Textbook Question
Cyclopropane (C3H6, a three-membered ring) is more reactive than most other cycloalkanes.
c. Suggest why cyclopropane is so reactive.
4. Alkanes and Cycloalkanes
Ring Strain
Problem 16b
Textbook Question
Which conformation in each of the following pairs has the least strain energy?
(b) 
Verified step by step guidance1
Analyze the two conformations provided in the image. The first conformation shows a cyclohexane ring with a bulky tert-butyl group in the axial position, while the second conformation shows the same cyclohexane ring with the tert-butyl group in the equatorial position.
Understand the concept of steric strain: Steric strain arises when bulky groups are positioned in a way that causes crowding or unfavorable interactions with other atoms or groups in the molecule. In cyclohexane, the axial position is more prone to steric strain due to interactions with other axial hydrogens on the same side of the ring.
Recall that the equatorial position in a cyclohexane ring is generally more favorable for bulky groups because it minimizes steric interactions. The tert-butyl group, being large, will experience less strain in the equatorial position compared to the axial position.
Consider the energy implications: The conformation with the tert-butyl group in the equatorial position will have lower strain energy because it avoids the steric clashes that occur in the axial position.
Conclude that the conformation with the tert-butyl group in the equatorial position has the least strain energy, as it is the most stable arrangement for bulky substituents on a cyclohexane ring.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Conformational Analysis
Conformational analysis involves studying the different spatial arrangements of atoms in a molecule that can be interconverted by rotation around single bonds. This analysis helps in understanding the stability of various conformations, as some arrangements may lead to steric hindrance or torsional strain, affecting the overall energy of the molecule.
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Strain Energy
Strain energy refers to the energy stored in a molecule due to the distortion of its normal bond angles and lengths. In organic molecules, factors such as steric strain (from repulsion between atoms) and torsional strain (from eclipsing interactions) contribute to the overall strain energy, influencing the stability of different conformations.
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Stability of Conformations
The stability of conformations is determined by the balance of various types of strain energy present in a molecule. Generally, conformations that minimize steric and torsional strain are more stable and have lower energy. Identifying the most stable conformation involves comparing the strain energies of different arrangements to determine which has the least energy.
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