Lecture 9 - Diastereoisomers

Question 1

If the sample rotates plane polarized light clockwise →

Answer 1

dextrorotatory (+)

Question 2

If it rotates plane polarized light CCW →

Answer 2

laevorotatory ( - )

Question 3

Optical rotation (a) is a:

Answer 3

quantitative measure of optical activity and a = [a]cl

[a] = specific rotation; c = concentration; l = path length

Question 4

properties of enantiomers:

Answer 4

often have identical chemical properties too but not if they’re interacting with another chiral molecule, such as an enzyme - having very different biological effects

Question 5

racemate:

Answer 5

a racemate is a mixture containing equal amounts of two enantiomers

Question 6

racemization:

Answer 6

the process of forming a racemate from a pure enantiomer is called racemization

Question 7

the separation of a pair of enantiomers is called:

Answer 7

resolution

Question 8

difference between racemates and pure enantiomers:

Answer 8

racemates typically have different physical properties from that of the pure enantiomers

Question 9

when are additional stereoisomers possible?

Answer 9

additional stereoisomers are possible when a molecule has 2 or more asymmetric carbons

Question 10

stereoisomers that are not enantiomers are called:

Answer 10

diastereomers

Question 11

diastereomers are not [x] and [x]:

Answer 11

diastereomers are not mirror images and differ in all their physical properties

Question 12

what relationships are present in a set of four diastereomers?

Answer 12

all four compounds are diastereomers with one another however there are also two pairs of enantiomers within the four of them

Question 13

meso compounds:

Answer 13

• meso compounds have two or more asymmetric carbons and are achiral

• they are not optically active

• they possess an internal mirror plane

Question 14

a molecule with n stereocenters can exist as:

Answer 14

2n stereoisomers

Question 15

a molecule with n stereocenters, can exist
as __ stereoisomers, this number is _______ if a ____ compound is present among the possibilities

Answer 15

(1) 2n

(2) reduced

(3) meso

Question 16

can allenes be chiral?

Answer 16

allenes (cumulated dienes) may be chiral even though they lack an asymmetric carbon

Question 17

amine inversion:

Answer 17

amines with an asymmetric nitrogen atom have the potential for chirality

Question 18

diastereoisomers are stereoisomers that are not enantiomers

Question 19

what governs the priority order in R&S stereochemistry?

Answer 19

the atomic number - higher the atomic number, higher the priority

Question 20

R&S - which one is clockwise and which one is anticlockwise?

Answer 20

S - ANTICLOCKWISE - LEFT

R - CLOCKWISE - RIGHT

Question 21

how do you view the lowest priority groups in R&S stereoisomerism?

Answer 21

you must always view the lowest priority group straight on so that it is projecting behind from your viewpoint

Question 22

what is different between group placements in R&S stereoisomers?

Answer 22

in S isomers the highest-priority group will be found on the left and carry on around rightwards

in R isomers the highest-priority group will be found on the right and carry on around leftwards

Question 23

what is E/Z isomerism used for?

Answer 23

determine the stereochemistry of double bonds

Question 24

what is a newman projection?

Answer 24

the projection with a circle in the middle that looks like a mini-pie-chart

Question 25

with R&S stereoisomers, when counting the priorities, what is different about double bonds?

Answer 25

with double bonds, you count their atomic mass twice O=O = 2(atomic mass)