Isomerism: Stereochemistry Flashcards
(35 cards)
What are isomers
Are compounds that have the same chemical formula, but the atoms in the compound are arranged differently
Two isomers will therefore have different properties
What are constitutional isomers
Have the same atoms but bond differently (atoms are connected with different atoms)
For example: C₄H₁₀
What are conformational isomers
Are those that differ as a result of a C-C bond rotation in alkanes.
For example: C₂H₆
What are Stereoisomers
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
Stereoisomers can be classified into which to categories
Enantiomers and Diasteroisomers:
Enantiomers are…
non-superimposable mirror images
Diasteroisomers are…
not superimposable but they are not mirror images either
fill in the gaps
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?
If something is achiral
There is symmetry and the two compounds are superimposable on another
If something is chiral
It doesn’t have symmetry and both compounds are non-superimposable
How can chirality related to an organic mechanism
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
Are C and D identical?
C and D are themselves identical, meaning they are achiral
One structure can be superimposed through rotating along a vertical plane
What is a molecules with a stereogenic or chiral centre
A carbon atom with 4 different atoms/groups attached
All molecules with one stereogenic centre of chiral
What is a racemic mixture
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)
If something is enantiomerically pure it …
is a single (pure) enantiomer of a chiral compound
Nature is a major source of enantiomerically pure compounds
What is the CIP system
Assigns a letter to each enantiomer (R or S) that describes the configuration of groups at a particu;ar stereogenic (chiral) centre
Using the CIP system, assign a letter to the following compound
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]
Chiral molecules are optically active meaning
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
How does a polarimeter work
- 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
Why do you use optical rotation to distinguish enantiomers
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
During polarisation, enantiomers will rotate either to the left or to the right
How is this shown through nomenclature
Rotating plane polarised light to the right = (+)
Rotating plane polarised light to the left = (-)
e.g. (S)-(+)-carvone
What can the exact angle of rotation (α) be affected by
How is this standardised
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
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
- 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
If we compare
What can we say about these two isomers of 3-aminobutan-2-ol
Both centres are swapped
They are mirror images of another and non-superimposable meaning they are enantiomers
