Textbook Question
Which conformation in each of the following pairs has the least strain energy?
(a)
4. Alkanes and Cycloalkanes
Ring Strain
4:29 minutesProblem 50c
Textbook Question
Cyclopropane (C3H6, a three-membered ring) is more reactive than most other cycloalkanes.
c. Suggest why cyclopropane is so reactive.
Verified step by step guidance1
Cyclopropane is a three-membered ring, which means it has significant angle strain. The ideal bond angle for sp3 hybridized carbon atoms is 109.5 degrees, but in cyclopropane, the bond angles are approximately 60 degrees.
The deviation from the ideal bond angle creates angle strain, which increases the potential energy of the molecule, making it less stable and more reactive.
In addition to angle strain, cyclopropane also experiences torsional strain due to the eclipsing interactions of the hydrogen atoms attached to the carbon atoms in the ring.
The combination of angle strain and torsional strain makes cyclopropane more reactive as the molecule seeks to relieve this strain by undergoing chemical reactions.
Cyclopropane can participate in reactions such as ring-opening reactions, where the strain is relieved by breaking one of the carbon-carbon bonds, leading to more stable, open-chain structures.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Ring Strain
Cyclopropane experiences significant ring strain due to its three-membered ring structure. The bond angles in cyclopropane are approximately 60 degrees, much smaller than the ideal tetrahedral angle of 109.5 degrees, causing increased angle strain. This strain makes the molecule less stable and more reactive as it seeks to relieve the strain.
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Bonding in Cycloalkanes
In cycloalkanes, the carbon atoms are typically sp3 hybridized, forming sigma bonds. However, in cyclopropane, the overlap of orbitals is less effective due to the ring's small size, leading to weaker bonds. This weaker bonding contributes to the molecule's higher reactivity compared to larger, more stable cycloalkanes.
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Torsional Strain
Cyclopropane also experiences torsional strain due to the eclipsing of hydrogen atoms attached to the carbon atoms. This eclipsing increases the energy of the molecule, making it more reactive. The combination of torsional and angle strain in cyclopropane results in a highly strained and reactive structure.
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