Chapter 4: 4.5 Stereoisomers IIIa - Chiral Molecules

Question 1

Define:

Chiral centre

Answer 1

An sp3 hybridized atom with four “different” groups connected to it

Question 2

What is a chiral centre also known as?

Answer 2

Stereocentre

Question 3

True or False:

Chirality can change how things smell

Answer 3

True

Question 4

True or False:

Nitrogen and phosphorus cannot be chiral, only carbon can

Answer 4

False, carbon is the most common one in organic chemistry but nitrogen and phosphorus can both be chiral too

Question 5

Define:

Cahn-Ingold-Prelog Rules

Answer 5

Allows you to assign the priority of the four different substituents to assign R/S for a stereocentre

Question 6

State:

Steps of the Cahn-Ingold-Prelog Rules

Answer 6

1. Find a chiral carbon (cannot be done with a non-chiral carbon)
2. Look at the atoms directly bonded to that chiral centre - highest atomic number/atomic mass atoms take the greatest priority
3. If there are two or more of the same atom, book-keep and rank the three atoms bonded to that one - travel until you find a difference and find an atom that “outranks” the others
4. When there are substituents that contain multiple bonds, you count them as such: C=X is X-C-X and a C(triple bond)X is C(X)3

Question 7

State:

The steps on how to assign R/S nomenclature

Answer 7

1. Find a chiral centre
2. Rank the four substituents
3. If group #4 is in the back, trace from 1 to 2 to 3. If clockwise it is R, anticlockwise is S
4. If group #4 is in the front, trace from 1 to 2 to 3. If clockwise it is S, anticlockwise is R
5. If group #4 is in the plane, redraw to get step #3 or #4