Question

Which cycloalkane is the most stable among the following according to Baeyer strain theory?

• A. Cyclopentane
• B. Cyclohexane
• C. Cycloheptane
• D. Cyclopropane

Hint:

Compare each ring's planar bond angle with the ideal tetrahedral angle of 109.5 degrees.

Answer 1

The Correct Option is A

Step 1: Baeyer's strain theory assumes cycloalkane rings are planar and calculates strain from how far each ring's internal bond angle deviates from the ideal tetrahedral angle of \( 109.5^\circ \).

Step 2: The internal angle of a regular planar polygon with n sides is \( \frac{(n-2)\times180^\circ}{n} \). For cyclopropane (n=3) this is \( 60^\circ \), for cyclobutane (n=4) it is \( 90^\circ \), for cyclopentane (n=5) it is \( 108^\circ \), for cyclohexane (n=6) it is \( 120^\circ \), and for cycloheptane (n=7) it is about \( 128.6^\circ \).

Step 3: The angular strain per corner is proportional to the deviation from \( 109.5^\circ \): cyclopropane deviates by about \( 49.5^\circ \), cyclobutane by \( 19.5^\circ \), cyclopentane by only \( 1.5^\circ \), cyclohexane by \( 10.5^\circ \), and cycloheptane by \( 19.1^\circ \).

Step 4: Since cyclopentane's internal angle is closest to the ideal tetrahedral angle, Baeyer's theory assigns it the least angle strain and therefore predicts it to be the most stable ring among those listed.

\[ \boxed{\text{Cyclopentane}} \]