unit 5 - stereochemistry

Question 1

Nomenclature

How do you name cis(z)/trans(e) to c=c

Answer 1

• Cahn-Ingold-prelog rules
  1. split the c=c and look at the higher atomic number
  2. first point of difference
  3. multiple bonds are equivalent to the same number of single bonds to the atom
• never use E/Z for rings!

Question 2

define chirality

Answer 2

• property of a molecule (object) not of an atom

Question 3

what is a stereogenic center and how do you know if a molecule is chiral?

Answer 3

• an atom where the interchange of the two substituents leads to a stereoisomer
• Achiral: there is a plane of symmetry regardless if it has a stereogenic center; “meso” if there is seteogenic center and plane of symmetry
• chiral: there is no plane of symmetry and has a stereogenic center

Question 4

What is an enantiomer?

define: racemate/racemic/racemic pair

Answer 4

• two molecules that are mirror images of each other but are not non-superposable
• each molecule is called an enantiomer of the other
• racemate: when equal numbers of each enantiomer are present
• have identical physical properties except for the way they rotate plane-polarized light (L) and (D) notation

Question 5

How to assing R or S stereochemistry?

Answer 5

• Cahn-Ingot-Prelog rules for assigning priority
• if priorities increase in a clockwise direction = R
• if priorities increase in counterclock wise direction = S

Question 6

What are Diasteromers?

Answer 6

• stereoisomers that are not mirror images of each other
• for a molecule with N stereogenic centers, there are 2^n stereoisomers
• draw the molecule and then its mirror image
• keep the substituents on one stereogenic center fixed and switch the two groups on the other sterogenic center then draw mirror image

Question 7

How can you rotate Fischer projections to dtermine the R and S designation?

Answer 7

1. rotate entire projection 180
2. hold one group fixed and rotat the other three either clockwishe or counterclockwise
   tip: rotate to put H (or lowest priority) to the bottom or the top then lab the others and assign stereochemistry
   - wanting H in the back

Question 8

How to assign priority when the sterogenic center are on atoms other than carbon?

Answer 8

• a lone pair is a group; it is of lowest priority, lower than H