Chap 11B - Isomerism

State conditions for EI

1. Often contain one or more chiral centres

2. Not possess any internal plane (or centre) of symmetry

• If a molecule has a plane of symmetry, then it is achiral -> its mirror image can be superimposed onto itself

• A superimposable mirror image is when the mirror image of an object (or molecule) can be perfectly placed on top of the original object so that all parts match exactly

• Hence , its must have a non-superimposable mirror image 

• The chiral molecule and its non-superimposable mirror image (Eg. Reflection of hands) form a pair of enantiomers

Describe chiral centre

• A carbon atom with four different atoms or groups of atoms attached

• A chiral centre is denoted with an * on the structure

• This lack of symmetry allows for two distinct, non-superimposable mirror images

• A molecule may contain chiral carbon atoms but overall the molecule may not be chiral

Describe molecules with 1 chiral centre 

• The molecules have the same structural formula

• The two molecules differ in the spatial arrangement of these groups around the chiral centre

• The molecules are non-superimposable mirror images of each other and are enantiomers

Describe molecules with more than 1 chiral centre 

• Can only exhibit EI if they do not possess any internal plane of symmetry 

• If there are m = chiral centres in a molecule + no. of C=C bond or C-C (part of ring structure) with cis-trans isomerism: maximum number of stereoisomers will be 2^m
  

Describe R and S configuration + molecule with 2 chiral centres

Each chiral center can exist in : 

1. R configuration : The groups are arranged clockwise 

2. S configuration : The groups are arranged counterclockwise

Eg. Molecule with 2 chiral centers 

• Each center can independently adopt either the R or S configuration

• There are 2^2 = 4 possible stereoisomers

• A: Left (R), Right (R)

• B: Left (S), Right (S)

• C: Left (R), Right (S)

• D: Left (S), Right (R)

Describe molecules without chiral centre

• Mirror images are superimposable -> enantiomerism does not exist 

• Other examples include molecules with bulky substituents preventing rotation around the single bond between the aromatic rings and helical molecules (Eg. DNA) 

Describe 1,3-dichloroprop-1,2-diene

• Functional group : C=C=C

• Due to the orientation of the overlapping p orbitals forming the two π bonds, the H and Cl atoms at one end do not lie in the same plane

• There is no internal plane of symmetry in the molecule

• Its mirror image is non-superimposable onto itself -> has two enantiomers, and is chiral

Describe properties of enantiomers

• Enantiomers have mostly identical physical and chemical properties, due to same structural formula

• When placed in a chiral environment or interacting with chiral molecules, the enantiomers will behave differently from each other

Describe enantiomers and plane-polarised light

1. A ray of normal light has waves which vibrate in many directions (2) at right angles to the direction of travel of the ray

• Polarisers have the ability to remove from normal light all waves except those vibrating in a single plane

2. The light is plane-polarised (4) after it passes through the polarise

3. Compounds that are optically active (chiral molecules) have the ability to rotate plane-polarised light (6) in either a clockwise or counter-clockwise direction, and to different extents

4. The direction and extent of rotation can be measured using an analyser (7)

5. The rotation of plane-polarised light is the result of interactions between light and the individual molecules in the sample that the light passes through

• Dextro-rotatory (+): light rotated right, clockwise 

• Laevo-rotatory (-): light roasted left, anti-clockwise

Describe observations of plane-polarised light of optically pure samples

• Contain either (+) or (-) enantiomer only

• For two optically pure samples, each containing the same concentration of either the (+) or (-) enantiomer, the observed rotation will be of the same extent but in opposite directions

Describe observations of plane-polarised light of racemic samples 

• Sample containing a 50:50 mixture of the (+) and (-) enantiomers

• The clockwise rotation caused by one enantiomer is exactly cancelled by the anti-clockwise rotation of the other enantiomer as the pair of equal amounts of enantiomers will rotate the plane of polarised light by the same extent but in opposite directions

• No net effect on passage of plane-polarised light and the sample is optically inactive

• A racemic mixture has the same chemical properties as the single enantiomers but differs in physical properties that depend on the packing of molecules in the solid (Eg. melting points and solubilities)

Describe chemical properties of enantiomers (And lactic acid and drug action example)

• Enantiomers have identical chemical properties except towards chiral reagents (their interactions with another chiral molecule) 

• Enantiomers have different biological properties 

  • Eg. Pure (+)-lactic acid is produced in our muscles when we exercise anaerobically while a racemic mixture of (+)-lactic acid and (–)- lactic acid is often produced when milk is fermented

  • Eg. Drug action: 

    • Drug molecules work by binding to targets like enzymes and receptors in the body via lock-and-key

    • As these targets are chiral in nature, only the drug molecule with the right spatial configuration (only one of the two enantiomers), can bind to the target and be effective

Explain how to count stereoisomers

• Stereoisomers include both cis–trans isomers and enantiomers

• If a molecule has m chiral centres and n double bonds that can give rise to cis–trans isomers, the maximum number of stereoisomers it can have = 2^(m+n)

• The actual number of stereoisomers is determined by considering any internal plane of symmetry in the possible structures