathrm{HBr}$ molecule, resulting in the final formation of n-propyl bromide.

The formation of ethylbenzene through the reaction of benzene with an alkene is achieved via Friedel-Crafts alkylation, which requires a Lewis acid catalyst such as anhydrous $\mathrm{AlCl}_{3}$ along with $\mathrm{HCl}$. When benzene ($\mathrm{C}_{6}\mathrm{H}_{6}$) is treated with ethylene ($\mathrm{H}_{2}\mathrm{C}=\mathrm{CH}_{2}$) at 95 °C, the electrophilic ethyl cation generated in situ effectively alkylates the aromatic ring to produce the desired ethylbenzene ($\mathrm{C}_{6}\mathrm{H}_{5}\mathrm{CH}_{2}\mathrm{CH}_{3}$).

The combination of $\mathrm{NaIO}_4$ and $\mathrm{KMnO}_4$ acts as an effective reagent for the oxidative cleavage of alkenes, breaking the carbon-carbon double bond to form higher oxidation state carbonyl products. When applied to $\mathrm{CH}_3 - \overset{\mathrm{CH}_3}{\overset{\vert}{\mathrm{C}}} = \mathrm{CH} - \mathrm{CH}_3$, the reaction splits the molecule at the site of unsaturation. The carbon atom carrying two methyl groups is oxidized to the ketone $\mathrm{CH}_3 \mathrm{COCH}_3$, and the carbon atom bonded to a hydrogen and a methyl group is oxidized to the carboxylic acid $\mathrm{CH}_3 \mathrm{COOH}$.

Electrophilic aromatic substitution depends fundamentally on the electron density available within the aromatic ring, as a more electron-rich system more readily attracts the incoming electrophile. The $-{NH}_2$ group present in aniline (I) acts as an activating substituent, donating electron density into the ring through resonance and making it highly susceptible to electrophilic attack. Benzene (II) maintains a standard electron distribution without any substituent-induced electronic effects, serving as the intermediate baseline for reactivity. In contrast, nitrobenzene (III) contains a strongly electron-withdrawing $-{NO}_2$ group that depletes the ring of electron density, creating a deactivating effect that renders the compound the least reactive of the three. This variation in substituent influence dictates the overall rate of reaction, establishing the reactivity order as I > II > III.

Free-radical substitution reactions, such as the photochemical chlorination of methane ($\mathrm{CH}_4 + \mathrm{Cl}_2 \xrightarrow{h\nu} \mathrm{CH}_3\mathrm{Cl} + \mathrm{HCl}$), occur when a halogen molecule undergoes homolytic cleavage in the presence of ultraviolet light to generate highly reactive radical species. These radicals initiate a chain process by abstracting a hydrogen atom from an alkane to create an alkyl radical, which subsequently reacts with a halogen molecule to form the substituted product and regenerate the radical. Unlike nucleophilic addition reactions involving carbonyl compounds, this pathway is characterized by the step-wise replacement of hydrogen atoms via radical intermediates.

When a geminal dihalide like $\mathrm{CH}_3\mathrm{CH}_2\mathrm{CHCl}_2$ encounters a strong base such as $\mathrm{NaNH}_2$, it undergoes a double dehydrohalogenation process where two molecules of $\mathrm{HCl}$ are eliminated from the molecule. This transformation facilitates the formation of a carbon-carbon triple bond, effectively converting the saturated starting material into the corresponding alkyne. Consequently, the reaction proceeds to yield $\mathrm{CH}_3\mathrm{C}\equiv \mathrm{CH}$ as the final product.

The acidity of phenols is governed by the stability of the resulting conjugate base, where electron-withdrawing groups such as $-NO_2$ enhance stability by dispersing the negative charge, while electron-donating groups like $-CH_3$ decrease stability by increasing electron density. Consequently, $p$-nitrophenol is more acidic than unsubstituted phenol, whereas $p$-cresol is less acidic because the $-CH_3$ group destabilizes the phenoxide ion. This hierarchy establishes that the nitro-substituted compound is the most acidic and the methyl-substituted compound is the least, resulting in the acidity order $p$-nitrophenol > phenol > $p$-cresol.

The catalytic dehydration of alcohols to alkenes using $\mathrm{Al}_{2}\mathrm{O}_{3}$ is an endothermic process that requires sufficient thermal energy to overcome the activation barrier needed to break the strong carbon-oxygen and carbon-hydrogen bonds. Since the reaction must reach a high temperature, such as 620 K for the conversion of ethanol into $\mathrm{CH}_{2}=\mathrm{CH}_{2}$, the thermal environment dictates the efficiency and success of the transformation. Consequently, temperature serves as the critical effective factor because the rate and extent of dehydration are primarily governed by the ability of the system to maintain these specific thermal conditions.