6.2.4: Chirality

Question 1

What are stereoisomers?

Answer 1

• A class pf isomers that have different arrangement of atoms in space.
• These isomers are generated because of a lack of rotation of the C=C bond.

Question 2

What is optical isomerism?

Answer 2

• Another type of stereoisomerism.
• The molecules are non-superimposeable mirror images of each other and are known as ‘enantiomers’.
• They have the same chemical properties but interact with polarised light differently.

Question 3

enantiomers have the same chemical properties and similar physical, but they may have different biological properties. Give an example of this.

Answer 3

• Limolene (1-methyl-4-(1-methylethenyl)-cyclohexane)
• It’ a chiral hydrocarbon that is naturally found in citrus fruit.
• One enantiomer smells strongly of oranges, while the other enantiomer smells of pine.

Question 4

How can each enantiomer be given a prefix to denote its effect on a plane of polarised light and how can this be determined?

Answer 4

• If the enantiomer rotates the plane of polarised light in the clockwise direction it is given the prefix ‘+’.
• The prefix ‘-‘ is given if the rotation is in the anticlockwise direction.
• Can only be determined by experiment and cannot be deduced from the structure.

Question 5

What is a 50:50 mix of enantiomers called?

Answer 5

A racemic mixture or a racemate.

Question 6

What effect does a racemic mixture have on polarised light?

Answer 6

-A racemic mixture will have no effect on polarised light as the rotation of the two isomers cancel each other out.

Question 7

What must happen for a optical isomers to be formed in an organic compound?

Answer 7

• there must be a carbon atom with four different groups attached.
• This carbon is known as the chiral centre and is often shown with a ‘*’ on diagrams of the molecule.

Question 8

What is the chiral centre?

Answer 8

• The carbon atom with four different groups attached known as the chiral centre and is often shown with a ‘*’ on diagrams of the molecule.
• some molecules will have more than one chiral centre.
• It is also possible for inorganic atoms such as nitrogen to be chiral centres.

Question 9

Why do optical isomers not have a plane of symmetry?

Answer 9

Due to the asymmetric, or chiral, carbon.