I made this video to
try and help some people understand how cis and trans
geometric isomers work.
I'm hoping it will be useful to some people, and
before we begin, some definitions. Structural isomers have the same
molecular formula but a different structural formula.
There are two examples of structural isomers here.
If you look at them, the sequence of atoms within the structures
are different, even though the molecular formulas are the same.
Geometric isomers are different.
They will have the same molecular and structural formula,
but a different arrangement of atoms in space.
Before we start
talking about alkenes and their geometric isomers,
I think a good starting point is a simple
alkane. This is 1,2-dichloroethane. It's
got single bonds between the two carbon atoms,
and if this was the structure of the molecule,
I could draw a picture of it. And,
it would look, in terms of fully expanded,
like the top diagram above. I've got two chlorines
up top, two hydrogens on the bottom and there's a hydrogen left and a hydrogen right
that are slightly obscured because of the geometry.
The top diagram is a expanded structural formula.
It shows exactly, all the bonds in the structure where the bottom one is a condensed
structural formula. It still shows us the sequence of all the atoms in the molecule, but
it's much, much faster to write.
The thing about this model is that
the molecule could be viewed from many different angles.
And, if I want to view it from say, this angle, I would actually draw
I would actually draw the
two chlorines left and right and the hydrogens up and down, up and down on each side.
It's still the same molecule, just viewed from a slightly different angle,
and to show that on paper,
I could have gone those two chlorines in in two different arrangements.
It could also set possibly with the two chlorines
both looking downwards, in which case
I would draw the structural formula like that. Every single one of these
are all equivalent. It's still the same thing,
just viewed from different angles. But, there's
one more trick that this molecule can do and that is is that
whenever there's a single bond present,
the atoms on each end of it can rotate about the axis of that bond,
that means that
this one here can spin on this axis of the bond, and this one here can also do
the same.
So, both of these atoms have that spin movement available to
spin around as they wish.
And that means that I don't necessarily have to have the two chlorines
sticking out side by side. They could be in any
other possible position relative to each other.
So, that means that this molecule could also be drawn
like this, in which case
there is a chlorine
above and below on each side and hydrogen are taking up
the other remaining spots. Still the exact same molecule
because all I've done to change that from
what it was originally to what it is now
is just rotated the molecule internally within its own structure.
The same thing would apply
to any simple single-bonded carbon chain.
This is four carbons all in a line. But, I don't necessarily have to draw them like that
because with all these bonds having free rotation, I could draw three
in a row and one down. Or, I could draw
one up and one down,
or I could draw the whole thing in some sort of "n" shape, or as
some sort of "u" shape.
Exactly which one I draw is really
irrelevant because it's all the same molecule, just twisted
internally. Of course, for clarity,
a structural formula will generally have all atoms in a row, or
if you draw the skeletal formulas,
generally, it will be drawn as a bit of a zig-zag.
Alkenes
are different
because they have a double bond between the carbon atoms
and that limits what kind of rotation is available
for those atoms. But, first of all, if we looked at this one here, it's
1,2-dichlorobutene (really 1,2-dichloroethene).
And, I could hold this
in a couple of different positions to get the different perspectives. So, this one here
probably matches this, yes it does. So,
on the left, we've got the chlorine up top and on the right, we've got the chlorine on the
bottom. But, the exact time molecule flipped over,
we could now draw it as having the chlorine on one side down
on the left, and on the right hand side, it's now up.
It's still the exact same molecule, just
one perspective versus another one. However,
what this molecule can't do is
rotate on the axis of this double bond. That means the two carbons on the
end of that double bond,
cannot twist, spin independently
of the other one. The double bond will not allow it. There is
no ability to twist around. That means that I can't get this structure
twisted around and get both of my chlorine atoms
facing either both
up or both down. So, at no stage
can I turn this structure into
this one. They both still have the same structural formula, in terms of
the sequence of atoms in the molecule. It has got a
hydrogen and chlorine on the left,
hydrogen and chlorine on the right, double bond between them. So,
in terms of the sequence of atoms and bonds, they are identical.
However, the arrangement in space of these atoms,
is different and
no amount rotation or manipulation
can ever get them to line up with each other.
So, these two here are two different molecules. We call them geometric isomers.
Same structural formulas, but different arrangements of atoms in space.
A requirement for this is that they must have a double bond between
carbon atoms. Structures like this
many different ways up of drawing those chlorine atoms
around those carbon atoms within the same molecule
because the bond can rotate to put them in
any position you wish. This double bond can't rotate
with means those two chlorines are forever one up
and one down. This one here, they're forever either both
that way, or they're both that way. So,
a geometric isomer must have a double bond for starters.
However, there's one more requirement, and that is
that each carbon within
the double bond has to have
two unique groups attached to it. So, that carbon there
has got a hydrogen and a chlorine, which are different from each other,
and also, this one here has got
a chlorine and a hydrogen that are different from each other.
It doesn't matter whether or not these ones match or
these ones match, it's one carbon at a time. Are they
the same or are they different for both sides?
If there's ever an instance where
on either side of that bond,
there is two groups that are the
same as each other then it is not possible
to have this molecule arranged
in any other
conformation other than this.
So, this one here. If I was to have this one
and this one.
On the surface they might look possibly
a little bit different because this one here has got the chlorine on the right
down and this one has the chlorine on the right up.
But, because the right-hand sides, sorry, left-hand sides of these
both have identical groups, two chlorines on that left-hand side
carbon that means that
even though I can't rotate the double-bond,
if I just take the whole structure and twist it over, then I've
got the exact same arrangement
in space. So, these aren't geometric isomers. It's the same molecule.
Possibly just originally viewed from a different angle.
So this one here can't have geometric isomers.
But, it does satisfy the double-bond requirements. One of the carbon atoms
has got two identical group attached.
That means that any different ways I can try and arrange,
these groups still represent the same molecule
from a different perspective. In this one here,
can exist as geometric isomers because
number one, there's a double-bond present and number two, each codon
has got two different groups attached to it. So I can have this structure drawn like this
or I could draw it so
the two chlorines are on the same side of each other.
I cannot manipulate this molecule to get their original arrangement back again.
No matter how I try because there's no rotation in here.
I can never get one chlorine up and one chlorine down.
So this is a different structure from the other one.
And, finally the original molecule we had
was an alkane. An alkane
by default cannot have, when it's in a single chain,
cis and trans geometric isomers because
this rotation means that
having the two chlorines on opposite sides or the same sides,
are actually just the same molecule with different amounts of internal rotation.
The lack of a double bond means any different ways of drawing this,
still represent the same molecule.