NEET Chemistry: Alkanes conformations and halogenation Practice Questions
15 free multiple-choice questions on Alkanes conformations and halogenation (Hydrocarbons) for NEET Chemistry, each with the correct answer and a step-by-step explanation. Attempt a question, then reveal the solution to check your reasoning.
Practice thousands more NEET questions in the app →Q1. The staggered conformation of a cyclohexane ring is generally preferred over the eclipsed conformation because it minimizes which form of strain?
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Correct answer: D — torsional strain
In cyclic systems like cyclohexane, the staggered conformation minimizes torsional strain (eclipsing interactions) and geometric strain, leading to greater stability.
Q2. During free-radical halogenation of an alkane, the halogenation always occurs at which type of carbon atom if the starting material is an unbranched alkane?
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Correct answer: B — Primary carbons
Free-radical substitution preferentially attacks the weakest C-H bond, which is typically found at primary carbon atoms in unbranched alkanes, leading to the primary halogenated product.
Q3. For a simple linear alkane, the ideal bond angle is approximately:
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Correct answer: B — 120°
The ideal tetrahedral bond angle for carbon atoms in unstrained alkanes is approximately 109.5 degrees.
Q4. Which conformation of an alkane possesses the lowest potential energy?
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Correct answer: A — Staggered
The staggered conformation minimizes torsional strain (eclipsing interactions) and is thus the most stable conformation of an alkane.
Q5. In the free-radical halogenation of a saturated alkane, the substitution reaction typically occurs at which type of carbon atom?
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Correct answer: A — primary carbon
Free-radical substitution reactions generally follow the stability order: tertiary > secondary > primary. Primary carbons are generally less reactive.
Q6. The energy difference between the anti and the gauche conformations of butane is approximately:
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Correct answer: D — 2.0 kcal/mol
The energy difference between the anti conformation (lowest energy, 0 kcal/mol) and the gauche conformation is approximately 3.8 kcal/mol (the energy difference between anti and gauche is often cited around 3.5-4.0 kcal/mol depending on the exact reference, but 2.0 kcal/mol is an approximation used in introductory settings for the primary gauche interaction).
Q7. Which conformation of butane is the most stable due to minimized steric hindrance?
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Correct answer: B — Anti
The anti conformation of butane is the most stable arrangement because it minimizes steric repulsion between the two methyl groups, resulting in zero torsional strain and minimal steric strain.
Q8. If a primary alkyl halide undergoes free-radical halogenation, the halogenation typically occurs preferentially at which position on the chain?
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Correct answer: C — Primary carbon
Free-radical halogenation involves the homolytic cleavage of the weakest bond, which generally occurs at the weakest C-H bond. Primary carbons are generally less sterically hindered than secondary or tertiary carbons, making them preferential sites for initial radical attack.
Q9. When a substituent is placed on a cyclohexane ring, the preferred conformation minimizes which type of strain?
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Correct answer: B — Steric strain
Cyclohexane prefers the chair conformation because it minimizes both angle strain (deviation from 109.5°) and steric strain (1,3-diaxial interaction).
Q10. When comparing the free-radical halogenation of an alkane, the rate of reaction is generally highest for which type of alkane?
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Correct answer: D — Alkane with only secondary carbons
Secondary carbons typically react faster than primary carbons in free-radical halogenation due to the greater stability of the intermediate radical formed at the secondary position.
Q11. The staggered conformation of butane is more stable than the eclipsed conformation because:
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Correct answer: D — less torsional strain
Staggered conformations minimize torsional strain, making it the preferred, most stable conformation due to reduced electron-electron repulsion.
Q12. Which geometrical arrangement facilitates the most favorable interaction during molecular rotation and subsequent reaction steps?
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Correct answer: A — Anti-periplanar
The anti-periplanar arrangement is crucial because it allows for the most stable and lowest energy transition state geometry, which is necessary for reactions involving bond rotation and alignment.
Q13. Which type of bond rotation leads to a change in potential energy, even in the absence of steric strain?
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Correct answer: D — Pi (π) bond rotation
Pi (π) bond rotation involves breaking and reforming π bonds, which is restricted in single bond rotation (free rotation), leading to potential energy changes based on orbital overlap.
Q14. Consider the effect of substitution on the stability of a substituted alkane compared to an unsubstituted alkane. Which factor contributes most significantly to the increased stability of a substituted alkane?
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Correct answer: B — reduced torsional strain
Substitution, especially with larger alkyl groups, introduces steric bulk but also allows for favorable interactions. The primary stability increase is due to the resulting molecular geometry and the distribution of van der Waals interactions, which often reduces unfavorable torsional strain compared to highly constrained systems.
Q15. In an alkane, the bond angles are approximately 180 degrees. Which factor primarily causes deviations from this ideal geometry?
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Correct answer: C — Steric hindrance from large substituents
Steric hindrance from large substituents forces the bond angles to deviate from the ideal 180° due to non-ideal spatial arrangements.
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