Structural (constitutional) isomers | Structure and bonding | Organic chemistry | Khan Academy
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Structural (constitutional) isomers | Structure and bonding | Organic chemistry | Khan Academy

August 24, 2019


– [Voiceover] Let’s
say we’re asked to draw all the structural isomers that have the molecular formula C5H12. The word “isomer” means same parts. And so we’re talking about
the same number of atoms. All of our structural
isomers are gonna have five carbons and 12 hydrogens. Our isomers are gonna differ in how those atoms are
connected to each other. So they differ in terms
of their structure. And that’s why we call
them structural isomers. We can also call them
constitutional isomers. So we need five carbons. So for our first isomer we could just draw five carbons in a chain. So here are my five carbons in a chain. And you should have already seen the video on bond line structures
before you watch this one. So let’s draw those five carbons and let’s double check and make sure we have the correct number of hydrogens. The carbon on the far
left has three hydrogens, so here we have our three hydrogens. Next carbon has two, same with the next carbons, so two for this one,
two for the next carbon, and finally three hydrogens
for the last carbon. So let’s count up everything and make sure we have to
correct molecular formulas. We have one, two, three,
four, five carbons. So that’s C5. And then we should have 12 hydrogens. Here’s three plus two gives us five, plus two gives us seven, plus two gives us nine, and then we have three
more for a total of 12. So, C5H12 is the molecular
formula for this compound. Let’s draw another structural isomer that has the same molecular formula. So instead of drawing
five carbons in a chain now we have to draw four. So let’s start by drawing four carbons. We need a total of five carbons so we need to show the fifth carbon branching off of our chain. So we could show the fifth carbon branching off of our chain here. Let’s draw in those five carbons. So here we have our five carbons. Let’s count up hydrogens. Carbon on the left has three, so three hydrogens here. Three hydrogens on this top carbon. There’s only one hydrogen on this carbon, two hydrogens on this one, and finally three
hydrogens on this carbon. So let’s count up our atoms. So let’s use red for this one. We have one, two, three four, five carbons. So that’s C5. And then for hydrogens we have three here plus three gives us six, plus one gives us seven, plus two gives us nine. And three more for a total of 12. So C5H12 is the molecular
formula for this compound. So these two drawings represent two different molecules. Both these molecules have
the molecular formula C5H12. But they differ in terms of
how those atoms are connected. They differ in terms of their structure. So we call them structural
isomers of each other. All right, to draw
another structural isomer, some students might say,
“We could start with “four carbons in our chain again.” And this time, instead of showing a
branch off of this carbon, we could show a branch off of this carbon. And so a student might draw this structure and say, “Okay, there’s a
different structural isomer.” But actually these are
just two different ways to represent the same molecule. If you analyze that second
structure that we just drew the connections are the same. We have a CH right here bonded to a CH3, bonded to a CH3, and bonded to a CH2. And the CH2 is bonded to a CH3. That’s the same structure as
what we drew out over here. So it looks like it’s
a different structure. It’s a different drawing
than the one up here, but actually this is
just two different ways to represent the same molecule. So we have two structural isomers so far. Let’s think about one more. So we can no longer do
four carbons in our chain so we go down to three carbons. So we start with three
carbons in our chain. We know we need a total of five carbons. So we need to show two more
carbons added to our chain. And these would have to
add those two carbons to our central carbon like that. Let’s draw out all of our carbons here. And let’s add in our hydrogen. So this carbon would have three hydrogens same with this carbon. And the same with this one, and finally the same for this carbon. The carbon in the center, this
carbon in the center here, already has four bonds. So it doesn’t have any hydrogens on it. Let’s count up everything. Let’s count our carbons first, one, two, three, four,
five carbons, so C5. And then we have three hydrogens plus three is six plus three is nine plus three is 12. So C5H12 is the molecular formula for this compound. And this is another structural isomer. So it’s a different
molecule from the other two. So we have a total of
three structural isomers that have the molecular formula C5H12. Now let’s draw all of
the structural isomers that have the molecular formula C3H8O. And we’ll start with the molecule we talked about in the
bond line structure video, so that molecule look like this. We have three carbons
