Hydrocarbons – NEET UG ChemistryPractice Questions & PYQs

The stability of isomers with the molecular formula $C_4H_8$ is primarily determined by the degree of steric hindrance between the substituents attached to the carbon-carbon double bond. In trans-2-butene, the two methyl groups are oriented on opposite sides of the structure, which minimizes the repulsion between them compared to the configuration in cis-2-butene. This arrangement results in the lowest potential energy, making it the most stable isomer in the group.

Kolbe's electrolysis is a highly effective method for synthesizing symmetrical alkanes by subjecting an aqueous solution of sodium or potassium salts of carboxylic acids to an electric current. During this process, the carboxylate ions undergo oxidation at the anode to form alkyl radicals, which then dimerize to yield a hydrocarbon product alongside carbon dioxide, as represented by the reaction $2\mathrm{RCOO}^{-}-2e^{-} \rightarrow \mathrm{R}-\mathrm{R} + 2\mathrm{CO}_{2}$. Because this pathway provides a direct and efficient route for coupling alkyl groups, it stands out as the primary method for producing these specific hydrocarbon products in good yields.

Geometrical isomerism occurs when the substituents attached to the carbons of a double bond are held in fixed spatial positions rather than rotating freely. In the molecule 2-butene ($CH_3CH=CHCH_3$), the presence of the double bond creates a rigid structure because the pi-bond prevents the atoms from spinning relative to one another. Because this rotation is restricted, the substituents are locked into place, which allows for the existence of distinct geometrical arrangements.

Acid-catalyzed hydration of 3,3-dimethyl-1-butene proceeds via the formation of a carbocation intermediate, which undergoes rearrangement to achieve higher thermodynamic stability. Protonation of the double bond initially yields a secondary carbocation, which rapidly undergoes a 1,2-methyl shift to form a more stable tertiary carbocation. Water then attacks this tertiary carbocation to yield 2,3-dimethyl-2-butanol as the major product.

When analyzing the rotation about the carbon-carbon single bond in $C_2H_6$, the molecule occupies various conformations to minimize steric repulsion between hydrogen atoms. The staggered form represents the most stable orientation, where the hydrogen atoms on one carbon are positioned in the gaps between those of the neighboring carbon. This geometry places the hydrogen atoms at a dihedral angle of 60 degrees relative to each other.

Electrophilic aromatic substitution reactivity is governed by the electron density of the benzene ring, which is altered by the electronic effects of substituents. The $-CF_3$ group acts as a strong electron-withdrawing agent that deactivates the ring, making it significantly less reactive than the parent benzene molecule. Conversely, $-CH_3$ and $-OCH_3$ groups increase electron density, thereby activating the ring, with the $-OCH_3$ group exerting a more significant activating effect than the $-CH_3$ group. This results in an increasing order of reactivity from the deactivated system through benzene to the successively more activated alkyl and alkoxy derivatives.

Hydroboration-oxidation is the characteristic reaction sequence used to achieve the anti-Markovnikov hydration of an alkene, which is necessary to convert propene into 1-propanol. Unlike standard acid-catalyzed hydration or oxymercuration methods that typically direct the hydroxyl group to the more substituted carbon, treating propene with $\mathrm{B}_2\mathrm{H}_6$ produces an intermediate that, upon oxidation with $\mathrm{H}_2\mathrm{O}_2$ and $\mathrm{OH}^-$, places the alcohol group at the terminal position.

The reaction begins with the electrophilic addition of hypochlorous acid to the double bond of $\mathrm{CH}_2=\mathrm{CH}_2$, yielding the chlorohydrin $\mathrm{CH}_2\mathrm{Cl}-\mathrm{CH}_2\mathrm{OH}$ as the intermediate $M$. In the subsequent step, the chlorine atom in $M$ acts as a leaving group and is replaced by a hydroxyl group through hydrolysis to produce ethylene glycol, a transformation facilitated by the mild basic conditions of aqueous $\mathrm{NaHCO}_3$ acting as reagent $R$.