Isomerism: Stereochemistry

Question 1

What are isomers

Answer 1

Are compounds that have the same chemical formula, but the atoms in the compound are arranged differently
Two isomers will therefore have different properties

Question 2

What are constitutional isomers

Answer 2

Have the same atoms but bond differently (atoms are connected with different atoms)
For example: C₄H₁₀

Question 2

What are conformational isomers

Answer 2

Are those that differ as a result of a C-C bond rotation in alkanes.
For example: C₂H₆

Question 2

What are Stereoisomers

Answer 2

are compounds that have the same atom connectivity but a different arrangement of atoms in space
For example in alkenes this is because there is no rotation around the double bond
And in chiral compounds because they are non-superimposable on another

Question 2

Stereoisomers can be classified into which to categories

Answer 2

Enantiomers and Diasteroisomers:

Question 3

Enantiomers are…

Answer 3

non-superimposable mirror images

Question 4

Diasteroisomers are…

Answer 4

not superimposable but they are not mirror images either

Question 5

fill in the gaps

Answer 5

Do the two molecules have the same constitution?
Can the two molecules be superimposed on one another after rotating about single bonds?
Is one molecule the non-superimposable mirror image of the other?

Question 6

If something is achiral

Answer 6

There is symmetry and the two compounds are superimposable on another

Question 7

If something is chiral

Answer 7

It doesn’t have symmetry and both compounds are non-superimposable

Question 8

How can chirality related to an organic mechanism

Answer 8

The reaction between an aldehyde and cyanide forming a cyanohydrin
The cyanide anion undergoes nucleophillic attack at the carbonyl carbon of the Pi antibonding orbital, which can occur on either side
This can allow two products to form: where the OH groups will alternate either pointing forward or backwards, both are enantiomers

Question 9

Are C and D identical?

Answer 9

C and D are themselves identical, meaning they are achiral
One structure can be superimposed through rotating along a vertical plane

Question 10

What is a molecules with a stereogenic or chiral centre

Answer 10

A carbon atom with 4 different atoms/groups attached
All molecules with one stereogenic centre of chiral

Question 11

What is a racemic mixture

Answer 11

A mixutre of enantiomers in equal proportions (50:50)
A reaction with achiral starting materials and reagents will always produce a racemic mixture (equal attack from both sides of the reactant)

Question 12

If something is enantiomerically pure it …

Answer 12

is a single (pure) enantiomer of a chiral compound
Nature is a major source of enantiomerically pure compounds

Question 13

What is the CIP system

Answer 13

Assigns a letter to each enantiomer (R or S) that describes the configuration of groups at a particu;ar stereogenic (chiral) centre

Question 14

Using the CIP system, assign a letter to the following compound

Answer 14

Step 1: Prioritise the 4 groups around a chiral centre according to the atomic number. The highest atomic number is assigned priority 1 and the lowest atmoic number kis assigned priority 4
Step 2: You must orientate the chiral centre such that the lowest priority group is pointing away from the viewer
Step 3: Draw a path from priority 1 - priority 2 - priority 3
If the path is clockwise, the chiral centre is assiged [R]
If the path is anticlockwise, the chiral centre is assiged [S]
Therefore for this compound the absolute configuration is [s]

Question 14

Chiral molecules are optically active meaning

Answer 14

they can rotate plane polarised light
Optical rotatation is the extent to which a molecule will rotate plane polaried light - it is measured in degrees and the instrument used is a polarimeter

Question 15

How does a polarimeter work

Answer 15

• The first step is to pass light (sodium lamp) through a polariser, that will only allow wave on one specific vertical plane to pass through the sample
• If the material is optically active, the plane of polarisation is rotated in either clockwise or anticlockwise direction. The angle of rotation is alpha

Question 16

Why do you use optical rotation to distinguish enantiomers

Answer 16

Enantiomers are identical in all physical properties except optical rotation (their boiling points, melting points, IR/NMR and mass spectra are identical)
Each enantiomer is a pair is optically active and rotates the plane polarised light by the same degree but in opposite directions. The angle (α) are equal and opposite

Question 17

During polarisation, enantiomers will rotate either to the left or to the right
How is this shown through nomenclature

