Stereochemistry

Question 1

What are the three categories of stereoisomers?

Answer 1

Structural isomers: different atoms/bonding patterns

Conformational isomers: differ by rotation around the single bonds

Geometric isomers: alkenes/cyclic compounds using cis or trans

Question 2

Define enantiomers.

Answer 2

Two non-superimposable molecules which are mirror images of each other.

Question 3

Define diasteromers.

Answer 3

Any pair of stereoisomers that are not enantiomers and are chemically/physically different.

Question 4

Define ligand.

Answer 4

A molecule that binds to a (usually larger) molecule.

Question 5

Define chiral.

Answer 5

Molecules containing at least one chiral carbon (one with four different elements attached).

Question 6

Define racemic mixture.

Answer 6

A mixture of equal amounts of L and D enantiomers of a substance, showing no rotation of plane polarised light.

Question 7

What are achiral diastereoisomers?

Answer 7

A chiral stereoisomer (with 2 out of 4 stereoisomers which are unidentical).

Question 8

How do you assign R/S configuration?

Answer 8

1. Identify an asymmetrical carbon and the attached groups.
2. Assign priorities using the following rules:
   
   • atoms of higher atomic number have priority
   • isotope of higher atomic mass receives higher priority
   • higher priority assigned to group with atom of higher atomic number or mass at 1st point of difference.
3. View molecule along the bond from the asymmetric carbon to the group of lowest priority.
4. Consider CW/CCW order of priorities:
   
   • if they increase clockwise it is R configuration
   • if they decrease clockwise is is S configuration

Question 9

How do you assign E/Z configuration?

Answer 9

Z = same sides
E = opposite sides

Question 10

What are the Fischer projection rules?

Answer 10

Solid line = in plane of paper.
Dashed line = going away from you.
Wedged line = coming towards you.