cycloalkane
/ SY-kloh-AL-kane /
A cycloalkane is an alkane whose carbon chain has been joined head to tail into a ring. Instead of two loose ends, the carbons close up into a loop — cyclopropane is a triangle of three carbons, cyclohexane a six-membered ring, and so on. Cyclohexane in particular is everywhere: it is the basic shape of countless natural products, steroids, and sugars.
Closing the chain costs two hydrogens (the two ends each give up one H to bond to each other), so a single ring with no double bonds follows CnH2n — two hydrogens fewer than the matching open-chain alkane. That missing pair of hydrogens counts as one degree of unsaturation even though there is no double bond; a ring and a pi bond each remove the same two hydrogens, which is why you can't tell them apart from the formula alone.
Rings introduce a brand-new concern: shape. Small rings like cyclopropane and cyclobutane are forced into bond angles far from carbon's comfortable 109.5 degrees, building up angle strain that makes them unusually reactive. Larger rings escape this by puckering out of flatness — cyclohexane folds into the famous strain-free chair conformation. So cycloalkanes are where the geometry of single bonds, ignored in floppy open chains, suddenly starts to matter.
Cyclohexane is C6H12, not C6H14: it has the same six carbons as hexane but two fewer hydrogens because the ring closure uses up one bond from each end.
One ring = one degree of unsaturation = two hydrogens lost, just like one double bond.
From the molecular formula alone, a ring and a double bond are indistinguishable — both reduce the hydrogen count by two. You need the structure (or a reaction test) to tell which you have.