17.1 chirality, carbonyl & carboxylic functional groups

Question 1

explain the term ‘chiral centre’.

Answer 1

• a chiral centre refers to a central carbon atom with four different functional groups attached to it.
• the arrangement of these functional groups around the central carbon forms a pair of isomers, known as enantiomers.

Question 2

enantiomers are non-superimposable. describe what is meant by the term ‘non-superimposable’.

Answer 2

• enantiomers are defined as a pair of stereoisomers that are non-superimposable mirror images of each other.
• they have the same atomic arrangement but different spatial arrangement, meaning that they cannot be superimposed without breaking and remaking bonds.

Question 3

optical isomers, such as enantiomers, are optically active. describe what is meant by the term ‘optically active’.

Answer 3

optical isomers are optically active - they rotate the plane of polarisation of plane-polarised monochromatic light.

Question 4

define the term ‘racemic mixture’, and explain why racemic mixtures do not rotate plane-polarised light.

Answer 4

• a racemic mixture (or racemate) contains equal quantities of each enantiomer in a chiral compound.
• in a pair of enantiomers, one enantiomer rotates plane-polarised light in a clockwise direction, whilst the other rotates it in an anticlockwise direction.
• racemic mixtures do not rotate plane-polarised light because the pair of enantiomers cancel out the other’s light-rotating effect.

Question 5

give the differences in the reactants used and products formed in an SN1 as opposed to SN2 mechanism.

Answer 5

SN1 mechanism

• reactant: single enantiomer.
• product: racemic mixture of two optical isomers.

SN2 mechanism

• reactant: single enantiomer.
• product: single enantiomer.