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Organic chemistry
Course: Organic chemistry > Unit 3
Lesson 2: Naming alkanes, cycloalkanes, and bicyclic compoundsNaming cubane
Systematic naming of cubane. Created by Jay.
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@2:50why did he split the cube/pentacyclo ring the way that he did?(16 votes)
Because of the symmetry of the molecule, there are many ways in which he could have done it. The point is that he had to draw the largest ring possible (8 C atoms) that contained two bridgehead carbons. The particular one he chose was probably the easiest to draw.(8 votes)
Now we know how to tell the IUPAC names of different polycyclic compounds but is there an easy way to go backwards? I meant telling the compound from the name itself . I , for one , would never be able to draw the structure of Cubane from its IUPAC name. Can you please give us a video teaching us that for poly-cyclic compounds ? Thankyou(30 votes)
I think naming cubanes may be quite difficult, especially going from name -> drawing.
But is it really going to be on the test?
Is it a big part of passing the class, lab, or getting a good understanding Org??
Keep your actual goal in mind and don't waste your time.(0 votes)
The structure which we named in video @3:00is not pentacyclic because it needs only three cuts to become a chain. Why did not we choose the structure where we made 5 cuts @1:20for naming?(15 votes)- I actually had the same question. I 'm assuming that either something got mixed up or I 'm just missing something...(1 vote)
- With cubane, it seems logical to split the molecule into two rings in this exact way. But still, what rule prevents this from being named pentacyclo[3.1.1.1(2,4,6).0(3,7)]octane? How do you decide which bonds to pick for the first two rings?
(sorry for the formatting, it doesn't do superscript)(7 votes) 
I don't quite understand the way you to come up with the overall name. In this example you counted all the C and said it is OCTANE but in the previous video there were two methyl groups sticking out and you did not take them into account when you coming up with the overall name.
So my question is what and how do you count to come up with overall name for the molecule (Last part of the name)?(3 votes)- It all comes from the rules for naming.
When you name alkanes you pick the longest continuous chain of carbon atoms, and the base name comes from that chain. Then you add the names of any alkyl substituents.
When naming polycyclic alkanes, you start at a bridgehead carbon and then trace a path along the longest possible ring you can find that brings you back to your starting point. Since the largest ring contains eight carbon atoms, the name is constructed as if it were a bicyclooctane. Then you add the locations of the other bridges. Finally, you add the names and positions of any alkyl substituents. For cubane itself, there are no substituents to take into account.
Note: the fact that you can see six different cyclobutane rings does not mean that cyclobutyl will be anywhere in the name.(8 votes)
Wouldn't the longest ring between the bridgehead carbons on cubane be going through 1, 8, 3, 2, 5, 4, 7, 6?(4 votes)- But there are 6 carbons between the bridgehead carbons in the one i mentioned, opposed to the four in the video.(2 votes)
At2:08a pentacyclo with 5 rings was determined for the cubane structure by 5 cuts to cut out all the rings. But in a cube there are 6 faces, each forming an independent ring. Isn't it a contradiction? The rule seems to determine less rings than appear in the structure. Can someone clarify? Many thanks!!(4 votes)- At2:24, what does it mean to pretende if it is a bicyclic compound and why does Jay do this? The right shape does show that it should be called a butacyclo something because this is missing four bonds. It's confusing.(2 votes)
- Because that is part of the rules for the systematic naming of polycyclic compounds. You start at a bridgehead carbon and trace a path along the carbon atoms to find the largest possible ring (in this case, 8 carbon atoms). Then you locate the largest bridge between two carbon atoms of the ring (in this case, it is a bridge of zero carbon atoms) . In this case, the base name becomes bicyclo[4.2.0]octane. If there are more rings (in this case, there are five rings) you change the prefix from bi- to penta- and put the locations of the other bridges inside the brackets.
Although there are six cyclobutane rings in this molecule, the systematic name considers only the largest possible ring (cyclooctane).(3 votes)
- At2:25Jay says "we are going to pretend like it is a bicyclic compound" - why do we pretend like cubane is a bicyclic compound? Do we do this for all compounds that have more than two rings or is cubane a special case?(3 votes)
But a cube has six faces so shouldn't this molecule be hexacyclic ? I really don't understand how cuts can make a difference?(2 votes)- Since our nomenclature has defined rules, one of them is that we define the number of rings as the number of cuts it takes to create a non-ringed structure.
