Alright, so it turns out that in addition to being branched, alkanes can also form cyclic structures, whereas the carbons sort of form a circle, or a different shape, where they come back and meet on each other. So the first, the smallest cycloalcane, and that's what they're called, that kind of curve is with three carbons. So instead of three carbons being in a straight chain, they actually form a triangle like this. And each of those carbons still needs two hydrogens, and so they're each bonded to two hydrogens. You get four bonds, always four bonds, the carbon. And we can even draw the skeletal form formula for this, or the skeletal structure, and it looks like a triangle. And of course we can go to four, which will be four carbons connected to each other. And we name these by just using the cyclo prefix for whatever alkanes they are. So these are cyclo, pentane, and then of course cyclo mutane. And it turns out that when carbons are sort of pushed together together like this, they actually create a little bit of instability. So cyclo pentane and cyclo butane are really reactive, and you don't see those very often. But once you get past the four carbons, they become much more stable, and you see them pop up in terms of organic chemistry, and even eventually biological chemistry, biological molecules. So a five carbon ring would look like this, and of course that would be cyclo pentane. And there are five carbon ring structures all around, very useful, and very common. And then of course we can even go up to higher ones, but we'll stop talking at cyclohexane. The cyclohexane is a very common organic solvent used to dissolve nonpolar substances because it's a nonpolar solvent. Okay, so that's cycloalkanes, and of course we could start introducing double bonds into the scenario, and even triple bonds, although double bonds are much more prevalent for cyclic structures. A very important cyclic structure with double bonds is benzene, and that sets up a new category of hydrocarbons, which are known as aromath carbon. Hydrocarbons. Okay, benzene is a six carbon cyclic structure, six carbons connected, and cyclic structure with alternating bonds, alternating double bonds. Okay, and so what this would look like is, we've got carbon, six carbons in a ring, and they've got alternating double bonds, so every other carbon has a double bond, or the double bond, every other bond. And so we know that each carbon has four bonds, so every carbon is also connected to one hydrogen, like so. And so the formula for benzene is C6, H6. Of course we could draw the skeletal form of this structure, and it would look like this. First we would draw the ring structure, and then we would put double bonds. Every other bond is a double bond. It turns out that these double bonds resonate or move around this hydrocarbon to create what is known as an aromatic ring, and so it goes back and forth between these two structures, where the double bond, this double bond would appear, this one moved down here, and last double bond moved over here. This happens all the time, and so other ways we can write this resonance structure is that we can draw dotted lines between the carbons, to show that the double bond is constantly moving around. And then we can even write it a little more simply as a circle in between that hexagon. And so that is the skeletal form of benzene. C6, H6 with alternating double bonds that are constantly moving, and so that's how we typically draw benzene. Now, when benzene is connected to any other carbon, what we call it is an aromatic ring. And so if we were to draw this molecule, got a benzene, and then off of this, instead of being connected to a hydrogen, it's connected to a, will be that one, two, three, a propyl group. This would be an aromatic ring on this hydrogen carbon. So essentially when we start talking about functional groups, aromatic rings are a functional group unto themselves. So anytime you see this hexagonal structure with a circle in the middle, really what they're trying to show is that it's a benzene, and if it's on a molecule, we call it an aromatic ring.