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- Solutions 20

The general molecular formula for alkanes is * C _{n}H*

(a) Cycloalkanes

Hey guys. So for this question, we're looking at a cycloalkane and the general formula for regular alkane is this CNH2N plus 2, what do you think the molecular form is going to be or the general formula for a cycloalkane. Now, one that now has a ring in it? Well, let's compare two different alkanes, we can say we have one such as C6H14 and that's going to look like this, we have 1, 2, 3, 4, 5, 6 carbons and then we have double the number of hydrogen's plus 2, okay? Now, for say a cyclohexane if this was a regular hexane what the molecular formula you think be if we now combine this and make it into a ring, let's take a look. So, if you have the six carbon ring that looks like this, what is the formula going to be? Well, we still have six carbons but I want you guys to remember that when we now have a ring structure that's going to give us an IHD of one and what that means is we have minus 2 hydrogens. So, our molecular formulas is going to be CNH2N, okay? So, how many hydrogen's do you guys expect to have here? Well, our n is going to be equal to 6, right? And now if we do 2 n is going to be 2 times 6, so we should expect 12 hydrogens I ring. So, C6H12. So, just remember that when we have a ring in our molecule that's going to decrease the hydrogens by 2, so the molecular formula that we expect for a cycloalkane is going to be CnH2n, so I hope that makes sense. Now, let's move on.