Question

How `"degrees of unsaturation"` are associated with a `C_8H_10` formula?

Answer 1

Well the formula `C_8H_10` has 4 degrees of unsaturation ......

Explanation:

The saturated formula would be `C_8H_18`, and thus the given formula `C_8H_10` has `"4 degrees of unsaturation"`.

And each degree of unsaturation CORRESPONDS to an olefinic bond, i.e. `C=C` OR a RING JUNCTION.

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And thus the pictured `"xylene"`, as well as any one of its other 2 isomers (which are?) could correspond to the formula. And by my count `"xylene"` has `8xxC-C` `sigma` bonds, `10xxC-H` `sigma` bonds, and, formally, `3xxC-C` `pi` bonds.........

By joining and fusing rings, we could form a structure where the distinction between `sigma` and `pi` bonds are less ambiguous, however, these would be less common structures than the benzene derivative.

Note that the `"degree of unsaturation"` is a very useful criterion with which to assess organic molecules.