Lecture 9 - Stereochemistry 2/4

Question 1

Name the two types of configurational isomers

Answer 1

geometric and optical isomers

Question 2

We know that we change conformations/conformational isomers by rotating bonds. However, how do we change configurational isomers?

Answer 2

BREAKING BONDS

Question 3

Configurational isomers are…

Answer 3

DIFFERENT COMPOUNDS

Question 4

Draw the configurational isomers of but-2-ene. Label them cis/trans and E/Z

Answer 4

Question 5

You can’t go from cis but-2-ene to trans without…

Answer 5

BREAKING A BOND

Question 6

Optical isomerism occurs between isomers that are…

Answer 6

chiral

Question 7

What is a chiral compound?

Answer 7

A compound with no plane of symmetry within it. They contain an stereoogenic centre - an atom bonded to four different groups.

Question 8

What do we call molecules with a plane of symmetry?

Answer 8

achiral

Question 9

Name and draw the only achiral amino acid. Why is it achiral?

Answer 9

• glycine/Gly/G
• has a plane of symmetry so it is achiral
• no chiral atom
  -central atom is bonded to two H atoms (therefore it isn’t bonded to all different groups)

Question 10

Alanine is an amino acid but unlike glycine it is ….. Why? Draw it?

Answer 10

chiral. It does not have ap lane of symmetry
It contains a carbon atom bonded to 4 different groups- stereogenic centre. The central carbon is bonded to amine group, carboxylic acid group, methyl group AND a hydrogen.

Question 11

What is another name for an optical isomer?

Answer 11

enantiomer

Question 12

Define enantiomers

Answer 12

Two non-superimposable mirror image structures

Question 13

Optical isomers/enantiomers have the same….

Answer 13

physcial properties- melting point, boiling point, NMR

Question 14

BUT what is the one difference between optical isomers

Answer 14

They rotate plane polarised light in equal but opposite directions.

Question 15

Draw the optical isomers of alanine

Answer 15

Question 16

Give the name and symbol for light that is moved anticlockwise

Answer 16

levorotatory - L- negative

Question 17

Give the name and symbol for light that is moved clockwise

Answer 17

dextrorotatory - D - positive

Question 18

When we label stereoisomers with +/d and -/l it has nothing to do with how we draw the molecule. It is something you have to…

Answer 18

measure. You cannot draw a molecule based upon how it rotates plane polarised light.

Question 19

The alpha D equation

Answer 19

Question 20

How do you get from cm to dm?

Answer 20

divide by 10

Question 21

Rotation of light only occurs in compounds not with achiral. Therefore, the alpha D of an achiral compound is…?

Answer 21

0

Question 22

What is a racemic mixture?

Answer 22

a 50:50 mixture of two enantiomers.

Question 23

What is the alpha D for racemic mixture?

Answer 23

0. This is because the two enantiomers rotate PPL in opposite direction. The movement of light in opposite directions cancel each other out.

Question 24

Calculate the Alpha D of pure cocaine when….
- 1.2g of cocaine in 7.5ml of CHCl3 solvent.
- the sample tube is 5cm
- the observed rotation is -1.3 degrees.

Answer 24

Question 25

We can use alpha D to figure out how much of each enantiomer is in a mixture. What is the percentage of enantiomer in a mixture of cocaine if the alpha D is -8? Remember we know that pure cocaine is -16.

Answer 25

Question 26

What does enantiomeric excess measure (ee)?

Answer 26

A measurement of how much one enantiomer is in excess over another in a mixture of chiral substances.

Question 27

What is the ee when a mixture contains 75% of negative/ L enantiomer and 25% of the positive/ D enantiomer?

Answer 27

ee = (x-y/x+y ) x 100%
((75-25)/ 100) x 100% = 50% ee of the negative enantiomer.