4. Stereochemistry and Isomers

Question 1

what are isomers?

Answer 1

two structures that have the same molecular formula but different structural arrangement in 3D

Question 2

what are constitutional or structural isomers?

Answer 2

two molecules that have the same molecular formula but are bonded differently

Question 3

what is the index of hydrogen deficiency?

Answer 3

-a calculation to find the degrees of unsaturation in a molecule

-2C+2+N-H-X/2

Question 4

what are conformational isomers?

Answer 4

two molecules that have the same molecular formula but are bonded differently in terms of rotation about the axis

Question 5

what is a condition where constitutional isomers can not be present?

Answer 5

when there is a double bond or pi bond

Question 6

what does it mean that due to steric repulsions, different constitutional isomers are favoured for enthalpic reasons?

Answer 6

reducing steric strain can lower enthalpy and thus increase stability

Question 7

what are staggered conformations?

Answer 7

an arrangement of Newman projections that minimizes steric repulsions because substituents are at the maximum distance from one another

Question 8

what are eclipsed conformations?

Answer 8

an arrangement of Newman projections where steric repulsions are not minimized because substituents are not at the maximum distance from one another

Question 9

what is anti-conformation?

Answer 9

when substituents are arranged as far as possible, typically 180 degrees apart to reduce steric strain

Question 10

what is gauche conformation?

Answer 10

when substituents are not arranged as far apart as possible, typically 60 degrees apart it has more steric strain

Question 11

what type of bond is needed for cis and trans conformations?

Answer 11

a double bond

Question 12

what is a cis conformation?

Answer 12

when higher priority substituent groups are added to the same side of the double bond

Question 13

what is trans conformation?

Answer 13

when higher priority groups are added to opposite sides of the double bond

Question 14

what geometric conformation is E the similar to?

Answer 14

trans

Question 15

what geometric conformation is Z similar to?

Answer 15

cis

Question 16

what is the chiral center or a stereocenter?

Answer 16

sp3 hybridized atoms with four groups connected to it

Question 17

what is the purpose of Cahn Ingold Prelog Rules?

Answer 17

allows you to assign priority to four different substituent groups

Question 18

what does it mean when substituents contain a double bond according to the Cahn Ingold Prelog Rules?

Answer 18

if carbon is double bonded to an atom, there are now two of the atoms carbon is bonded to

Question 19

how do you assign R and S nomenclature?

Answer 19

1. find a chiral center
2. rank the four substituents
3. draw an arrow tracing from highest to lowest priority

• if the arrow is clockwise this is R

-if the arrow is counter-clockwise, this is S

Question 20

what is the rule when assigning R and S nomenclature if hydrogen is pointing out of the page?

Answer 20

the answer is the opposite nomenclature

Question 21

what are optical isomers also known as?

Answer 21

enantiomers

Question 22

what are enantiomers?

Answer 22

-when comparing two molecules, if you flip one you will get the other molecule

-two molecules that are flipped mirror images of one another

Question 23

to have enantiomers, what must a molecule have?

Answer 23

one or more chiral centers

Question 24

what is the optical activity of enantiomers?

Answer 24

enantiomers rotate plane-polarized light in the opposite direction, one rotates light to the left and the other rotates light to the right

Question 25

what are diastereomers?

Answer 25

two molecules that have the same formula but are bonded differently and are not enantiomers

Question 26

to have diastereomers, what must a molecule have?

Answer 26

two or more chiral centers

Question 27

what is the optical activity of diastereomers?

Answer 27

there is no optical activity

Question 28

what is a meso compound?

Answer 28

a molecule where if you draw a line through the middle of it, one half mirrors the other half

Question 29

what is the optical activity of meso compounds?

Answer 29

there is no optical activity

Question 30

what does a molecule name with R or S stereochemistry look like?

Answer 30

the letter comes first and is in brackets

Question 31

when naming a molecule, what should you do if there are two stereocenters?

Answer 31

give the position of the stereocenters with numbers

Question 32

when naming a molecule, what should you do if there is one stereocenter?

Answer 32

you do not have to give the position of the stereocenters with numbers, you can just put the letter

Question 33

what does a molecule name with E or Z stereochemistry look like?

Answer 33

the letter comes first and is in brackets

Question 34

how can you find the number of possible stereoisomers?

Answer 34

2 to the power of n, where is the total number of chiral carbon atoms and double bonds

Question 35

what is the exception to find the total number of stereoisomers of meso compounds?

Answer 35

use the number of possible stereoisomers formula but subtract two from the total

Question 36

what is the formula for finding the number of geometric isomers when n is odd?

Answer 36

2^(n-1) +2^(n+1/2-1)

Question 37

what is the formula to find the number of geometric isomers with n even?

Answer 37

2^(n-1)+2^(n/2-1)