Hydrocarbons – JEE Main ChemistryPractice Questions & PYQs

The starting material, ethyne ($C_2H_2$), is the lowest molecular weight alkyne, which undergoes cyclic trimerization when passed through a red-hot copper tube to form benzene ($C_6H_6$) as molecule B. This benzene molecule then participates in a Friedel-Crafts alkylation reaction with methyl chloride ($CH_3Cl$) in the presence of anhydrous $AlCl_3$ to yield toluene ($C_7H_8$) as molecule C. Within the resulting structure, the six carbons of the aromatic ring and the single carbon atom of the methyl group all lie in the same plane. Since the five hydrogen atoms attached to the ring and one hydrogen atom from the methyl group can also align within this plane, the maximum number of atoms present in one plane is 13.

The addition of hydrogen halides to alkynes adheres to Markovnikov's rule, where the electrophilic deuterium ion preferentially adds to the terminal carbon to stabilize the developing carbocation. In the first step, $CH_3C\equiv CH$ reacts with one equivalent of $DCl$ to produce the vinyl chloride intermediate $CH_3-C(Cl)=CH-D$. During the subsequent reaction with $DI$, the deuterium ion again attaches to the carbon atom bearing the hydrogen, promoting the formation of a stabilized carbocation at the internal carbon, which is then captured by the iodide ion to yield the geminal product $CH_3C(I)(Cl)CHD_2$.

The rate of an $S_N1$ reaction is determined by the stability of the intermediate carbocation formed in the rate-determining step, where stability follows the order of methyl < primary < secondary < tertiary or resonance-stabilized carbocations. Reactivity increases as the carbocation becomes more stable due to inductive, hyperconjugative, and resonance effects, which lower the activation energy for the dissociation of the leaving group. Thus, the specific sequence (B) < (A) < (D) < (C) reflects the increasing order of carbocation stability derived from the structural features of the provided compounds.

Acidic strength in hydrocarbons is primarily determined by the stability of the conjugate base formed after the loss of a proton, which is directly influenced by the hybridization of the carbon atom involved. Carbon atoms with higher s-character are more electronegative, meaning $sp$ hybridized carbons in $HC \equiv CH$ and $CH_3 - C \equiv CH$ are more acidic than the $sp^2$ hybridized carbon in $CH_2 = CH_2$. Between the two alkynes, $HC \equiv CH$ is the stronger acid because the electron-donating +I effect of the methyl group in $CH_3 - C \equiv CH$ destabilizes its conjugate base, resulting in an overall acidity order of $HC \equiv CH > CH_3 - C \equiv CH > CH_2 = CH_2$.

The reaction follows regioselectivity principles where, during the electrophilic addition of $\mathrm{Cl}_2$ to $\mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}_2$, water acts as a nucleophile to attack the more substituted carbon atom of the cyclic chloronium ion intermediate. This attack occurs at the central carbon because it is better able to accommodate the partial positive charge developing during the transition state, resulting in the hydroxyl group adding to this central position. Consequently, the chlorine atom adds to the terminal carbon, yielding $\mathrm{CH}_3 - \mathrm{CH}(\mathrm{OH}) - \mathrm{CH}_2\mathrm{Cl}$ as the major product.

The $S_N1$ reaction mechanism proceeds through a stepwise pathway where the departure of the leaving group generates a carbocation intermediate. Because the reaction occurs in two distinct elementary stages-the formation of the carbocation followed by the subsequent attack of the nucleophile-the potential energy profile must feature two activation energy humps separated by a local energy valley. This valley corresponds to the energy state of the carbocation intermediate, identifying the double-humped graph as the characteristic representation of the $S_N1$ mechanism.

The stability of the intermediate carbocation governs the regioselectivity of this reaction, as the pathway proceeding through the most stable carbocation intermediate possesses the lowest activation energy. Following the principles of electrophilic addition, the electrophile adds to the less substituted carbon to generate this stable intermediate, which subsequently undergoes nucleophilic attack to yield the product represented in structure C.

The vigorous oxidative cleavage of an alkene using a strong oxidizing agent like hot alkaline $\mathrm{KMnO}_4$ results in the complete breakage of the carbon-carbon double bond. When but-2-ene ($\mathrm{CH_3-CH=CH-CH_3}$) reacts under these conditions, the two double-bonded carbon atoms are oxidized directly into carboxylic acid groups. This process splits the molecule into two identical fragments, ultimately yielding 2 moles of $\mathrm{CH_3COOH}$ upon acidification.

The compound 'A' is identified as propyne, which is a terminal alkyne characterized by the structural formula $\mathrm{CH}_3-\mathrm{C}\equiv\mathrm{CH}$. In typical reaction pathways involving the double dehydrohalogenation of terminal vicinal or geminal dihalides, the strong base used facilitates the removal of two equivalents of hydrogen halide to establish the triple bond at the end of the carbon chain. This terminal triple bond distinguishes propyne from internal alkynes or alkenes, granting it unique chemical reactivity such as the ability to form metal acetylides with strong bases.

Huckel's rule establishes that for a planar, cyclic system to exhibit aromaticity, it must contain $(4n+2)$ delocalized $\pi$ electrons, while structures failing to meet these specific bonding and electron density criteria are categorized as non-aromatic. Since only one of the provided examples fulfills these conditions to be aromatic, the compounds identified as B, C, and D do not meet the standard, making them non-aromatic.

