Cyclic and Acyclic Alkanes

As I mentioned in my Functional Groups post, alkanes are hydrocarbon molecules with no π-bonds. They can be straight chains of carbons with attached hydrogens, or there can be branches or rings or both. All of the fourth chapter in my textbook is dedicated to alkanes. But the first part is just about getting acquainted with them. Alkanes are something of a baseline in organic chemistry. It's when functional groups are added that the chemical properties behind so much of our world come into play. Lacking functional groups, alkanes are not particularly reactive. They can react, though. And I know we'll come to that eventualy. There's a lot to learn from alkanes, though.

Firstly, let's distinguish between acyclic alkanes and cyclic alkanes. If it has a ring, it's cyclic. If it does not have a ring, it is acyclic. Simple, right? It better be. No, two rings is still cyclic. What counts as a ring? Oh, good question. A ring is pretty much what it sounds like. Three or more atoms bonded to each other with a loop that can be formed from the bonds between them. Carbon #1 is attached to Carbon #2 and Carbon #2 is attached to Carbon #3, which is itself attached to Carbon #1. Three atoms is the minimum, but larger rings are more common.

For an acyclic alkane, the number of hydrogens will always be two plus double the number of carbons. H = 2C+2. Actually, a little logic should demonstrate this point. No amount of branching chains changes the formula. But a single ring does. I shall illustrate with some examples. First, here is hexane...

Name: n-hexane
Molecular formula: C₆H₁₄
Skeletal structure:
Well, that's a nice, simple acyclic one. How about an acyclic alkane?

Name: Cyclohexane
Molecular formula: C₆H₁₂
Skeletal structure:
I Know I've shown this one at least once here, once upon a time. Hexagons should hopefully be pretty recognizable. And notice that it has two fewer hydrogens than the last one? That's because of the ring. What? You want to know how the ring makes it so that there are two fewer hydrogens in the molecule? Really? Look, just pretend we sever the bond between two carbons. Any two. Now those two carbons need a new bond to something else because, remember, carbon forms four bonds. So we stick a hydrogen onto each of them, and look at that, it's n-hexane, the same molecule I already showed you just before this one. Amazing. And that is why the ring makes it so that there are two fewer hydrogens than in an acyclic alkane. Simple.