Chirality and Stereochemistry

Question 1

What are molecules that are arranged differently in 3 dimensions, but have the same molecular formula, and have different properties called?

Answer 1

Stereoisomers

Question 2

What are the two types of isomers?

Answer 2

Constitutional Isomers

Stereoisomers

Question 3

When are two molecules constitutional isomers?

Answer 3

Two molecules are constitutional isomers if they have the same molecular formula but different connectivity

Question 4

In what direction is a bond represented by a dashed line pointing?

Answer 4

Into the page

Question 5

In what direction is a bond represented by a bold line pointing?

Answer 5

Out of the page

Question 6

What can be said about the relationship between the properties of stereoisomers?

Answer 6

They have the same chemical properties (melting point, boiling point, solubility and density) but different biological properties.

Question 7

What are stereoisomers?

Answer 7

Stereoisomers are molecules that have the same molecular formula, and the same connections between atoms, but a different 3-D shape

Question 8

What do we refer to stereoisomers as being?

Answer 8

We refer to stereoisomers as being chiral

Question 9

Can stereoisomers be superimposed?

Answer 9

No.

Question 10

What does it mean for molecules to be superimposable?

Answer 10

They can be overlaid to show the same structure.

Question 11

If a carbon only has 3 different substituents, can it be a stereoisomer?

Answer 11

No.

Question 12

What is a molecule considered to be if it is not superimposable on its mirror image?

Answer 12

A molecule that is not superimposable on its mirror image is called chiral

Question 13

What are a pair of chiral molecules said to be in relation to each other?

Answer 13

A pair of chiral molecules are enantiomers of each other

Question 14

What do we do to determine if something is chiral?

Answer 14

We look for a stereocentre (or “chirality” centre)

Question 15

What is the Cahn-Ingold Prelog system used for?

Answer 15

The Cahn-Ingold-Prelog system is used to describe configuration at chiral centres using the notation “R” and “S”

Question 16

When are the three steps to assigning which form of a stereoisomer is present?

Answer 16

Step 1. Assign priority to the substituents from 1-4
Step 2. Turn molecule so the lowest priority atom (4) faces away from you
Step 3. Connect 1 to 3
If the order is clockwise, it is an “R” configuration
If the order is anticlockwise, it is an “S” configuration

Question 17

What is the first thing you should always do before assigning priority to substituents of a stereoisomer?

Answer 17

Check if it is actually a chiral carbon. Look at the four atoms directly attached to the carbon centre and see if they’re different.

Question 18

How do you determine which atoms have the higher priority?

Answer 18

The higher atomic number has higher priority (or hybridisation state sp>sp2>sp3 - if there are only carbon substituents)

Question 19

If a carbon atom has two atoms attached that are the same type of atom, can the carbon be chiral?

Answer 19

Yes, if the substituents have further differences - like what they are attached to or if they have a different bond order

Question 20

If you’re looking to give priority to one of two substituents that are the same atom, but have different attachments, how do you choose which has priority?

Answer 20

The one with the attachment(s) with the greatest atomic number have priority

Question 21

If you’re looking to give priority to one of two substituents that are the same atom, but one is double bonded, how do you choose which has priority?

Answer 21

The one with the double bond is given priority

Question 22

If a pair of stereoisomers are non-superimposable mirror-images, what is this called?

Answer 22

They are “enantiomers”

Question 23

Which has a higher preference: an atom with a higher bond order, or with heavier molecules attached?

Answer 23

The molecule with the higher bond order

Question 24

How do you draw an enantiomer of a molecule?

Answer 24

When you draw an enantiomer, you just flip the sides. The into/out of the page orientations remain the same, you just flip from left to right.

Question 25

Can bonds in chiral carbons be exchanged through rotation?

Answer 25

Bonds in chiral carbons cannot be exchanged through rotation. This can only be achieved through breaking a bond and physically reattaching it.

Question 26

Can chiral molecules switch between R and S configuration?

Answer 26

Configurations cannot change from R to S (or vice versa) without physically changing

Question 27

Can a double-bonded carbon be a chiral centre?

Answer 27

No

Question 28

Can a nitrogen atom be a chiral centre?

Answer 28

No.

Question 29

When is a compound considered to be an enantiomer?

Answer 29

A compound is only an enantiomer if its mirror image is non-superimposable

Question 30

What are diastereomers?

Answer 30

Diastereomers are non-superimposable, non-mirror images

Question 31

If compounds with the same molecular formula have 4 enantiomers, how are these broken down?

Answer 31

If compounds with the same molecular formula have 4 enantiomers, they are not all enantiomers of each other, but there are 2 pairs of enantiomers.

Question 32

Are CH3 and CH2 different constituents in stereochemistry?

Answer 32

CH3 and CH2 are different substituents in stereochemistry

Question 33

What can be said about the chirality of symmetrical molecules?

Answer 33

As soon is there is symmetry, a compound may have chiral centres, but will definitely not be chiral

Question 34

If a molecule has chiral centres, but is completely superimposable, is it chiral?

Answer 34

If a molecule has chiral centres, but is completely superimposable, it is not chiral.

Question 35

What are mesocompounds?

Answer 35

Molecules that have chiral centres, but are completely superimposable.

Question 36

Do mesocompounds have chiral centres?

Answer 36

Yes

Question 37

Are mesocompounds completely superimposable?

Answer 37

Yes

Question 38

Are mesocompounds chiral?

Answer 38

No

Question 39

Does electronegativity effect stereochemistry?

Answer 39

No.