position numbering in cyclic molecules

position numbering in cyclic molecules


Welcome to Chem Help ASAP. Let’s talk about how rings affect numbering
when we name molecules. As it turns out, when you have a ring – all
these structures have some kind of cyclohexane or cyclohexene – when you have a ring, numbers
are not required for your top priority group. This structure. There’s no reason to call it 1-methylcyclohexane
because wherever the methyl is, it is by definition going to be the 1-position. So, we can just call this methylcyclohexane. We would never do this for hexane. We’d have to say where on that chain is…
the methyl is. But, with cyclohexane, all the carbons are
equal until you put the methyl group on there, then the methyl group by default is the 1-position. So, it’s redundant to put in the 1. Let’s see what happens when we put in an alkene. The alkene becomes the top priority functional
group, so it by definition gets the 1-position. We will number this as 1, and going through
the alkene, the second carbon of the alkene is 2. That means the methyl group is at the 4-position. We do have to say where the methyl group is
because it could be other places on the ring. But, we’ll call this 4-methyl and simply say
cyclohexene. Now that may seem a little bit barren. It’s like, “I want to see the number for where
that hexene is.” But, in a ring structure, you don’t have to
specify if that functional group is your top priority functional group. It, by definition, must get the 1-position. Let’s go to our last example. Now we have an alcohol. That is now a new high priority group. So, the alcohol must take the 1-position. Then we number around as quickly as possible
to hit the alkene. So, go around in a clockwise fashion, not
counterclockwise. Now we’re going to have to give the number
for the alkene, but we don’t have to give the number for the alcohol. The alkene is at the 3-position. This is going to be a 3-cyclohexen… We don’t need a 1 there. In fact, it’s such a habit, I almost put it
in there. The dash… This is just 3-cyclohexenol. What does the 3 refer to? The 3 refers back to the alkene because we
have to say where the alkene is relative to the alcohol. We know where the alcohol is. It’s at the 1-position. Rings put in kind of some quirks into this
numbering. We want to explicitly number things but we
don’t have to, and it’s considered redundant if you do, when you number the top priority
functional group in a ring.

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