Chapter 5: Stereochemistry

Question 1

What are Nitriles?

Answer 1

Nitriles have the general scheme: R-CtriplebondN

• Name this compound by adding the word Nitrile at the end of the chain
• If the Nitrile is Cyclic then use; carbonitrile instead

Question 2

What is a Constitutional Isomer?

Answer 2

Isomers that have the same molecular formula, but the atoms connect differently.
- Both physical and chemical properties are different

Question 3

How do you choose the most favourable Resonance Structure?

Answer 3

1. One must assign formal charges to all atoms (#of actual electrons - #electrons present) in the structure
2. Then choose the strucuture with the lowest formal charge on each atom

Question 4

What does Achiral mean?

Answer 4

These are molecules that are superposable on its mirror images

Question 5

What does Chiral mean?

Answer 5

Chiral are molecules that do not have an internal plane of symmetry, thus they are nonsuperposable mirror images

Question 6

What is a Diastereomers?

Answer 6

Diastereomers are stereoisomers that are not mirror images of each other

Question 7

What are Enantiomers?

Answer 7

Enantiomers are stereoisomers that are non-superposable mirror images

Question 8

What are Fischer Projections?

Answer 8

Fischer projections are representations of a 3D molecule as a flat structure, in which the carbon atom is at the centre of a cross

Question 9

What is an Isomer?

Answer 9

Isomers are compounds that have the same molecular formula, but have different properties and arrangements of atoms in space

Question 10

What are Meso isomers?

Answer 10

Meso compounds are achiral compounds that have chiral centres

Question 11

What does Racemic mean?

Answer 11

A racemic mixture is a mixture of equal amounts of two stereoisomers of an optically active substance

Question 12

What is a Stereogenic Centre?

Answer 12

A stereogenic centre is one in which an interchange of two different groups will result in stereoisomer

Question 13

What is a Stereoisomer?

Answer 13

Stereoisomers are that have the atoms linked in the same way in the molecule, but they differ in spatial arrangements of the atoms

Question 14

What are R,S Systems?

Answer 14

Configuration of molecules in which the stereocenter is either R=Clockwise or S=counterclockwise. A stereocentre is a carbon atom that has 4 different groups attached to it

Question 15

What is the %Enantiomer Excess formula?

Answer 15

%Enantiomer Excess = (moles of one enantiomer - moles of other enantiomer)/ Total moles of both enantiomers * 100
%Enantiomer Excess = (Observed specific rotation) /Specific rotation of pure enantiomer * 100

Question 16

What are Fischer Projections?

Answer 16

Fischer projections are representations of a 3D molecule as a flat structure, in which the carbon atom is at the centre of a cross
- We are permitted to rotate this structure only by 180° , we cannot flip the structure

Question 17

How do we Draw Fischer Projections?

Answer 17

1. An easy way to remember is that the two horizontal bonds are projecting out of the plane towards you
2. The two vertical bonds are projection into the plane, away from you

Question 18

How do we assign Priority to Fischer Projections (R&S)?

Answer 18

Vertical = Very Good
Horizontal = Horizontal
If smallest priority group is Horizontal then flip the configuration