MCQ
There is no ring strain in cyclohexane, but cyclobutane has an angle strin of $9^\circ 44\ ′$. If $\Delta\text{H}_\text{C}^\circ $​ of cyclohexane per $\ce{(CH_2​)}$ group is $660\text{ kJ mol}^{-1}$ and $\Delta\text{H}_\text{C}^\circ $ of cyclobutane is $2744 \text{ kJ mol}^{-1}$, what is the ring strain in $\text{kJ mol}^{-1}$ of cyclobutane?
  • A
    $-104$
  • $104$
  • C
    $-2084$
  • D
    $2084$

Answer

Correct option: B.
$104$
The $\Delta\text{H}_\text{C}^\circ $​ value of cyclobutane is given. From this if we substract the $\Delta\text{H}_\text{C}^\circ $ value of cyclobutane calculated when there is no angle strain, we will get the value of ring strain of cyclobutane.
The $\Delta\text{H}_\text{C}^\circ $​ value of cyclobutane when there is no ring strain can be calculated from $\Delta\text{H}_\text{C}^\circ $ value of cyclohexane per $\ce{CH}_2$ group.
Ring strain $= (\Delta\text{H}_\text{C}^\circ $ of cyclobutane $- 4 \times \Delta\text{H}_\text{C}^\circ $​ of per $\ce{(CH_2​)}$ group of cyclohexane $) $
$= 2744 - (4 \times 660) = 104 \text{ kJ mol}^{-1}$.

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