Chapter 2 - Isomers

Question 1

Describe constitutional isomers

Answer 1

AKA structural isomers

• only share molecular formula (do not have the same connectivity)
• have different chemical and physical properties

Question 2

If an isomer shares the same connectivity and can convert one molecule into another by rotation what type of isomer is it? If you can’t by rotation what is it?

Answer 2

Conformational isomer

If you can’t by rotation it is a configurational isomer

Question 3

What is an eclipsed, gauche, and anti conformation? Describe the positions of groups

Answer 3

Eclipsed conformation- groups are on top of each other (0 degrees)

Anti conformation- biggest groups are the farthest away from each other (180 degrees)

Gauche conformation- groups are near each other but not on top of each other (60 degrees apart)

Question 4

What is torsional strain?

Answer 4

A strain that results from the repulsion of the electrons forming the bonds of two adjacent atoms

Question 5

Dihedral angle

Answer 5

Angle between the two largest groups on either side of the projection

Question 6

Describe anti, gauche, and eclipsed conformations in terms of torsional/ steric strain and stability

Answer 6

Anti confirmation: most stable, low steric and torsional strain

Eclipsed confirmation: least stable, max steric and torsional strain due to electron clouds of substituents repelling each other

Gauche conformation: in between eclipsed and anti in terms of stability and steric/torsional strain

Question 7

When a molecule twists into a chair what happens to the strain?

Answer 7

Angle strain and torsional strain are now effectively zero

Question 8

Axial vs equatorial in terms of where the groups project out of a bond? And stability

Answer 8

Axial: follows the direction of the corners it’s on

Equatorial: are the best and most stable, do not follow the direction of the corners

Question 9

What does a dash line and a wedge mean?

Answer 9

A dashed line means it is going into the page

A wedge means it is going out of the page

Question 10

Chiral definition

Answer 10

Non-super impossible mirror images

Question 11

What is a chiral center?

Answer 11

An atom, typically a carbon that has 4 different groups attached to it

(So it’s non superimposable on its mirror image)

Question 12

Depending on the number of chiral centers what type of stereoisomers are possible?(0-2)

Answer 12

0 - Achiral

1- enantiomers

2- enantiomers or diasteriomers

Question 13

What are the steps to assigning R and S to chiral centers?

Answer 13

1- identify 4 atoms directly attached to chiral center

2- assign priority based on atomic number, highest receives priority 1, lowest priority 4 (usually hydrogen)

3- if 2 atoms have same atomic number, move away from the chiral center looking for the first point of reference

4- rotate molecules so that the 4th priority is on a dash line

5- determine if sequence 1-2-3 follows a clockwise order (R) or a counterclockwise order (S)

Question 14

What do you do if the 4th priority group is right next to a dash line but not on one?

Answer 14

Assign it the opposite of what you should if you kept the sequence how it was. So if the 1-2-3 sequence was clockwise (R) change it to counter clockwise (S)

Question 15

If the 4th group is across from the dash line what do you do? (Opposite side of chiral center)

Answer 15

Just adding R/S based on sequence without change. Like you would if it was on a dash line already.

Question 16

E and Z forms are for what?

Answer 16

Double bonds in stereochemistry

Question 17

How do you know if it is a Z or E conformation?

Answer 17

Z order- if highest priority substituents are on the same side of the double bond

E- if highest priority substituents are on opposite sides of the double bond

Question 18

What are enantiomers? How do you identify them?

Answer 18

Non superimposable mirror images that cannot appear identical simply by rotation

NEED atleast 1 chiral center

If the same chiral center(s) have different R/S the molecules are enantiomers IF there is NOT A PLANE OF SYMMETRY

Question 19

If isomers have the same chiral centers and same R/S configuration on all chiral centers then what are they?

Answer 19

Identical isomers

Question 20

What are diasteriomers and how do you identify them?

Answer 20

Non superimposable and not mirror images

NEED atleast 2 chiral centers

They are a mix of same S/R and opposite S/R at same chiral centers IF there is not a plane of symmetry

Question 21

What are meso compounds and how do you identify them?

Answer 21

They are Achiral compounds that have multiple chiral centers

They NEED atleast 2 chiral centers

An internal plane of reflection indicates meso compounds

Question 22

What is optical density?

Answer 22

The ability of a molecule to rotate plane polarized light

Question 23

What is the trick for determining the max number of stereoisomers that can be found in a compound?

Answer 23

2^x

X is the number of chiral centers but if there is an internal plane of reflection watch for meso compounds which would change the number of stereoisomers

Question 24

Do chiral molecules have optical activity?

Answer 24

Yes

Question 25

What is most stable, a chair, bondline, or boat?

Answer 25

A chair

Question 26

What is more stable: axial or equatorial?

Answer 26

Equatorial is more stable than axial

Question 27

A relative confirmation is retained if what happens?

Answer 27

The bonds of the stereoisomer are not broken

Question 28

What does E and Z indicate?

Answer 28

E- trans

Z- cis

Question 29

What is the equation for specific rotation?

Answer 29

[Alpha] = alpha/ c • l

[alpha] = specific rotation

Alpha = observed optical rotation

c = concentration

l = path length in decimeters