What are cis and trans geometric isomers

This video goes over the basic background behind cis and trans geometric isomers in alkenes.

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.