Important definition! Stereoisomers Molecules with the same STRUCTURAL FORMULA but a different ARRANGEMENT OF ATOMS IN SPACE.

Using model kits see if you can build two different versions of 1,2-dichloroethene

cis and trans cis = two biggest groups on the same side of the double bond (above or below) trans = two biggest groups on opposite sides of the double bond (above and below)

geometrical isomerism Most commonly found in alkenes. Each C in the double bond must have 2 different groups bonded to it. C=C has restricted rotation So groups are stuck in a certain position relative to each other.

More examples Build and draw the cis-trans isomers of; a)but-2-ene b)2-chlorobut-2-ene

But-2-ene

2-chlorobut-2-ene The biggest atom bonded to each carbon decides the isomer. Cl > C Now, come up with an alkene that you think will have geometrical isomerism. Get your neighbour to draw the 2 isomers.

Cyclic compounds Build 1,2-dichlorocyclopropane Can you see how it can have geometrical isomers? Like alkenes, small rings have restricted rotation. cis, 2 Cl above or below the plane of the ring trans, 1 Cl above and 1 Cl below the plane of the ring

dichlorocyclobutane Draw the FIVE isomers of this

But why does it matter? Geometric isomers can have different physical properties (i.e boiling point). They can also react differently.

1,2-dichloroethene Build or draw both isomers. See if you can determine the kind of intermolecular force each will have.

cis isomer Permanent dipole-dipole Polar molecule due to both Cl atoms being on the same side.

trans isomer Cl-C dipoles cancel out. No overall dipole, so van Der Waals’ forces present only. So, the cis isomer has a higher boiling point than the trans isomer

Reactivity We will use but-2-ene-1,4-dioc acid. Draw the two isomers. Both these isomer can react form this cyclic compound.

Which will require a higher temperature to react? It’s the trans isomer as we need to break the double bond to allow the groups to get close enough.

Optical isomerism Your hands are optical isomers of each other! They look the same don’t they? But if you try to make all the parts of your hand match up, you can’t do it.

Optical isomerism A pair of non-superimposable mirror images. We can achieve the same effect with simple carbon molecules. See if you can build the two isomers of 1-bromo-1-chloroethane

Optical isomerism arises whenever we have a carbon atom bonded to 4 DIFFERENT atoms or groups. Try coming up with another example.

Facts A pair of optical isomers are called enantiomers (from the Greek enantio meaning ‘opposite’). Your hands are enantiomers!

Facts Enantiomers are said to be CHIRAL (Greek cheir, means ‘hand’) A carbon with 4 different groups is described as being a chiral carbon.

Facts Chiral molecules will be asymmetric. You can denote a chiral C in a molecule with an asterisk * Draw HOCH 2 CH(OH)CH(OH)CH(OH)CHO Put an asterisk by each chiral carbon.

HOCH 2 CH(OH)CH(OH)CH(OH)CHO

Physical properties Will physical properties differ? They are identical, melting point, boiling point, density, solubility. Apart from one thing. The direction of rotation of plane-polarized light!

Now for the complicated bit! Light is messy, it oscillates in an infinite number of directions, they just have to be at right angles to the direction of light. Polarized light is when we cut the light down to just ONE direction of oscillation. We basically use a gate which only allows ONE direction through (its what 3D glasses do).

Now for the complicated bit!

Chiral molecules rotate the plane of the polarized light.

Now for the complicated bit! A pair of enantiomers will rotate the light by an equal angle. BUT in opposite directions. If we have a 50:50 mixture of the 2 isomers, what will happen to the light? Nothing, they cancel each other out. This type of mixture is called RACEMIC.

Lets try it out with a glucose mixture and a polarimeter.

Chemical Properties Do you think chemical properties will be different? Only when reacting with another optical isomer. Optical isomers are abundant in biological systems. Let’s look at some examples.

Aspartame – sweetener l-aspartame tastes sweet d-aspartame tastes bitter

Glucose d-glucose sweet and nutritious l-glucose is tasteless and cannot be metabolised

Lactic acid d-lactic acid produced in muscle tissue during exercise. l-lactic acid is found in off milk.

Amino acids. The are all l-amino acids. This is why we have to be VERY careful with medicine compounds.

Thalidomide

One isomer of this drug is an effective treatment for morning sickness. The other isomer causes deformaties in unborn babies. Unfortunately before they realised this it had already been given to thousands of women across the world.

This is why chemistry is important