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Studying for a test? Prepare with these 5 lessons on Alkanes, cycloalkanes, and functional groups.

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# Naming a cycloalkane

Video transcript

Let's see if we can name
this guy right over here. And so like always
we always want to look for the longest carbon
chain or the longest carbon cycle, and I think it's pretty
obvious from this picture that we have a very long carbon
cycle here, that we can start, or we can just look at this. We have one, two, three,
four, five, six, seven, eight carbons in a
cycle right over here. And so we can call
this backbone, which is the backbone of our
molecule right over here, this is cyclo octane. It's an octane in a cycle. It's an alkane because it's all
single bonds right over here. So that is cyclo octane. Now let's think about
the different groups that are attached to this
backbone right over here. So first we can look at
this one right over here, and we can ask ourselves, well,
how many carbons are there? Well, there's one, two,
three, four carbons that are attached
to the backbones. So our prefix for when we're
dealing with four carbons is bute, and it is in a cycle. So this is a cyclobutyl group. And we also have another
cyclobutyl right over here. So this is another
cyclobutyl group. Now let's think about
this one right over here. How many carbons are there? We have one, two, three carbons. So our prefix when we're dealing
with three carbons is prop, and they're in a cycle as well,
so this is a cyclopropyl group. And now we can jump
over to do these two groups right over here. We see that these are
clearly the same groups. Let's count the number
of carbons they have. They have one, two,
three, four carbons. So if we're dealing
with four carbons we would think about the prefix
bute, but this isn't in a cycle and it's not in just a straight
chain, it has this "Y" shape. And so the common
name, when we end up with this "Y" shape, when
we're attached down here, we're attached to
one carbon here, and that's attached to kind of
this carbon right over here, this is then attached
to these other two, this is an isobutyl
group, which we've seen multiple times before. And of course this one is
an isobutyl group as well. So these are both
isobutyl groups. Now the next thing we
need to think about is how we can number it. We want to go back to our
main carbon chain, which in this case is a
cycle, and think about how we want to number. And the convention
is to start numbering at the thing that is first
in alphabetical order. And when we consider
alphabetical order we think even the
"C" in cyclo and we think of the "I" in isobutyl. If this was tert
or sec you wouldn't think about the "T" or the
"S" in secbutyl or tertbutyl, but for isobutyl you do consider
the "I", for the cyclo's you consider the "C." And so when you
look at it that way, all of the "C"s come
before the "I"s, and then amongst the
"C"s these all have cyclo's, but the cyclobutyl
comes before cyclopropyl or cyclo octane. "B" comes before "P" or "O." So the thing that is
first alphabetically are both of these cyclobutyls. So we can start numbering
at where either of them attach to the main
carbon backbone. I guess we should say
those are both options, but we want to number
from the one that is going to give us the
total lowest numbering. So let's think about
that a little bit. So we could say, let's
number from here, let's make this the one carbon,
and so that we hit something else as soon as possible,
because remember we're trying to minimize
the numbers here. We might want to go clockwise
so we hit two right over here, three, four, five, six, seven. So in this situation where
do we hit interesting things? We hit interesting things
at the one, at the two, or where do we have groups
attached to the chain? One, two, four, six, and seven. So that's one option. We'll have groups attached at
one, two, four, six, and seven if we start right over there,
and if we were to go clockwise. Our other option,
is to start, and let me erase those, at this
other cyclobutyl group here on the left. Let me erase these so I don't
mess up the diagram too much. The other option
is to start here, make this the one,
number this as one, and then we would
also go clockwise to hit the cyclopropyl,
to hit something else as soon as possible. So two, three, four,
five, six, seven. So now where we are
getting interesting things? We have groups at the one
carbon, the two carbon, the four carbon, the
five carbon, and at the seven carbon. So actually this second
numbering is preferable. Both of them have something
at the one, the two, the four, and the seven, but the second
one has something at a five while the first one
had something at a six. So we would actually want to do
this numbering right over here, and this is the numbering
that we have now listed right over here. Let's now write what the name
of this molecule actually is. So once again, we start
first in alphabetical order. So we're going to start
with the cyclobutyl groups. And we have two of them,
one at the one carbon, and one at the four carbon. So we could say 1 comma 4. And since there are
two cyclobutyl groups we would say dicyclobutyl. 1, 4 dicyclobutyl. And then what comes next
in alphabetical order is the cyclopropyl. That comes before isobutyl. "C" comes before "I." That's at the two carbon. So then we can
say 2 cyclopropyl. And now we can get
to the two isobutyls. So there's an isobutyl
at the five carbon and at the seven carbon. So we can say 5, 7,
and since there's two of them we'd
say di isobutyl. 5, 7 di isobutyl. And then we just give
the name of the backbone. We've done the butyls, the
dicyclobutyl right there, the cyclopropyl, the
two isobutyls, and now we just write cyclo octane. And we are done. We have successfully
named the molecule. And this kind of fun. It's like a secret
code or something. You wouldn't expect
naming organic molecules to be, I think quite so
satisfying, but it is.