Answer 17

Rotating plane polarised light to the right = (+)
Rotating plane polarised light to the left = (-)
e.g. (S)-(+)-carvone

Question 18

What can the exact angle of rotation (α) be affected by
How is this standardised

Answer 18

The exact value of [α] depends on concentration of sample, path length, temperture and wavelength of light used
To standardised this [α] is in square brakets and the temperature of light (usually sodium D-line) are quotated

Question 18

When a molecule has two or more stereogenic centres, its can have two different types of stereoisomers
For example, how many isomers are possible and how many combinations of R and S are possible for 3-aminobutan-2-ol

Answer 18

• Both the OH and the NH₂ groups can pointing forward arrising an S configuration for both groups
• Both the OH and the NH₂ groups can face backwards arrising an R configuration for both groups
• The OH can point forwards (S configuration) and the NH₂ point backwards (R configuration)
• THe OH can point backwards (R configuration) and the NH₂ point forwards (S configuration)
• This gives 4 possible isomers

Question 19

If we compare
What can we say about these two isomers of 3-aminobutan-2-ol

Answer 19

Both centres are swapped
They are mirror images of another and non-superimposable meaning they are enantiomers

Question 20

If we compare
Two isomers of 3-aminobutan-2-ol

Answer 20

Both centres are not swapped, only one centre is swapped (OH group attached) hence we don’t produce mirror images and is not enantiomeric
Instead they are diasteroisomers

Question 21

Comparing enantiomers and diasterisomers
How do melting points and [α] value differ

Answer 21

When comparing Enantiomers or Diastereomers they will have very similar melting points and thir [α] values will be a positive and negative or the same value
When comparing Enantiomers and Diastereomers they will have very different melting points and their [α] values will be very different too

Question 22

What is an easy way to work out how many possible isomers a chiral compound will have

Answer 22

If there is a compound with n-stereogenic centres, there will be 2n possible isomers
Some of these will be pairs of enantiomers and pairs of diateroisomers

Question 23

What is relative stereochemistry

Answer 23

When there are two or more sterogenic centres in a molecule, the position in space of an atom/group in a molecule can be described relative to another atom/group

Question 24

For rings, the notation cis/trans is used
Relative stereochemistry: Which compound is cis and trans

Answer 24

Cis = same side, trans = opposite side
Left = cis 4-t-butyl-cyclohexanol
Right = trans 4=t=butyl-cyclohexanol
It depends if the tertiary butyl group and OH group is pointing the the same or different directions

Question 25

For open chains, the notation syn/anti is used
Relative stereochemistry: Which compound is syn and anti

Answer 25

Depends on if the bromine groups are pointing in the same direction or different directions
Left = syn
Right = anti

Question 26

What is the resolution of Enantiomers

Answer 26

The process of separating enantiomers
Since almost all of the physiochemical properties of each enantiomers are identical, you must temporarily convert them into diasteroisomers (which have different properties) by derivatisation with an enantiomerically pure reagent

Question 27

For example: how could you seperate two chiral alcohols

Answer 27

We could react them with a homochiral enantiomerically pure carboxylic acid forming a pair of diasteromeric esters
These could be separated then the ester hydrolysed, to creat the original alcohol product

Question 28

What is Chiral Chromatography and why would you use it

Answer 28

To seperate to chiral compounds
The stationary phase of the column is itself made up of a homochiral material
As the two enantiomers pass down through the column, flowing with the mobile phase such as a solvent, the two enantiomers will react differently with the homochiral stationary phase of the chromatography column
One enantiomer will take longer to come out of the column than the other, thus they are separated - creates a temporary disatereomeric interaction

Question 29

What are double bond isomers

Answer 29

Because there is no rotation about the double bond, two alkenes may have the same bonds but different arrangement
This makes them diasteroisomers as they are not superimposable and not mirror images

Question 30

Using the CIP rules, explain how you could allocate either E or Z to a pair of alkenes

Answer 30

Step 1: Select one of the carbon atoms of the double bond and assign priorities for the groups/atoms attached using atomic number
Step 2: If the two atoms directly attached to the double bind carbon are the same, move along to the next atom till you find a difference
Step 3: Of the substituents contain a double/triple bond, treat them as through they are linked to 2 or 3 bonded atoms