Bond energy of CH₄ is 396 k cal and that of ${\mathrm{C}}_2{\mathrm{H}}_6\text{ is }680$ k cal. The C – C bond energy in ${\mathrm{C}}_2{\mathrm{H}}_6\text{ in }$  K cal is ____

$$\begin{gathered} \mathrm{Methane} {\mathrm{CH}}_4 \mathrm{has} 4\mathrm{\sigma C}-\mathrm{H} \mathrm{bonds} \\ \therefore \mathrm{Bond} \mathrm{energy} \mathrm{of} {\mathrm{CH}}_4=396\mathrm{kcal}\mathrm{. } \end{gathered}$$
Bond energy of $\sigma \mathrm{C}-\mathrm{H}\text{ bond }=\frac{396}{4}=99\mathrm{kcal}$ 
$$\begin{gathered} {\mathrm{C}}_2{\mathrm{H}}_6 contain 6\sigma \mathrm{C}-\mathrm{H} bonds \mathrm{an}d \mathrm{one} \sigma \mathrm{C}-\mathrm{C} \mathrm{bond} \\ \mathrm{Bond}\mathrm{energy}\mathrm{of} {\mathrm{C}}_2{\mathrm{H}}_6=680 K\mathrm{cal} \\ \mathrm{i}.\mathrm{e}., 6\mathrm{\sigma C}-\mathrm{H} \mathrm{bonds} +1\mathrm{\sigma c}-\mathrm{c} \mathrm{bond}\mathrm{energy} =680 K cal \end{gathered}$$
$6\times 99 + 1\sigma c-\mathrm{c}\text{ bond energy }=680$
$1\mathrm{\sigma c}-\mathrm{c}\text{ bond energy }=680-594=86\mathrm{kcal}.$