and then we have an OH coming off of the central carbon. Let’s expand that out and make sure that this has the correct
molecular formula. We have our three carbons. And on the middle carbon we have an OH. So an oxygen bonded to a hydrogen. I’ll go ahead and put lone pairs of electrons on this oxygen. How many hydrogens do we need to add to the carbon on the left? Well, we need to add three hydrogen. So we go ahead and draw
in those three hydrogens. The carbon in the center
already has three bonds so it needs one more
so we add one hydrogen to that carbon. And the carbon on the right
needs three hydrogens. So let’s count everything up now. So we’ll start with our carbons. We have one, two, three carbons. So that’s C3. We have three hydrogens
here and three here, so that’s six plus one is seven, and don’t forget about the
hydrogen on the oxygen for eight. So we have eight hydrogens. And obviously we have one oxygen here. So I went ahead and put in lone pairs of
electrons on that oxygen. So the molecular formula for this molecule is C3H8O. And if I number this, if
I said this was carbon 1 and this was carbon 2,
and this was carbon 3, that helps us to draw the
next structural isomer because we could think about instead of that OH group
coming off of carbon 2, what if that OH group
came off of carbon 1? And so let’s draw out
our three carbons here. And now we put our OH group
coming off of carbon 1. And let’s expand this out and draw the Lewis dot structure and make sure that this has the correct molecular formula. So we have three carbons, again, in a row. And then the carbon on the left is bonded to the oxygen. The oxygen is bonded to a hydrogen. I’ll put in lone pairs of
electrons on the oxygen. Now we need to add in
carbon hydrogen bonds. So this carbon needs two. The next carbon also needs two. And the carbon on the
end would need three. So that’s one, two, and three. When we add everything up let’s use blue for that, that’s one, two, three carbons. We have C3. We have three hydrogens
here, plus two is five, plus two is seven, and one here is eight. So C3H8. And then, of course, our oxygen. So C3H8O is the molecular formula. Next. Some students might think, “Okay, well, “we put an OH coming off of carbon 1 “but what if I put an
OH on the other side?” So, over here on the other side. So let’s see what would that give us. If I put an OH coming off of that carbon, hopefully it’s obvious that these two represent the same molecule. There’s no difference in terms of how those two are connected structurally. So this is the same molecule, so two different ways
to draw the same one. So this is not a new structural isomer. Just a new way of
looking at this molecule. Now let’s draw one more. So we can’t put the OH
on the other carbon. So now we have to figure out
something else that we can do. Well, we could, this time, put two carbons in a row and
put an oxygen in between, so putting an oxygen to
break up our carbon chain. So now this would be
carbon bonded to carbon bonded to oxygen, bonded to carbon. And then we fill in our hydrogen, so there would be three on this carbon. There would be two on this carbon. There would be three on this carbon. And I could put in lone pairs of electrons on the oxygen like that, and can everything up. So we have one, two, three carbons, so that’s C3. We have three hydrogens plus two is five, plus three is eight. So we have the H8. And then, of course, the one oxygen. So this is another structural isomer. Again, some students might say, “Well, we could go like this,” and this would be yet another
structural isomer like that. But really this is just another
way to draw this molecule. So it’s not a new structural isomer. It has the same connections. So we have a total of
three structural isomers that have the molecular formula C3H8O. And as you go further in organic chemistry you’ll learn that the first
two isomers we talked about, so this one and this one, the ones that have an OH on it, those are called alcohols. And the last structural
isomer is called an ether. So we’ll worry about that
more later in other videos.

Only registered users can comment.

  1. what about when there is multiple heteroatoms?
    i have a the formula C4H8O
    and one of the isomers is cyclic.
    how do i find cyclic isomers?

  2. Was hoping for help figuring out number of isomers at speed, with tips such as looking at symmetry in the molecule? Good beginners intro though.

  3. Why can the 3rd isomer for C3H8O be broken by the carbon chain by an oxygen, but it wouldn't work for something such as C3H7Br where it is deemed a new molecule?

  4. thank you for this video, you're better than my teacher but it would have been better if you had named every molecule

  5. Isn't the molecule shown at the end of video a ketone, for it has two alkyl groups at the ends of oxygen?

  6. Dumb question I'm sure but… is there an explanation for why an alkyl can be attached to the end carbons? I'd like to know the explanation

  7. Cool I subscribed and liked and even downloaded khan Academy my dad suggested me this app it is the worlds best study app

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