Leah here from leah4sci.com and in this video we're going to look at Cis and Trans Isomerism.
Let's backtrack to alkene naming for a moment.
In a molecule like this we identify that we have 4 carbons in the parent chain giving
me but.
A pibond occurring between 2 and 3 giving me 2-ene.
For a name of 2-butene.
The number tells us what we have on the molecule but it doesn't give us that much about the
3-dimensional structure.
If I draw this out in line structure I have the option of drawing two butene this way,
or this way.
Which is correct?
The answer is that both are 2-butene, but the first one is the trans isomer and the
second is the Cis-isomer.
You've already learned that isomer comes from Iso meaning the same, and mer units.
This is when two molecules have the same molecular formula but they're connected differently
specifically we're looking at the geometric stereoisomers where geometric tells us that
there's something different in the geometry between the two molecules we're comparing.
If we try this with the molecule butane which is an alkane all single bonds, then we can
rotate the bond between carbons 2 and 3 so that the molecule at any point in time will
look like one of the two configurations.
In fact, this is what you study when looking at the newman projections which is linked
below.
But to get a geometric isomer they can't be able to move.
You should be be able to turn one into the other.
And that means they have to be locked in place.
How do you lock the molecule in place?
We have two options.
A pibond or a ring.
If I look at this molecule with the single bond between carbons 2 and 3, I have free
rotation.
If I add a pibond between the two carbons, pibonds remember are above and below the atom,
they can't move, the molecules locked in place, and the only way to rotate to bring this methyl
group down would be to break the pibond, move the methyl down, and then reform the pibond.
Let's take a look at the molecule.
Here we have an alkene with Sp3 carbon atoms.
Notice that the red and blue substituents are next to each other but because of sp3,
I can rotate and now they're opposite of each other.
And then I rotate they're next to each other again.
The substituents are not locked in place because the carbons are sp3.
If I look at a similar molecule but this time I have two sp2 carbons pibound pulling the
substituent.
The red and blue substituents are now locked in place opposite each other and if I try
to rotate I'll break the model kit.
It doesn't want to rotate.
The only thing I can do to get them next to each other is to break the bond and manually
move the substituent to the other side.
And again, now they're locked in place, can't rotate the molecule.
We have a similar situation with the cyclohexane where even though all the carbons in the ring
are sp3, notice that the carbon holding the substituents are sp3 and they can rotate.
Watch what happens if I try to move these substituents away from each other.
The ring doesn't allow it.
The ring is locked in place and this is entire thing is gonna break if I keep going which
means even though the substituents are sp3 carbons in a ring they're still locked in
place and the only way to move them from having two substituents on the same side is to break
a bond and reform it.
To identify Cis or Trans, first, identify the pibond and then look for the substituents
that are coming directly out of that pibond.
We don't care about anything else on the molecule except for the very group coming out of the
pibond on one side compare that to the other side.
Once again, identify your pibond, look at the group coming out on the one side, and
on the other side.
How do they relate to each other?
If they're on the same side, we have the Cis configuration which I like to think of as
'cisters'.
Cisters are together, they're on the same side.
If they're on opposite sides on the pibond then we have Trans.
I think of it as transfer away from one side to the other.
A note on Cis, I drew both of the substituents down.
I could just as easily have drawn the molecule this way with the two substituent pointing
up and it's exactly the same thing because I can either rotate or flip the molecule and
it'll look like this one.
To name Cis and Trans alkenes, first learn basic alkene naming from the tutorial linked
below, and then just add Cis or Trans to the beginning of the molecule.
We'll start with our cis molecule.
We have a total of four carbons, giving me a first name of but.
A pibond in the molecule giving me a last name of ene, the pibond occurs at carbon 2,
always use the lower number giving me 2-ene, and the substituents are on the same side,
they're cisters that gives me Cis.
To put the name together , Bring cis in front of the name, then drag the 2 in front of but,
giving me cis-2-butene.
Now let's try the Trans version.
Once again we have 4 carbons in the parent chain.
First name of but, a pibond starting at carbon 2 for a last name of 2-ene.
The substituents on the pibond are facing away from each other or transferred away from
each other giving us Trans.
Put the name together, we have Trans, bring the 2 in the front butene.
What if we have more than one pibond in the molecule?
We start the same way.
First we'll number to get the total lowest set of numbers, in this case 7 which gives
us the first name of hept.
We have not one but two pibonds so the last name would be diene.
Di to specify 2 and then a number for each one so we know where they are.
First one starts at carbon 2, second starts at carbon 4, giving me 2 comma 4 diene.
And last but not the least, are they Cis or Trans?
The pibond starting at 2 has the substituents facing on the same side.
Remember we're looking at just the carbon coming out of that pibond.
The pibond starting out at 4 has the substituents on opposite sides giving me Trans.
For this name, we can't simply put cis and trans on the front coz we have to specify
which pibond is cis and which pibond is trans.
For this we put a number in front of the designation.
That means we need 2 cis and 4 trans at the beginning of the name.
Putting this altogether, we have the pibond designations, 2-cis dash-trans, then we bring
these numbers out front 2 comma 4, dash first and last name, heptadiene for a final name
of 2-cis-4trans-2,4-heptadiene.
We said there are two ways to lock the geometric isomers.
The first was a pi bond, second is a ring.
With the ring, we can have different types of substituents because we're just looking
at the relationship.
In this case, the substituents on the cyclohexane would be upward down where you should remember
that a wedge tells you the substituent is up and dashes tell you the substituent is
down.
A good way to remember it is dashes and down start with a d.
If I add 2 methyl groups to this cyclohexane, you'll notice one is a wedge and one is on
a dash.
They are on the opposite sides, they are transferred away from each other.
This molecule has a methyl group trans to each other that's because one is up and one
is down.
It doesn't have to be just methyls.
What if I have 2 halogens?
say a chlorine and a bromine.
In this case I can say that two halogens are cis to each other, that's because the chlorine
is up and the bromine is up.
They're on the same side, they're like sisters together going up making them Cis.
This is covered in more detailed when you study cyclohexanes and chair conformations
which is linked below.
We know that professors don't like to keep it simple.
So let's try a trickier example.
For this molecule, start from the left and identify the configuration of each pibond.
Pause the video and see what you come up with.
Did you get them all right?
For the first pibond, we have the two substituents on the same side of each other.
They are like sisters, together making this a cis pibond.
For the second pibond, once again we have the substituents on the same side making this
a cis pibond.
And finally for the last one, this is a tricky question.
I have one substituent going up, but on the other side I appear to have nothing.
Well I don't have nothing, they're actually two invisible hydrogen atoms but when it comes
to cis and trans we don't look at hydrogen.
When you don't have something to compare up or down, the answer is neither.
It's not cis and it's not trans.
To make sure you don't fall for this on exams, just remember, a terminal alkene without any
groups, without any functional groups is going to be neither cis nor trans.
What about a scenario where you have different types of substituents like halogens, functional
groups, or more than one group.
For example, on this molecule, they identify the pibond.
On the left, my substituent is going up.
On the right, what do I use?
Do I use the methyl or the ethyl?
So if you think about it, the methyl is cis to the left side, but the ethyl is trans to
the left side, so is it cis or trans?
Once again, the answer is neither.
This is where you have to go to the e and z configuration which I'll cover in the next
video.
You can find this video along with more examples by visiting my website leah4sci.com/CisTrans.