Fundamentals 1 Flashcards
Define chirality
- The geometric property of a molecule (or rigid object) of being non-superimposable on it’s mirror image
- Contain no rotation-reflection axis- Sn
- Including mirror planes- S1 and centre of inversion S2
What are the 4 Molecular symmetry types
- Proper rotation
- Plane of symmetry
- Center of inversion symmetry
- Improper roation
Give the symbol, symmetry element and symmetry operation of proper rotation
- symbol = Cn (n= degree of rotation 360/n)
- Element = Proper rotation axis
- Operation= rotation by 360/n degrees
- Principle axis - highest order proper rotation axis
Give the symbol, symmetry element and symmetry operation of plane of symmetry
- Symbol= sigma
- Element = reflection plane
- Operation = reflection in the plane - not the same as reflection in external mirror as that would have undergone translation
Give the symbol, symmetry element and symmetry operation of Center of inversion symmetry
- Symbol= i
- Element = inversion centre - always at centre of molecule
- Operation = inversion of point x,y,z to -x,-y-z
Give the symbol, symmetry element and symmetry operation of Improper Rotation
- Sn
- ELement- improper rotation axis
- Operation- rotation by 360/n then reflection perpendicular to reflection axis
What is S1 and S2 equivalent to
- S1= reflection- as rotating 360 degrees so just reflection
- S2= inversion
Define assymetric
- Used for a molecule devoid or any symmetry elements - such as tetrahedral carbon atom surrounded by 4 different substituents
Is asymmetric and chiral synonymous
- No
- Chiral molecules can still possess Cn axes even when they are non-superimposable on their mirror image
What does dissymmetric mean
- Lacking one particular symmetry element
- For chiral dissymmetric molecules this is the Sn axis
What is difference between chiral molecule and enantiomerically pure
- Property of chirality is to do with the molecule and is unrelated to the enantiomeric composition- which is make up of collection of molecules
Define enantiomer
- Either of a pair of chemical compounds whose molecular structures have a nonsuperimposable mirror-image relationship to each other
- A molecule that possesses the property of chirality with have two possible mirror-image, non-superimposable stereoisomers known as enantiomers
Define stereoisomer
- Is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space.
What are optical isomers
- Sometimes used as a synonym of enantiomers but it is a redundant term and strongly discouraged
- Not all chiral, non-racemic compounds have measurable optical rotations
What is a stereogenic centre
- An atom for which permutation of any two ligands generates a stereoisomer
- Its substituent groups give rise to chirality
Describe physical properties of enantiomers
- Show identical scalar physical properties in an achiral environment
- In a chiral environment (enzyme interior or chiral stationary phase) individual enantiomers may show different scalar physical properties
Why should you not use chiral centre
- Refers to stereogenic centre in a chiral molecule
- Chirality is property of molecule as a whole not the atoms in it
- May have stereo centres but not be a chiral molecule
Can you get chiral conformations of achiral compounds
- Yes- racemic distributions of chiral conformations lead to macroscopic achirality
give example of an achiral compound with chiral conformers
- Butane - achiral compound but has chiral conformers
- Two achiral co-planar conformations - syn and anti
- But, the enantiomeric chiral conformations being rapidly interconvertible and equal in energy are always present in a 50:50 ratio- racemic
- So from a macroscopic perspective butane is an achiral compound because its enantiomeric forms are far too short-lived for separation of its enantiomeric conformers to be possible.
Can a chiral molecule have achiral conformations
- No
- If it did it would forget which enantiomer it came from and have a 50:50 chance of then converting to the opposite enantiomeric conformation
What is used to specify the absolute configuration of a particular stereocentre
- R/S stereodescriptors
- Rank groups in order of priority
- Lowest group pointing away
- Highest to lowest priority- clockwise= R and anticlockwise= S
How do you assign R or S to a stereogenic axis
- Look down from one end of axis
- Substituents that are further away are lower priority
- Clockwise= R etc
What determines a substituent priority
- Cahn-Ingold-Prelog (CIP) priority rules
- Based on atomic weights
What happens when assigning priorities when there are multiply bonded groups e.g. -CH=O
- Each atom in the multiple bond is regarded as being associated with a phantom (duplicate atom) at the other end of the double bond
- e.g. C=O is treated as though the carbon has two single bonds to two oxygens
How are priorities assigned to cyclic compounds
- The structure is converted to an acyclic tree diagram by turning the ring into two separate chains that each terminate in a phantom atom- represents the other ring atom attached to the stereocentre
Are melting points of a racemate or an pure enantiomer higher or lowe
- Lower as mixture
Describe optical activity of chiral molecules
- When a beam of polarised light passes through a solution of chiral organic molecules the plane of polarisation is rotated
- The amount of rotation can be measured with a polarimeter
- Amount of rotation, alpha, in degrees is measured
What are the names for enantiomers which rotate plane of polarised light left or right
- Rotate plane of polarised light left/ anticlockwise = laevorotatory
- Opposite enantiomer will rotate it to the right = dextrorotatory
How do we make the expression of optical rotations meaningful
- Specify standard conditions
- THe specific rotation [alpha]D of a compound is defined as the observed rotation when
- The sample pathway, l, is equal to 1dm (10cm)
- The sample concentration c, is equal to 1 g per 100 mL
- Light of 589 nm wavelenggth is used
- The temperature is specified in superscript
What is the equation for the specific rotation
- [alpha]TD = alpha/c*l
- Units= 10^-1 deg cm^2 g^-1
- Alpha in formula = angle
- [alpha]TD = amount of rotation
Define enantiomerically pure
- A sample of chiral compounds with only one enantiomer
Define Enantiomerically-enriched
- A sample of chiral compounds which contains both enantiomers but one predominates
What happens if there is a racemic mixture
- If a 50;50 mixture of enantiomers is present then the rotation effects will cancel out and the mixture will appear to have no optical activity
What are psuedoscalar properties (chiroptical properties) and give examples
- Changes sign but not magnitude on reflection
- Optical rotary dispersion-ORD
- Circular dichroism (CD) - absorption spectroscopy method based on differential absorption of left and right circularly-polarised light - Optically active molecules will preferentially absorb one direction of the circularly polarised light
- Vibrational circular dichroism (VCD)
