ring structure vs chain

First, BOTH n-hexane and cyclohexane are SATURATED hydrocarbons. They contain only single bonds, no double or triple ones. They do not undergo addition or hydrogenation or so.

Degree of unsaturation gives SUM of no. of double bonds, ring structures and triple bonds(x2); not meant to suggest only unsaturation.


Second, what do you mean by "stable"? Stable in what sense?
Chemically? Thermally? Addition? Substitution? Toward engine combustion? Or what?

In engine combustion, cyclohexane has octane rating of 97, while that of n-hexane is 25. Cyclohexane has higher activation energy in combustion (when mixed and compressed with air), and is less prone to knocking. In this sense, cyclohexane is more stable than hexane.

Hexane will be more stable since it is saturated but cyclohexane is unsaturated (ring structure has a degree of unsaturation = 1, just like C = C bond)