Do you find it overwhelming to draw every carbon and hydrogen and bond when drawing
organic molecules?
In this video, we're gonna simplify the process and learn how to draw skeletal structures
for organic molecules.
Skeletal structures give you a simple way to quickly represent organic molecules.
Organic compounds are molecules that have carbon and hydrogen, but they can also have
other atoms like oxygen, nitrogen, sulfur and even phosphorous.
Why do organic molecules have carbon?
Carbon in its hybridized state as an sp3 molecule, has 4 valence electrons, capable of forming
up to 4 different bonds.
If an atom can bind 4 different things and say one of those things is another carbon,
and that carbon combine 3 more atoms and that combine more atoms and more atoms, you go
from something really simple to something big and complex that can ultimately support
life.
But when you want to draw this molecules, that's where it helps to know how to do it
simply.
Let's start with something simple.
For example, a molecule like butane which is C4H10.
We have many ways to represent it, for example the condensed structural formula which gives
us different groups in that molecule.
So we have a CH3 bound to a CH2, another CH2 and CH3.
If we want more information about how the atoms are bound, we can use a Lewis Structure.
A Lewis Structure gives us a little more information because we actually see exactly how the atoms
are bound to each other.
These are all very helpful except that it takes a really long time to draw it and we
are only looking at a 4-carbon chain.
Now imagine if you have to draw the same thing for a molecule like Octane, which is 8 carbons
long, or Decane which is 10 carbons long.
Don't worry about the naming for now because I cover that in a separate video series http://Leah4Sci.com/Naming.
If I want to draw these molecules, Octane is eight, we take twice as long to draw as
butane and decane at 10 carbons will take even longer.
Now imagine you're doing the mechanism and you have to draw a version of the molecule
once and again and again and again!
Not only is it gonna take so much time, can you imagine how messy your paper's going to
be?
And you're under enough pressure as it is with your exams, so you need a simpler and
easier way and that's why we have Skeletal Structure so we can quickly and easily represent
these molecules and still know exactly what's going on.
The trick with skeletal structure is that we want to make this as simple as possible
and that means we're going to assume rather than show a lot of what is going on.
You don't show any hydrogen atoms that are on carbon.
You only show the bonds between the atoms again, unless it's carbon to hydrogen, which
we don't show.
The only atoms you show are the non-carbon and hydrogen atoms: Oxygen, Nitrogen, Sulfur,
Phosphorous, and so on.
So how does this work?
If we have molecule like butane, we're not going to show the carbon atoms but we're going
to show the bonds between carbon atoms.
That means we have one, two, three, and four.
Four carbon atoms and three bonds between them.
We're not gonna show any hydrogens we're going to simply assume that they're there and so
you put pen to paper and draw a line.
This line represents the bond between two carbon atoms unless something else is shown.
And then we draw another line, and another line.
The problem is, that a lot of students count it when they draw a line rather than the first
atom you write.
So they draw a line and call that carbon 1, 2, and 3.
But that is incorrect.
When you're drawing it, remember you have fewer bonds than you have atoms.
Four carbons have three bonds between them and the trick to recognize is that every corner,
every angle, every bend represents another hidden carbon atom.
So with that same structure even though we only have three lines we have a total of 1,2,3,4
carbon atoms.
I'm showing you the dots to help you count them but I caution you not to get used to
it, it is a crutch that wastes time and even though you think it's easier now, you're going
to be hurting yourself down the line.
Imagine that those dots are there and instead of drawing them, number it.
So we have 1,2,3,and 4, that's a 4-carbon chain.
What about hydrogen atoms?
honestly, we don't care!
They're there, but we don't need to show them.
If your professor asks you how many hydrogens you have or you're trying to figure it out
for the molecular formula, use this trick.
Knowing that a carbon atom has a total of four bonds, we take the number 4 and then
we subtract the number of visible bonds.
Visible bonds means it's not connected to hydrogen, and whatever is missing is a hidden
hydrogen atom.
If we look at carbon number 1, we see one bond to carbon number 2.
4 minus 1 is 3, that right there would be our three hydrogen atoms.
Carbon number 2 has a bond to 1 and a bond to 3.
That's two bonds, 4 minus 2 is 2 and we have 2 invisible hydrogen atoms.
And again for 3, 4 has three more.
I'm showing this to you so you understand that it's there but I want you to forget the
hydrogens and simply focus on the skeleton at hand.
Let's try another example, in this case looking at pentane given a molecular formula.
First thing we do is count the number of carbon atoms: 1,2,3,4,5.
Now we need a skeleton to represent these 5 carbons and only the 5 carbons.
When you first touch pen to paper that is your first carbon, so that'll be 1,2,3,4,5.
Always, always number especially when you're just starting out to make sure you don't make
a mistake.
1,2,3,4,5 and we are good to go.