I know that sounds weird, because clearly there are 6 "rings" on that cube, but it only takes 5 cuts to split it into a non-ring structure, meaning we label it as a pentacyclic compound.(2 votes)
2:50Video transcript
So here's a picture of
the cubane molecule, which is an alkane that's
shaped like a cube. And for that reason, it's
one of my favorite molecules. I just think it's
really cool looking. And you can see that at each
of the corners of the cube, there's a carbon. So there are eight
carbons total. And then there's also
a hydrogen coming off of each of those carbons for
a molecular formula of C8H8. At first, a lot
of chemists didn't think this molecule could be
made because of the high amount of angle strain that's
present in this molecule. But it was made
starting in the 1960s. And it's being looked at
for a lot of potential uses in medicine and
explosives these days, and because you can
nitrate it and make things like octanitrocubane and
heptanitrocubane, which are potential explosives
for the future. And we're going to look and
see if we can name cubane using IUPAC nomenclature
in this video. So let's first think about the
rules we learned in the video on bicyclic nomenclature. And if you're
trying to figure out how many rings
are in the system, you have to make
cuts and figure out how many cuts does it take to
get to an open chain alkane. So if we start with this yellow
version of cubane over here on the left, I'm going
to start cutting bonds. And let's see how
many cuts it takes to get to an open chain alkane. For example, I could start
by cutting right here. So we'll say that's
our first cut. And then our second cut, we
could make a cut right back here like that. So we could make that my
second cut here on my cubane. And then for my third cut, I'm
going to go for this one right up here. So we'll take care of that one. So that's three cuts so far. And then, if I just go ahead
and take out of this one, this bond right here, and then
this bond right here, so that's cuts four and five. I now get an open chain alkane. So it took five cuts
for us to do that. So there are five rings in
cubane, so it's pentacyclo. So that's just not immediately
obvious, to me anyway, as to why there are
five rings in cubane. So let's go ahead
and write pentacyclo to start the IUPAC name here, so
pentacyclo, meaning five rings. And then we start our brackets,
just like we did in the video on bicyclic nomenclature. And to finish
naming cubane, we're going to pretend like it
is a bicyclic compound. And the first thing
we do is identify our bridgehead carbons. So the carbons that are common
to both of the two rings here. So hopefully it's obvious
those are two rings and those are the
bridgehead carbons that connect those two rings. When you number a
bicyclic compound, you start at one of
the bridgehead carbons and then you go the
longest path first. So I'm going to
start at this carbon, and I'm going to go the longest
path, which would be up here. So this would be
number 2, this would be number 3, carbon
number 4, carbon number 5, carbon number 6, which takes me
to the other bridgehead carbon. And then you name your
next longest path. So I'm just going to continue
around and make this carbon 7, and then make this one
back here carbon 8. So those are my eight
carbons of cubane. And so, once again,
I can continue to pretend like it's
a bicyclic molecule. And the next thing
I would do is I would name the number of
carbons in my longest path. So the number of carbons
in my longest path would be this one, so there'd
be 1, 2, 3, 4 carbons. Remember, you exclude
the bridgehead carbons when you're doing this, so we're
going to start with a 4 right here, like that. Next, you do the
number of carbons in your second longest path. So we can see my
second longest path would be this one right here. And there are two carbons
in my second longest past. So I go ahead and put a
2 over here like that. And then finally,
it's the number of carbons between the
bridgehead carbons, which in this example,
of course, there are no carbons between my
two bridgehead carbons. So I would put a
0 here like that. But of course, cubane is
not a bicyclic compound, so we have to keep going. We have to figure out how I can
continue naming this molecule. And the way to do
it is to next think about how many carbons are
there between carbons 2 and 5. So if I draw a
dashed line in here, so I can pretend like
I'm connecting those two right there. And of course, there are
no carbons between 2 and 5. So I can keep going. I could make this is 0, and
I could put a 2 comma 5, saying there are no carbons
between carbons 2 and 5. And I can continue on. I can do that
between the 3 and 8. So if I were to connect the 3
and the 8 back here like that, there are no carbons
between 3 and 8, so I can write 0,
and then 3 comma 8. And, of course, I can do
the same thing over here on the right. So between 4 and 7
there are no carbons. So I can write 0,
4, and 7, like that. So let me just clear
up that 7 there. And we're done
with our brackets. So the last thing you
need to do when you're naming a polycyclic
alkane like this is to figure out how many total
carbons are in the molecule. Well, of course, there
are 8 So this is octane. So I can go ahead and write
octane down here, like that. And I have my IUPAC
name for cubane. It is
pentacyclo[4.2.0.0(2,5).0(3,8)0(4,7)]octane. And there are several
other molecules that are similar
to cubane, which are also very interesting. So molecules like
dodecahedrane would be one to check out as well if you
like the structure of cubane.