Q.68. - Organic Chemistry

Question

Question: Cyclopropane and cyclobutane have similar strain energy despite the fact that the C-C-C bond angles of cyclopropane are much smaller than those of cyclobutane. Suggest an explanation for this observation, considering all sources of strain discussed in Chapter 4.

Step-by-Step Solution

Answer

Answer

The strain energies of cyclopropane and cyclobutane are similar because cyclobutane has low angle strain and high torsional strain.

 

Step 1: Angle strain

When the angle is less than the desired angle, calculated from the hybridization of the compound, the energy of the compound rises due to increased instability. This phenomenon is called an angle strain.

For example, according to the $\mathit{s}{\mathit{p}}^{\mathbf{3}}$ hybridization of carbon atoms, the bond angle between the carbon and its four attached groups must be $\mathbf{109}\mathbf{·}{\mathbf{5}}^{\mathbf{0}}$ due to tetrahedral geometry.

When an angle in any compound with $\mathit{s}{\mathit{p}}^{\mathbf{3}}$ carbon is smaller than $\mathbf{109}\mathbf{·}{\mathbf{5}}^{\mathbf{0}}$ , there arises an angle strain.

Step 2: Torsional strain

It is also called an eclipsing strain, and this strain leads to an increase in energy due to eclipsing conformation. 

Step 3: Explanation

The interior bond angle of cyclopropane is ${60}^0$ , which is too low than the desired angle, i.e., $109·5^0$. Due to this, the energy of cyclopropane rises to a great extent.

The interior bond angle of cyclobutane is ${90}^0$, which is only $~{19}^0$ less than the expected angle. Hence, the angle strain of the cyclobutane ring is low.

However, cyclobutane has an eclipsing conformation due to which the energy of the molecule rises, and one of the carbon atoms from the ring puckers up. The puckered ring of cyclobutane has angles less than ${90}^0$, which increases the angle strain in cyclobutane. 

Hence, the ring strain energies of cyclobutane and cyclopentane rise and attain almost similar values,that ar., 27 kcal/mol and 26 kcal/mol, respectively.

Cyclopropane ring with high angle strain

Puckered cyclobutane ring with an interior angle less than ${90}^0$