Question

In Baeyer strain theory, which cycloalkane is more stable?

• A. Cyclopropane
• B. Cyclobutane
• C. Cyclopentane
• D. Cyclooctane

Hint:

Remember: Cyclopentane has minimal angle strain and is considered most stable according to Baeyer's angle strain theory — important for understanding ring stability in GPAT.

Answer 1

The Correct Option is C

- Baeyer Strain Theory (proposed by Adolf von Baeyer) explains the relative stability of cycloalkanes based on angle strain. According to this theory, the most stable ring systems are those in which the bond angles are closest to the tetrahedral angle of \( 109.5^\circ \).
- The ideal bond angle in a tetrahedral molecule is \( 109.5^\circ \), and any deviation from this causes angle strain.
Let’s analyze the ring strain in each option:
- (A) Cyclopropane: Bond angle = \( 60^\circ \) → High strain
- (B) Cyclobutane: Bond angle = \( 90^\circ \) → Considerable strain
- (C) Cyclopentane: Bond angle = \( 108^\circ \) → Closest to 109.5° → Least strain and most stable
- (D) Cyclooctane: Angle strain is reduced, but suffers from torsional strain due to its flexible ring, making it less stable than cyclopentane
- Hence, Cyclopentane is considered the most stable cycloalkane as per Baeyer strain theory.

Answer 2

Baeyer strain theory judges the stability of a cycloalkane by how close its internal bond angle sits to the ideal tetrahedral angle of about 109.5 degrees, since any deviation from this angle puts strain on the ring. Comparing the internal angle of each ring given in the options against this ideal value picks out the most stable one.

1. Cyclopropane: This three membered ring forces an internal angle of only 60 degrees, a huge deviation of almost 50 degrees from the ideal, which packs a large amount of angle strain into the ring and makes it the least stable of the four.
2. Cyclobutane: This four membered ring has an internal angle of 90 degrees, still about 20 degrees away from the ideal angle, so it also carries considerable strain, though less than cyclopropane.
3. Cyclopentane: This five membered ring has an internal angle of about 108 degrees, extremely close to the ideal 109.5 degree tetrahedral angle. This small gap means almost no angle strain, making it the most stable ring among the four according to Baeyer's theory.
4. Cyclooctane: Baeyer strain theory treats rings as flat, and by that flat model, an eight membered ring would have larger internal angles that overshoot the ideal value again. Even setting that aside, real cyclooctane suffers from torsional strain caused by eclipsing interactions around its larger, flexible ring, which keeps it from being the most stable option.

Measuring how far each ring's internal angle strays from the ideal tetrahedral angle shows cyclopentane sitting closest to that ideal, with the least angle strain of the four rings.

So the correct answer is Cyclopentane.