So, up until this point, we've been looking exclusively at linear compounds
but, molecules can also form cyclic structures that we call rings
here we can see some kinds of cycloalkanes, which end up just looking like these fun shapes.
basically, all the rules are the same, but for a cyclic compound, we are going to utilize the prefix "cyclo"
to indicate that it is cyclic, but the rest is the same.
a 3-carbon ring is a cyclopropane
4, cyclobutane, cyclopentane, cyclohexane, et cetera
we would use the same prefixes indicating the number of carbons that we already know.
now let's see what happens when we're looking at cyclic compounds with substituents.
let's take a look at a simple example, here we have a cyclohexane with a methyl substituent.
things are a little bit different with a cyclic compound
compared to a linear compound because of the symmetry
now, there's only one group here and it doesn't really matter where it is
because all six carbons on a cyclohexane are identical.
so this is simply methylcyclohexane, we do not have to indicate the location of the methyl group
because if it were on any other carbon, we could simply rotate it and it would be the same molecule.
so this is methylcyclohexane, that is sufficiently unambiguous.
however, when there are multiple substituents, we are going to have to use a numbering system
that is a little bit different from a linear molecule
because we no longer have a situation where it is left to right or right to left.
now there are actually 12 possible ways to number this ring, because we could start
on any of the six carbons, and then go either counterclockwise or clockwise.
however, we will choose the numbering system that gives the lowest set of locants possible
for the substituents that are present.
so let me show you what I mean, basically, let's say arbitrarily if we began here with carbon 1
and then went clockwise, this would be 1, 2, 3 and 4.
so we would have substituents on carbons 1, 3, and 4.
let's say we started here, and went counterclockwise. we would have a substituent on 3, 4, and then 6.
so you can see how there are many possible sets of locants, but the one that offers the lowest set of locants
is if we begin here and go counterclockwise
now the locants are 1, 2, and 4. so it doesn't matter what the substituents are
at least if they are alkyl or halogen
those are equal priority. so here, 1, 2, and 4 is the lowest set of locants possible.
then, by a separate algorithm, we are going to just list them alphabetically as we always do.
4-ethyl is going to be first
then actually isopropyl is before methyl because iso is the only prefix that is taken into account for alphabeticity
so that's 1-isopropyl
and then 2-methyl
cyclohexane
so once again, we chose 1, 2, 4 as the numbering system because that is the lowest possible
set of locants for those substituents, then we simply list them alphabetically as we normally would.
so we just looked at some cyclic compounds with alkyl substituents, but what if we were looking at
cycloalkenes or cycloalkynes, or cyclic compounds with hydroxyl groups.
these are the substituents that take priority over alkyl and halogen in terms of the numbering scheme.
let's look at how we're going to apply prioritization for cyclic compounds.
here we have a cycloalkene and the presence of the double bond means that we have to
give prioritization to that functional group in terms of the numbering scheme.
so when we have a double bond present in a cyclic compound, the two carbons that are participating
in the double bond are going to be carbons 1 and 2 in some order.
we have two options, either carbon 1 and 2, or 1 and 2.
we will choose one of those two directions so as to give any other substituents occurring sooner.
so we are going to have to number this 1, 2, 3, 4, because then the bromo group will occur on carbon 4
as opposed to 1, 2, 3, 4, 5, occurring on carbon 5.
so that is why we will number it this way
and so now, we are going to list the substituent, we have 4-bromo
but then we can simply say cyclohexene
and that is sufficiently unambiguous because it is a cyclic compound so we are
talking about a cyclohexene, and listing the location of the other substituent in relation to the double bond
so that is sufficiently unambiguous without saying where the double bond is
it is implied that the double bond is between carbons 1 and 2, because that's the convention we've used
to number the ring.
let's look at another compound with a hydroxyl group.
as you would expect, this hydroxyl group is going to take priority over the alkyl group in terms of numbering
so that means that the carbon that bears the hydroxyl will be carbon 1.
we're going to choose the direction to number so as to give the other substituent occurring sooner.
in this case, it's going to be clockwise because that will give the ethyl group on carbon 3 instead of 4 if we went the other way.
once again, we're going to say 3-ethyl and again it is implied that the hydroxyl occurs on carbon 1
so we can simply say cyclopentanol.
3-ethylcyclopentanol.
thanks for watching, guys. subscribe to my channel for more tutorials
and as always, feel free to email me with questions