chirality

Define stereoisomers

Isomers with the same molecular formula and same connectivity of atoms but different arrangement of atoms in space

Define isomers

Different compounds with the same molecular formula

Define constitutional isomers

Isomers whose atoms have a different connectivity

Define enantiomers

Stereoisomers that are non superimposable mirror images of each other

Define diastereomers

Stereoisomers that are not mirror images of each other

Define geometric isomerism

Isomers with the same formulae and same connectivity of C skeleton but different spatial arrangement of substituents around alkene

Outline cis and trans geometric isomerism

Two substituents on opposite ends of pi bond

On same face = cis

On opposite faces = trans

Can you convert between cis and trans isomers at body/ambient temperature?

No - no rotation around double bond

Outline how you would use the Cahn-Ingold-Prelog (CIP) rules

consider the ends of the alkene separately

For each end, prioritise the two substituents according to the atomic number of the atom directly attached to the sp2 hybridised C (if necessary look at adjacent atoms until groups can be differentiated)

If priority 1 groups are on the same face of the alkene, use the prefix Z: if on opposite faces, use E

Define a racemic mixture

A 1:1 mixture of enantiomers

Define optically inactive

Achiral or racemic mixture

Describe chirality

Usually the result of a sp3 hybridised chiral carbon which has 4 different atoms/groups attached to it

Molecules have no internal plane of symmetry

The mirror image of the molecule is non-superimposable

What is the meaning of wedge and dash bonds?

Wedge approach you, dash move away from you

How would you determine D or L for carbohydrates?

Present most oxidised carbon at the top in the Fischer projection

Look at the bottom hydroxyl

To the left = L, right = D

How would you determine D or L for amino acids?

Look at position of amine group in Fischer projection compared to central carbon

Left = L, right = D

Describe how you would name molecules as R and S from perspective (3D) formula

1. Ensure chiral C is drawn in 3D with wedge bonds

2. Prioritise atoms directly attached to the chiral C using atomic number

3. Make sure priority 4 atom is at back (on dashed wedge bonds)

if order 1, 2, 3 is clockwise = R, if 1, 2, 3 is anticlockwise = S

(If priority 4 is at front, assign as usual then reverse answer)

Outline the similarities and differences between R and S isomers

Have identical physical and chemical properties (eg, m.p and b.p, solubility, pKa)

But will rotate plane polarised light in opposite directions in equal amounts and interact differently with other chiral molecules

Enantiomers of biological molecules and pharmaceuticals will bind with different affinity due to 3D fit

What is enantiomeric excess?

The amount of pure enantiomer in excess of the racemic mixture

If EE = 50% then the observed rotation will be only 50% of the rotation of the pure enantiomers

% rotation observed = % of enantiomer in excess - % of other enantiomer

Give the equation for % enantiomeric excess

((Moles of one enantiomer - moles of other enantiomer) / total moles of both enantiomer) x100

OR

(observes specific rotation/specific rotation of pure enantiomer) x100

Why are all enzymes and receptors in the body chiral?

They are formed from amino acids

How would you name enantiomers with more than one stereocentre?

Place carbon number before R or S (eg, 2R, 3S-name)

What is a molecule called if it has an internal plane of symmetry?

A meso compound

When and why would a medicine be administered as the racemate?

If benign

It is economically better not to separate the enantiomers

What does the 3-point bonding model explain?

How the medicinal activity often resides in only one of the enantiomers