The regioselective addition of electrophiles to unsymmetrical alkenes is governed by the relative stability of the intermediate carbocation, which dictates the reaction pathway. Following the formation of the more substituted and energetically favorable carbocation, a nucleophilic attack occurs at this position to minimize activation energy, preferentially yielding the more substituted, thermodynamically stable product. This mechanistic pathway ensures that the reaction proceeds to form the isomer with the greatest degree of substitution, which corresponds to the structure of the major product.

The dihedral angle, also known as the torsional angle, defines the spatial relationship between two intersecting planes sharing a common bond axis, in this case the carbon-carbon bond. Determining the angle for this specific skew conformation involves accounting for the angular deviation from a linear reference point. Subtracting the defined 29 degrees from the standard 180 degree arrangement provides the precise dihedral angle:

$180^\circ - 29^\circ = 151^\circ$

In a Friedel-Crafts alkylation, described by the general scheme $Ar-H + R-Cl \xrightarrow{AlCl_3} Ar-R + HCl$, the newly attached alkyl group serves as an electron-donating substituent on the aromatic ring. Because alkyl groups increase the electron density of the system, the resulting alkylated product is inherently more reactive and susceptible to subsequent electrophilic attack than the original starting reactant. This rapid further reaction ensures that the electrophile continuously targets the already substituted product, making polysubstitution a major, characteristic drawback of this method.

When internal alkynes undergo reduction using sodium metal dissolved in liquid ammonia, the reaction proceeds via a radical anion intermediate that allows for the stable positioning of substituents in a trans geometry. This process, commonly referred to as Birch reduction, forces the bulky organic groups to reside on opposite sides of the newly formed double bond to minimize steric hindrance during the transition state. By providing electrons to the alkyne system rather than relying on a surface-catalyzed hydrogenation mechanism, the $\mathrm{Na} / \text{liq.} \mathrm{NH}_3$ reagent effectively produces the trans-alkene isomer, whereas other methods like the use of Lindlar's catalyst typically result in the cis configuration.

Dehydrohalogenation proceeds via an $E2$ mechanism where the reaction rate is primarily governed by the stability of the forming alkene transition state, which is influenced by hyperconjugation and alkyl substitution. Tertiary alkyl halides are the most reactive in this process because the resulting alkene is more highly substituted and thermodynamically stable, significantly lowering the activation energy for the elimination reaction. Consequently, the tertiary halide substrate undergoes the elimination process most readily compared to primary or secondary isomers, which form less stable alkenes.

Determining the major product of a chemical reaction requires knowledge of the specific reactants, reagents, and mechanisms, which were omitted from the prompt. Please provide the complete reaction equation, including any relevant stereochemical or regiochemical details, so that the factors influencing the formation of the major product can be accurately explained.

The reduction of 2-butyne with $\mathrm{H}_2/\mathrm{Lindlar}$'s catalyst yields cis-2-butene (X) through syn-addition, while reduction with $\mathrm{Na}/\mathrm{liq. NH}_3$ produces trans-2-butene (Y) through anti-addition. The cis-isomer X possesses a net dipole moment because the bond dipoles do not cancel out, resulting in stronger dipole-dipole intermolecular attractions compared to the symmetric trans-isomer Y, which has a near-zero net dipole moment. Consequently, X exhibits both a higher dipole moment and a higher boiling point than Y.

The reaction proceeds via electrophilic addition, where the protonation of 3-methylbut-1-ene initially forms a secondary carbocation intermediate, $CH_3-CH^+-CH(CH_3)_2$. This carbocation undergoes a 1,2-hydride shift to rearrange into a more stable tertiary carbocation, $(CH_3)_2C^+-CH_2CH_3$, which is then attacked by the bromide ion to yield 2-bromo-2-methylbutane as the major product.

Markovnikov's rule governs the regioselectivity of electrophilic addition reactions to unsymmetrical alkenes, stating that the proton adds to the carbon atom already bonded to the greater number of hydrogen atoms to form the most stable carbocation intermediate. This reaction proceeds via the formation of this stable carbocation, which is subsequently attacked by the nucleophilic halide ion to yield the major product, prioritizing tertiary over secondary or primary carbocation intermediates due to inductive and hyperconjugative stabilization. By identifying the most stable carbocation intermediate achievable through the reaction mechanism, the structure of the final major product is consistently determined.

Since temperature and pressure remain constant, we can apply Avogadro's Law, which dictates that the volume ratio of gases participating in a reaction is equivalent to the ratio of their molar stoichiometric coefficients. The general balanced combustion equation for any alkane is:

$\text{C}_n\text{H}_{2n+2} + \frac{3n+1}{2} \text{O}_2 \rightarrow n\text{CO}_2 + (n+1)\text{H}_2\text{O}$

Because 5 L of the alkane reacts with 25 L of oxygen, the volume ratio is 5. Equating this volume ratio to the stoichiometric coefficient of oxygen from the balanced equation gives $\frac{3n+1}{2} = 5$. Solving this expression results in $3n + 1 = 10$, meaning $3n = 9$, which leads to $n = 3$. This value of $n$ confirms the alkane is propane, with the chemical formula $\text{C}_3\text{H}_8$.