If by mistake you drew something like this, 1,2,3,4,5 notice you didn't count when you
put your pen down but instead you drew the first line and then counted to 1, you will
catch this mistake when you number it, 1,2,3,4,5,6 then you'll realize that's not what I was
looking for.
That's why you always wanna double check your work and put the numbers to see what's going
on.
As a beginner orgo student and even when doing it with advanced mechanisms in organic chemistry
2.
What about the hidden hydrogen atoms?
we don't care!
We know they're there, we're not asked about it so we're not worrying about it.
Let's take a minute to break it down and understand why we're drawing it like this.
Remember that carbon is our central atom and when sp3 hybridized carbon will have a bond
angle of 109.5 and a shape that is tetrahedral.
There are many different ways to draw this in three dimensions.
Here is the format that I prefer: If we have a carbon atom with 2 bonds in the plane of
the page, a line means it's not coming out or going into the page but instead it's flat
parallel to the page.
The other two will have to be in the opposite dimensions.
One will come straightforward and the other will go back.
Remember that wedge means it's getting bolder and bigger and coming out at you.
The dashes it's fading away into the distance into the page.
If we go back to our example of pentane, we can imagine that this carbon is one of the
five in the chain.
Here we have another one and again two bonds in the plane of the page, one coming forward,
one going back.
We could do this for all 5 carbons.
These are the five carbons of the molecule, that means the missing bonds have to be hydrogen
atoms.
Hydrogens were typically invisible in line structure but we're showing them here to complete
the molecule.
Notice that when we have the carbons at 109.5, it's kinda difficult to show that on paper
so we've settled for approximately 120 degree bond angle.
The hydrogens are going in and out of the page but we keep it simple, we specifically
chose all the atoms in the plane of the page to represent the carbon bonds and anything
going forward or back to be the hydrogens because they're difficult to draw and we don't
wanna draw them anyway.
This gives us the zigzag structure that you're going to see when drawing skeletal structure.
To simplify this, we simply start with the first carbon and draw a zigzag to represent
that same molecule, faster, easier, and so much less confusing.
But don't forget when you have a sigma bond, that's a single bond there's a free rotation
about the bond.
This gives me a different way to represent the molecule.
And when that rotates, this group which is attached to the carbon that’s turning will
swing around and ultimately go towards the top of the molecule.
To represent that inline structure I start with everything that hasn't moved and then
the group that moved upward I simply draw that upward.
If I count this, 1,2,3,4,5, and 1,2,3,4,5 you’ll recognize that it's the same exact
molecule, just drawn in a different way.
In fact if you're not sure, use the highlighter trick.
If you can put your highlighter down and then keep it down as you trace the path through
the entire molecule you know that it's one continuous chain and in this structure even
though it's different I can trace that same path 1 through 5 so I know it's the same exact
thing.
Let's look at an even more complex structure: Cyclohexane.
In watching me draw this, did you get a little bit of a sense of panic?
if so, let me know in the comments below.
I find it tedious and I bet you find it tedious as well.
If I had to draw this in the exam, I could lose my mind.
Not to mention it would be messy and waste of a whole lot of time.
So what do we do?
We number it, 1,2,3,4,5,6.
I have 6 carbons attached to hydrogen.
Let's do the same thing in skeletal structure.
Let's show where the carbons are going to be.
We have 1,2,3,4,5,6.
This is a helpful trick to get an idea of where to draw the atoms when you're still
learning how to do skeletal structure.
1,2,3,4,5,6 but don't forget because this molecule is a cyclic compound, a ring, we
have to connect 6 back to 1.
Isn't that so much faster and so much easier?
Later in the course you'll gonna learn yet another way to draw a cyclohexane as chair
conformations which is something I cover in the tutorial link below.
If your molecule contains sp2 hybridized carbons, you're likely going to see a double bond.
For example, let's take a look at Trans-2-butene.
This molecule has 4 carbons but the 2 carbons in the middle have a second bond between them,
that's a pi-bond or a double bond.
Sp2 hybridized carbons have a bond angle of 120 degrees which is much easier to represent
in two dimensions.
To draw this in skeletal structure we do the same thing as with sp3 hybridized carbons.
Start by figuring out how many carbons you need in your chain.
In this case we have a total of 4 so we draw a zigzag of 4 carbons 1,2,3,4.
If you're not confident, just add the dots and count them or number accordingly.
And then, to show that double bond, well we simply show a double bond by drawing a second
line or that second bond between carbons 2 and 3.
Triple bonds is where it gets tricky.
A triple bond has an sp hybridized atom with a 180 degree bond angle.
The geometry for an sp3 hybridized atom is linear and this can get really confusing.
Let's take a look at 2-butyne.
Once again, we have 4 carbon atoms attached in a row but this time the two central carbons
have 3 bounds between them, one sigma and two pi.
