Chapter 5- Stereochemistry

Enantiomers

• Are mirror images of each other

• Have four different substituents

• Hence are not able to make a plane of symmetry.

• This is also known as a chiral compound

Example of lactic acid as a chiral

• Lactic acid exists in two forms

• One is more active and more healthy

• This is because one chiral might have different biological activity. Because for example it doesn’t fit in a receptor.

• Imagine trying to shake a right with a left hand, doesn’t work

Most common cause of chirality

• A carbon atom bonded to four different groups in an organic molecule.

Properties of chiral molecules

• Different biological activity

• Optical activity

Optical activity in chirals

• Using a light source and a filter to make the light polarized and you shine this on an enantiomer

• If the light is now angled left if will get a minus sign = levorotatory molecule

• If the light is now angled right it will get a plus sign = dextrorotatory molecule

• This is a property of an enantiomer

• Two enantiomers will turn angle the light with the same value just in opposite directions so one will be + and one will be -

• If you don’t have a chiral compound it won’t angle

RS system

• R configuration = 123 clockwise

• S configuration = 123 counterclockwise

• Side note there is NO correlation between R and S and +/-

Sequence rules for specifying configuration (choosing between R and S)

1. Ranking: Look at the four atoms directly attached to the chirality center and assign priorities based on decreasing atomic number

2. If they atoms in the first atoms in the substituents are the same: look at the second, third or fourth atoms outward until a difference is found.

3. In case of double bonds: multiple bonded atoms are equivalent to the same number of single-bonded atoms. look at picture

Diastereomers

• A molecule with multiple chiral atoms.

• If a molecule has two chiral atoms, it means there will be four isomers.

• formula = 2ⁿ where n is chirality centers.

Epimers

• Diastereomers that differ in only 1 chiral center.

Meso compounds

• 2 chirality centers

• When the top part of the carbon is the same as the bottom

• They are identical → and so are not enantiomers

• Meso compounds DO NOT give optical activity

• Check configuration, if chiral centers have different configurations then they are meso if the configurations are the same they are not meso.

Allenes

When there is optical activity without chiral C atoms.