Chirality 1+2

Question 1

Define steroisomers.

Answer 1

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

Question 2

Define constitutional (structural) isomerism.

Answer 2

Molecules with the same formula but different connectivity of C skeleton

Question 3

State the differences between isomers.

Answer 3

• Different physical properties e.g. m.p., b.p., solubility, density
• Different chemical and pharmaceutical properties

Question 4

What can constitutional isomers not do?

Answer 4

Interconvert

Question 5

Define geometric isomerism.

Answer 5

• Same formulae and same connectivity of C skeleton
• Different arrangement of substituents around alkene
• Two substituents on opposite ends of pi-bond
• on same face = cis
• on opposite faces = trans

Question 6

Why is geometry important for tamoxifen?

Answer 6

Z-tamoxifen is complementary in size and shape to oestrogen receptor causing inhibition of the signalling that would result in cell growth, thus breast cancer cell can be inhibited whereas E-tamoxifen is the incorrect shape so there would be no inhibition

Question 7

When does stereoisomerism arise?

Answer 7

• Arises due to asymmetry around a saturated carbon atom ( 4 diff groups)
• Rarely arises due to restricted rotation

Question 8

State features of a chiral molecule.

Answer 8

• No internal plane of symmetry
• The mirror image of the molecule is non-superimposable

Question 9

Describe the hybridisation of a chiral carbon.

Answer 9

A chiral carbon atom is sp3 hybridised and has
four different atoms or groups attached to it

Question 10

How can we test for a chiral carbon?

Answer 10

• Whether or not its mirror image is superimposable
• If non-superimposable it is chiral
• non-superimposable mirror image forms are called enantiomers

Question 11

Compare the properties of R and S isomers.

Answer 11

• Have identical physical and chemical properties e,g. m.p., b.p., solubility, pKa

Question 12

How is plane polarised light produced?

Answer 12

By passing light through a polarising filter

Question 13

What is a racemic mixture?

Answer 13

• A mixture which contains an equal amount of the two enantiomers

Question 14

Describe the optical activity of a racemic micture.

Answer 14

Optically inactive

Question 15

Describe the rotation of PPL by enantiomers.

Answer 15

• Rotate PPL in opposite directions by equal amounts

Question 16

What is enantiomeric excess? (ee)

Answer 16

Amount of pure enantiomer in excess of the racemic mixture

Question 17

How can we measure ee?

Answer 17

% ee = moles of one enantiomer - moles of the other/total moles of both x 100

or

Observed specific rotation/specific rotation of the pure enantiomer x 100

Question 18

How do R and S enantiomers interact with other chiral molecules?

Answer 18

(R)-enantiomer active binds
correctly to receptor to trigger
response
(S)-enantiomer cannot bind
correctly, does not trigger
response

Question 19

How are all enzymes and receptors chiral?

Answer 19

Because they are formed from amino acids which are chiral

Question 20

If a molecule has an internal plane of symmetry what is it called?

Answer 20

A meso compound