One of my pet peeves is to see this drawn as an sp2 or an sp3 molecule with simply two
extra pi-bonds as a triple.
This is absolutely incorrect.
Remember, 180 degrees gives us linear molecule, it should be a straight line not a zigzag
especially at those sp hybridized carbons.
When you do this on line structure, it's counter intuitive because the molecule is simply a
line.
So how would you know where the carbons start and end?
Let's not forget our triple bond.
We have first pi and second pi.
This is the proper way to draw a triple in line or skeletal structure but I understand
that it can get confusing.
As you've just starting out, draw a little dot to remind you that there is an extra carbon
atoms sitting on other side of triple bond before the start and end of the molecule.
That means we have 1,2,3,4 carbons total.
For even more examples make sure you try the skeletal structure practice quiz linked on
the description.
Let's look at an example of working backwards where you're given a skeletal structure of
the molecule and asked to turn it into a molecular formula, a lewis structure, or any other format
where you have to show all the atoms.
The first thing you want to do is to understand what you're looking at.
And my first step, if I need to know what's going on is to number it.
I see a total of 5 carbons in a row.
Since we're not naming it, it doesn't matter how you number it, right to left or left to
right.
It's simply about understanding what's going on.
I see a total of 5 carbons so I will draw a structure that has 5 carbons and then I'll
number it so I'll know what's what when I'm comparing between the skeletal and the lewis.
On carbon 2 and carbon 3, I see a line coming out with nothing on it so I know that's a
carbon atom with invisible hydrogens so we'll draw one coming up on carbon 2, and one coming
down on carbon 3.
Now we have to fill in the hydrogen atoms.
Remember the formula, 4 minus the number of visible bonds is equal to your hydrogen atoms.
Carbon 1 has one visible bond, that means 4 minus 1 equals 3 hydrogen atoms.
Carbon 2 has three visible bonds that's one hydrogen atom.
The purple carbon on top has one visible with 3 hydrogens same with this one.
Carbon 3 has three visible, 1 hydrogen.
4 has two visible, 2 hydrogen and 5 has one visible, therefore 3 more hydrogen.
If you want to verify, quickly count the total number of bonds on each carbon atom 1,2,3,4.
1,2,3,4 and so on to verify that it's correct.
If you're asked to condense this even more, just count your groups and then draw them
out.
But instead writing bonds, take every group as carbon followed by the attached hydrogen.
So for example, we're starting with the C bound to 3 hydrogens that's a CH3.
The next carbon is a CH2, the next carbon is a CH but it also has a CH3 coming off it
so that would be CH parenthesis CH3 showing that the CH3 is a substituent off that carbon.
Same thing for the next one, CH bound to CH3.
And finally a CH3 at the end of the molecule.
Till now we’ve looked at skeletal structures with just carbon and hydrogen but what happens
if you have another atom like oxygen, or nitrogen?
For example, how do you draw the skeletal structure from molecule like Ethanol?
Ethanol is CH3CH2OH.
If you see the carbon, you start the same way.
What is my carbon chain have?
1,2 so I draw a line that represents one carbon bound to its second one and it definitely
helps if you can number it.
The hydrogens when bound to carbon are invisible.
Oxygen is not invisible so we have to draw a bond from the skeleton of carbon to the
hetero atom that is attached so meaning non carbon or hydrogen.
In this case I draw an Oxygen.
And because Oxygen has a hydrogen that is not bound to carbon, we have to show that
hydrogen atom.
So what do we have here?
2 carbon atoms and then a bond going to oxygen and hydrogen.
This is where students tend to make a mistake.
They count this and they say Oh, that's 1 carbon, 2 carbon, 3 carbon.
That is not a third carbon atom.
If that line terminates in nothing meaning the line ends that's a carbon atom.
If the line terminates and there's another atom visible, it's not an imaginary carbon,
it's not a self understood carbon, it's a very very non imaginary, visible atom in this
case oxygen.
Let's look at another one.
Say we are given Butanamide which is CH3CH2CH2, C double bound to O, NH2.
We treat this the same exact way.
It doesn't matter what is on that carbon atom, count your carbon atoms and start there.
We have 1,2,3,4.
So we draw a skeleton 1,2,3,4.
Number it to make sure all your atoms work, count to 4, forget your hydrogens unless they're
not sitting on carbon and then add in everything else.
We have a double bond between carbon and oxygen so from that imaginary carbon at the end of
the line, we draw double bond and add oxygen because it's not imaginary, we have to show
that.
Same thing for Nitrogen but it's only a single bond.
Nitrogen has two hydrogen atoms, that means there are hydrogens not sitting on carbon
so we draw them in as 1 and 2.
For even more practice going from molecules in any form to skeletal structure or taking
that skeleton and converting it back, be sure to try the practice quiz on my website leah4sci.com/Skeletal
or visit the link in the description