Fundamentals 1

Question 1

Define chirality

Answer 1

1. The geometric property of a molecule (or rigid object) of being non-superimposable on it’s mirror image
2. Contain no rotation-reflection axis- Sn
3. Including mirror planes- S1 and centre of inversion S2

Question 2

What are the 4 Molecular symmetry types

Answer 2

1. Proper rotation
2. Plane of symmetry
3. Center of inversion symmetry
4. Improper roation

Question 3

Give the symbol, symmetry element and symmetry operation of proper rotation

Answer 3

1. symbol = Cn (n= degree of rotation 360/n)
2. Element = Proper rotation axis
3. Operation= rotation by 360/n degrees
4. Principle axis - highest order proper rotation axis

Question 4

Give the symbol, symmetry element and symmetry operation of plane of symmetry

Answer 4

1. Symbol= sigma
2. Element = reflection plane
3. Operation = reflection in the plane - not the same as reflection in external mirror as that would have undergone translation

Question 5

Give the symbol, symmetry element and symmetry operation of Center of inversion symmetry

Answer 5

1. Symbol= i
2. Element = inversion centre - always at centre of molecule
3. Operation = inversion of point x,y,z to -x,-y-z

Question 6

Give the symbol, symmetry element and symmetry operation of Improper Rotation

Answer 6

1. Sn
2. ELement- improper rotation axis
3. Operation- rotation by 360/n then reflection perpendicular to reflection axis

Question 7

What is S1 and S2 equivalent to

Answer 7

1. S1= reflection- as rotating 360 degrees so just reflection
2. S2= inversion

Question 8

Define assymetric

Answer 8

1. Used for a molecule devoid or any symmetry elements - such as tetrahedral carbon atom surrounded by 4 different substituents

Question 9

Is asymmetric and chiral synonymous

Answer 9

1. No
2. Chiral molecules can still possess Cn axes even when they are non-superimposable on their mirror image

Question 10

What does dissymmetric mean

Answer 10

1. Lacking one particular symmetry element
2. For chiral dissymmetric molecules this is the Sn axis

Question 11

What is difference between chiral molecule and enantiomerically pure

Answer 11

1. Property of chirality is to do with the molecule and is unrelated to the enantiomeric composition- which is make up of collection of molecules

Question 12

Define enantiomer

Answer 12

1. Either of a pair of chemical compounds whose molecular structures have a nonsuperimposable mirror-image relationship to each other
2. A molecule that possesses the property of chirality with have two possible mirror-image, non-superimposable stereoisomers known as enantiomers

Question 13

Define stereoisomer

Answer 13

1. 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.

Question 14

What are optical isomers

Answer 14

1. Sometimes used as a synonym of enantiomers but it is a redundant term and strongly discouraged
2. Not all chiral, non-racemic compounds have measurable optical rotations

Question 15

What is a stereogenic centre

Answer 15

1. An atom for which permutation of any two ligands generates a stereoisomer
2. Its substituent groups give rise to chirality

Question 16

Describe physical properties of enantiomers

Answer 16

1. Show identical scalar physical properties in an achiral environment
2. In a chiral environment (enzyme interior or chiral stationary phase) individual enantiomers may show different scalar physical properties

Question 17

Why should you not use chiral centre

Answer 17

1. Refers to stereogenic centre in a chiral molecule
2. Chirality is property of molecule as a whole not the atoms in it
3. May have stereo centres but not be a chiral molecule

Question 18

Can you get chiral conformations of achiral compounds

Answer 18

1. Yes- racemic distributions of chiral conformations lead to macroscopic achirality

Question 19

give example of an achiral compound with chiral conformers

Answer 19

1. Butane - achiral compound but has chiral conformers
2. Two achiral co-planar conformations - syn and anti
3. But, the enantiomeric chiral conformations being rapidly interconvertible and equal in energy are always present in a 50:50 ratio- racemic
4. 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.

Question 20

Can a chiral molecule have achiral conformations

Answer 20

1. No
2. 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

Question 21

What is used to specify the absolute configuration of a particular stereocentre

Answer 21

1. R/S stereodescriptors
2. Rank groups in order of priority
3. Lowest group pointing away
4. Highest to lowest priority- clockwise= R and anticlockwise= S

Question 22

How do you assign R or S to a stereogenic axis

Answer 22

1. Look down from one end of axis
2. Substituents that are further away are lower priority
3. Clockwise= R etc

Question 23

What determines a substituent priority

Answer 23

1. Cahn-Ingold-Prelog (CIP) priority rules
2. Based on atomic weights

Question 24

What happens when assigning priorities when there are multiply bonded groups e.g. -CH=O

Answer 24

1. Each atom in the multiple bond is regarded as being associated with a phantom (duplicate atom) at the other end of the double bond
2. e.g. C=O is treated as though the carbon has two single bonds to two oxygens

Question 25

How are priorities assigned to cyclic compounds

Answer 25

1. 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

Question 26

Are melting points of a racemate or an pure enantiomer higher or lowe

Answer 26

1. Lower as mixture

Question 27

Describe optical activity of chiral molecules

Answer 27

1. When a beam of polarised light passes through a solution of chiral organic molecules the plane of polarisation is rotated 2. The amount of rotation can be measured with a polarimeter 3. Amount of rotation, alpha, in degrees is measured

Question 28

What are the names for enantiomers which rotate plane of polarised light left or right

Answer 28

1. Rotate plane of polarised light left/ anticlockwise = laevorotatory 2. Opposite enantiomer will rotate it to the right = dextrorotatory

Question 29

How do we make the expression of optical rotations meaningful

Answer 29

1. Specify standard conditions 2. THe specific rotation [alpha]D of a compound is defined as the observed rotation when 3. The sample pathway, l, is equal to 1dm (10cm) 4. The sample concentration c, is equal to 1 g per 100 mL 5. Light of 589 nm wavelenggth is used 6. The temperature is specified in superscript

Question 30

What is the equation for the specific rotation

Answer 30

1. [alpha]TD = alpha/c*l 2. Units= 10^-1 deg cm^2 g^-1 3. Alpha in formula = angle 4. [alpha]TD = amount of rotation

Question 31

Define enantiomerically pure

Answer 31

1. A sample of chiral compounds with only one enantiomer

Question 32

Define Enantiomerically-enriched

Answer 32

1. A sample of chiral compounds which contains both enantiomers but one predominates

Question 33

What happens if there is a racemic mixture

Answer 33

1. 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

Question 34

What are psuedoscalar properties (chiroptical properties) and give examples

Answer 34

1. Changes sign but not magnitude on reflection 2. Optical rotary dispersion-ORD 3. 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 4. Vibrational circular dichroism (VCD)