So in our continued discussion of hydrocarbons, we're now going to take a look at bond rotation and spatial orientation. Here we're going to say carbon-carbon bonds in alkanes can rotate freely, and that's because of the presence of a single bond. So here, this is a carbon and this is another carbon, and we rotate around this single bond. Now by rotating that, we're going to keep one side stationary, meaning we're not going to move it. We're going to keep this side still, so it matches up with this side. And we're going to rotate the other part, the right side. When I'm rotating, this red ball is up here, so when I rotate, it'll rotate down to where the blue is. So there it goes right there. And then when I rotate that down, this blue comes to where this part is, so there goes the blue part right there. And then that green thing has to shift up to this, to where the red was, so there's the green part right up here.
How would this look like in terms of a skeletal formula? So again, there's free rotation around the single bond, so we rotate, we keep this side stationary, so it stays where it is. Everything else is spinning. This O would rotate down to where the H is, so that's why it's here. This H would rotate to where this methyl group is, and then this, this CH3 group here would rotate up here.
Now, if we have a double bond, which is in alkenes, we cannot rotate. There's no free rotation around a pi bond, a double bond. This leads to two different spatial orientations and therefore two different compounds. These two carbons are double bonded, so we can't rotate. That means that these two blues are locked on the same side, and these two greens are locked on the same side. So it would not be equal to this molecule here, where the blues are opposites of each other and the greens are on opposite sides of each other. If we show this as a skeletal formula, these two H's are locked on the same side, but here they're on opposite sides. There's no free rotation, so these will represent two different compounds.
So just keep in mind when we're dealing with double bonds and also triple bonds, there's no free rotation around that bond. Things are locked where they are.