Adolf von Baeyer suggested that, since carbon prefers to have tetrahedral geometry with bond angles of approximately `109^@`, ring sizes other than five and six may be too strained to exist. Baeyer based his hypothesis on the simple geometrical notion that a three-membered ring (cyclopropane) should be an equilateral triangle with bond angles to `60^@`, a four membered ring (cyclobutane) should be a square with bond angles of `90^@` and so on. According to Baeyer's analysis, cyclopropane, with a bond angle compression of `109^@ - 60^@ = 49^@`, should have a large amount of angle strain and must therefore be highly reactive. Cyclohexane becomes puckered to releave its strain. The angular derivation of cycloalkane is` (-11^@)`. Greater is the angular deviation more is the torsional strain.
Which among the following is most strained cycloalkane?

Options

A. Cyclopropane
B. Cyclobutane
C. Cyclopentene
D. Cyclohexane

Answer

To determine which cycloalkane is the most strained, we will analyze the bond angles and the resulting strain for each of the given cycloalkanes: cyclopropane, cyclobutane, cyclopentane, and cyclohexane.

### Step-by-Step Solution:

1. **Understanding the Geometry of Carbon:**
- Carbon prefers a tetrahedral geometry with bond angles of approximately 109.5° due to its sp³ hybridization.

2. **Analyzing Cyclopropane:**
- Cyclopropane has a three-membered ring structure.
- The bond angles in cyclopropane are approximately 60°.
- The angle strain can be calculated as:
\[ \text{Angle Strain} = 109.5° - 60° = 49.5° \]
- Cyclopropane is expected to have a large amount of angle strain due to this significant deviation from the ideal tetrahedral angle.

3. **Analyzing Cyclobutane:**
- Cyclobutane has a four-membered ring structure.
- The bond angles in cyclobutane are approximately 90°.
- The angle strain can be calculated as:
\[ \text{Angle Strain} = 109.5° - 90° = 19.5° \]
- Cyclobutane also experiences angle strain, but it is less than that of cyclopropane.

4. **Analyzing Cyclopentane:**
- Cyclopentane has a five-membered ring structure.
- The bond angles in cyclopentane are approximately 108°.
- The angle strain can be calculated as:
\[ \text{Angle Strain} = 109.5° - 108° = 1.5° \]
- Cyclopentane has minimal angle strain and is relatively stable.

5. **Analyzing Cyclohexane:**
- Cyclohexane has a six-membered ring structure.
- The bond angles in cyclohexane are approximately 120° in its chair conformation.
- The angle strain can be calculated as:
\[ \text{Angle Strain} = 109.5° - 120° = -10.5° \]
- Cyclohexane is stable and has no significant angle strain.

6. **Conclusion:**
- Comparing the angle strains:
- Cyclopropane: 49.5° strain
- Cyclobutane: 19.5° strain
- Cyclopentane: 1.5° strain
- Cyclohexane: -10.5° strain
- The cycloalkane with the greatest angle strain is cyclopropane.

### Final Answer:
**Cyclopropane is the most strained cycloalkane.**