>>Dr. Mark Morvant: In this video we'll look at organic isomers. Isomers are molecules
with the same molecular formula but different chemical structures. Isomers can be placed
in two broad categories: structural isomers, where the connections between the atoms has
changed, and spatial isomers, where the connections between the atoms are the same but in spatial
space they are different. If we think about isomers, again separating into structural
and spatial, we can look at this flowchart. If we have a structural isomer, the connections
between the atoms is not the same, these will be constitutional isomers. If we had four
carbons and a straight chain, that would be one isomer. If we had those same four carbons
but now we have three carbons in a straight chain and one carbon attached to the middle,
the connections between the atoms are not the same. These are constitutional isomers.
With stereoisomers, those where we have different spatial arrangements but the same connections
between atoms, we have enantiomers, which are optical isomers, and diastereomers. Enantiomers,
optical isomers, we can think about a left hand and a right hand. All things being equal,
these would be optical isomers. To have an optical isomer I must have a chiral molecule.
One way to have a chiral molecule is to have a carbon with four different things attached.
We can visualize a generic carbon with A, B, C, and D attached. We can have two species
that are mirror images of each other. Enantiomers will have a mirror image, and enantiomers
are defined as two non-superimposable molecules that are mirror images. Diastereomers are
also stereoisomers. You can have a diastereomer that is based on a cis-trans isomerization,
where we have a component like a double bond where we cannot rotate the two groups. The
cis and trans, cis being on the same side, trans being on the opposite side, if we think
about a multiple bond, this is cis and trans. Conformers are actually rotations. We can
have different conformations of different rotations. The thing about conformations is
they are spatial arrangements, but they can be converted back and forth by just rotating
around a bond.
Let's look at some problems. Label the following pairs of isomers as constitutional, cis-trans,
conformational, or optical isomers. In the first pair, we see that we have five carbons
in a straight chain. Each carbon is connected to one other carbon. In the other structure
we see that we have four carbons in a straight chain and one carbon that's attached to the
second carbon. That carbon has three connections to carbon, therefore the connections are different
and these will be constitutional isomers. There's a significant structural difference.
We've changed the way the atoms are connected. Let's look at the next. Here we have a double
bond species. When we have a double bond, we cannot rotate around that bond. That means
that in the first case, where we have the chlorine facing up and the methyl facing down,
there's no way to change that into the second object we have where we have the chlorine
going up and the methyl going up. The connections are the same; each atom is connected to the
same atoms. But where they are connected in space is different. Since we have the double
bond and we have, in the first case opposite sides, trans, in the second case same side,
cis, this would be a cis-trans isomer.
In the next one you can see here that we have two molecules. All the connections are the
same. And what's highlighted is the carbon in the middle that has four different groups
attached. This is our hint that this is an optically-active molecule, this is a chiral.
Chiral meaning hand, left and right. Notice that we have a mirror plane between these
two. Because these two are mirror images of each other, these are enantiomers. In the
last case, again notice that all of our connections are the same, so this is not going to be a
structural isomer, or a constitutional isomer. It looks like a cis-trans, but notice that
we have a single bond. Because we have a single bond we can rotate around that single bond
and we can convert the first conformation into the second. Now there's some energy to
do this, to convert from one conformation to another. But since we can convert, these
are conformations. There are lots of isomers in organic chemistry. Carbon can make four
bonds, and it makes very stable covalent bonds. Because of that, there are lots of different
isomers. Some of them are easy to recognize by just trying to name the molecule, which
almost always works, and others by just being observant. As you can see in the two diasteromers,
conformations, and cis-trans isomers, it is also important to understand the bonding structure,
that you cannot rotate a double bond but you can rotate a single bond. Practice recognizing
isomers, and become confident.