Option D Medicinal Chemistry HL

Question 1

What is taxol?

Answer 1

Taxol is an anti-cancer drug which is primarily used for the treatment of breast and ovarian cancer. Its properties are as a result of its ability to halt the cell cycle.

Question 2

How was taxol first isolated and what are the challenges with doing so?

Answer 2

Taxol was first isolated from the bark of Pacific yew trees. However the bark contains very small amounts of taxol (1kg of bark = 10g of taxol) and the process of stripping the bark kills the trees (which take 200 years to mature).

A major challenge is that taxol has many chiral carbon centres (11), so there are a large number of enantiomers. Only one enantiomer has the desired therapeutic effect and isolating this enantiomer is a wasteful process.

Question 3

Describe asymmetric synthesis.

Answer 3

In asymmetric synthesis, a chiral molecule binds to the reactant, blocking one reaction site through steric hindrance. Therefore the reaction can only take place from one side, forcing the reaction to occur with a specific stereochemistry so the desired enantiomer is produced.

Question 4

Give an example when a racemic mixture was used with detrimental effects.

Answer 4

The thalidomide tragedy of the 1950’s-60’s. The drug was sold as a racemic mixture. However only the R isomer had the desired effect of inducing sleep on pregnant women. The S isomer caused serious deformities in the womb.

Question 5

Describe the semi-synthetic route of producing taxol.

Answer 5

A precursor of taxol (10-DAB) is extracted from the needles and leaves of yew trees and then chemically modified to form taxol. This gives a higher yield and is a more sustainable approach.

Question 6

How are different enantiomers distinguished between?

Answer 6

Plane-polarised light is passed through the solution. An analyser is used to determine the angle of rotation of the plane of the plane-polarised light. The different enantiomers rotate the plane-polarised light by the same angle but in opposite directions.

Question 7

Discuss side effects of radiation.

Answer 7

Side effects of radiation/radiotherapy occur because it can also damage healthy cells and tissues near the area being treated.

• hair loss
• nausea
• fatigue
• skin disorders
• loss of appetite
• sterility

Question 8

Define half-life.

Answer 8

The time taken for the mass of a reactant to decrease to one half of its original value.

Question 9

What is technetium-99m used for?

Answer 9

Technetium-99m is used in most diagnostic radiotherapy. This is because it has a short half-life so it decays quickly which minimises exposure to the patient. (Gamma decay -> Tc-99, then beta decay). It is a chemically reactive and versatile transition metal so bonds to a range of biologically reactive substances.

Question 10

What is iodine-131 used for?

Answer 10

Iodine-131 is used for the treatment of thyroid disease and types of thyroid cancer. It undergoes gamma then beta decay.

Question 11

What is yttrium-90 used for?

Answer 11

Used for targeted radionuclide therapy to target liver tumours. Tiny beads are imbedded with Y-90. These get stuck in the small blood vessels in the liver giving a targeted treatment.

Question 12

Describe MRI.

Answer 12

MRI’s are used to give detailed images of the body and are used for cancer detection, diagnosis of soft tissue injuries and to monitor degenerative diseases. It does not use ionising radiation and instead uses radio waves which are low energy so MRI’s are low risk.

Question 13

Describe targeted alpha therapy.

Answer 13

Targeted alpha therapy (TAT) is a form of cancer treatment for the blood (leukemia) which uses radioactive isotopes that undergo alpha decay. The alpha particle is highly ionising so it is effective at killing cancer cells. It also has a short range so damage is minimal to surrounding healthy cells.

Question 14

Describe boron neutron capture therapy.

Answer 14

Boron neutron capture therapy (BNCT) is a form of cancer treatment used in the head and neck. Boron-10, a non radioactive isotope of boron, is injected. The patient is then irradiated with neutrons which convert the boron to an isotope of lithium and an alpha particle is emitted. The alpha radiation acts directly on the tumour.

*The effectiveness of the treatment depends on the uptake of boron by the cancer cells.

Question 15

What are methods of purification of organic products?

Answer 15

• recrystallisation
• distillation
• fractional distillation
• solvent extraction

Question 16

How are steroids detected in athletes?

Answer 16

Gas chromatography can be combined with mass spectroscopy to detect the presence of steroids in urine samples of athletes. Gas chromatography separates the chemical mixture into pure chemicals. Mass spectroscopy identifies the components.

Question 17

How can the presence of alcohol be tested in the human body?

Answer 17

1) Breathalyser (detects alcohol by a redox reaction that produces a colour change).
2) Alcosensor (detects alcohol using fuel cell technology).
3) Intoximeter (detects alcohol by infra-red spectroscopy).

Question 18

Describe how breathalysers work.

Answer 18

In a breathalyser, breath is blown through a tube containing crystals of potassium dichromate. The ethanol in the breath is oxidised to form ethanoic acid. The dichromate ion is reduced to form the Cr3+ ion. The colour change is from orange to green.

Question 19

Describe gas chromatography.

Answer 19

Compounds of a solution have different affinities for the mobile and stationary phase so move at different rates through the instrument. This separates the mixture into pure chemicals.

Question 20

Describe how alcosensors work.

Answer 20

An alcosensor contains a fuel cell with two platinum electrodes and a porous acid-electrolyte material between them. Exhaled air is passed over the cell and any ethanol present is oxidised to ethanoic acid at the anode. Electrons pass through the external circuit to the cathode where O2 is reduced. The electric current produced by the cell is proportional to the concentration of alcohol.