Stereochemistry: 3D Structures of Molecules

Question 1

Isomers

Answer 1

Different compounds with the same molecular formula.

Question 2

Constitutional Isomer

Answer 2

Isomer with different atomic connectivity

Question 3

Stereoisomers

Answer 3

Isomers with the same atomic connectivity but with different 3D arrangements

Question 4

Chirality

Answer 4

is the ability of objects to exist as non-superposable mirror images of each other
Often originated from an atom that is connected to 4 different substituents

Question 5

Objects that contain a mirror plane are

Answer 5

achiral

Question 6

Objects that do not have a mirror plane are

Answer 6

chiral

Question 7

Achiral

Answer 7

Can be superimposed with its mirror image

Question 8

Chiral

Answer 8

Cannot be superimposed with its mirror image

Question 9

Enantiomers

Answer 9

Chiral molecules and their non-superimposable mirror images

Question 10

Stereogenic center

Answer 10

Chiral center or an atom that is connected to 4 other substituents

Question 11

Enantiomers physical properties

Answer 11

They are identical physical properties including MP, BP, IR, NMR, density, etc.

Can be differentiated from each other;
- Interactions with other chiral molecules (ex. biological receptor sites only accept 1 enantiomer)
- Optical activity (interactions with polarized light). Means that they rotate plane-polarized light.

Question 12

How do we test for enantiomers with polar light?

Answer 12

A light source is shown through a polarizing filter (ex. the polarized light is vertical).
Then this polarized light is shown through a sample of pure enantiomers and as it bounces of and hits these molecules it begins to rotate.
The angle of this rotation, denotated as α (aka optical rotation), can be measured.

Question 13

How do we classify enantiomers?

Answer 13

Enantiomers polarize light by the same angle but in opposite directions (+ α and - α)

Question 14

Optical Rotation α

Answer 14

the angle that a sample rotates plane-polarized light.

Question 15

Dextrorotary

Answer 15

α is greater than 0 and so is positive
The light is rotated clockwise

Question 16

Levorotary

Answer 16

α is less than 0 and so is negative
The light is rotated counterclockwise

Question 17

Racemic Mixture

Answer 17

A 1:1 mixture of 2 enantiomers
Does not rotate polarized light, because the rotations are cancelled out

Question 18

Absolute Configuration

Answer 18

a method of indicating the permanent arrangement of groups attached to the chiral center.
Denoted as either R or S

Question 19

Determining whether a center is R or S

Answer 19

1. Identify the chirality center
2. Determine the priorities of attached groups (Cahn-Ingold-Prelog Rules)
3. Rotate molecule to put the lowest priority group (4) at the back
4. Determine the direction of the priority group numbers 1, 2, and 3

Question 20

R

Answer 20

Clockwise

Question 21

S

Answer 21

Counterclockwise

Question 22

CIP (Cahn-Ingold-Prelog) Rules

Answer 22

Help assign priority to groups attached to a chiral center based on atoms directly attached to the center

*Higher Atomic number = Higher Priority

Question 23

Diastereomers

Answer 23

Stereoisomers that are non-superimposable, non-mirror images

Question 24

Difference between enantiomers and diastereomers

Answer 24

Changing all stereocenters (and substituents) at once give you enantiomers.
Changing some stereocenters gives you diastereomers

Question 25

Diastereomers physical properties

Answer 25

Diastereomers have different physical and chemical properties (including BP, MP, IR, NMR, solubility, etc.)

Question 26

How do you find the maximum number of stereoisomers

Answer 26

2^n Where n = number of chirality centers Note: the number of stereoisomers will be lower if molecules have symmetry elements

Question 27

Meso Compounds

Answer 27

Molecules that have chirality centers but are achiral due to an internal mirror plane The molecule and its mirror image are superimposable (so achiral)

Question 28

Double bond Isomerism

Answer 28

If the two highest priority groups are opposite to one another (trans), the double bond is designated E (entgegen, German for “opposite”). If the two highest priority groups are on the same side of the double bond (cis), the double bond has a Z configuration (zusammen, German for “together”).