Question 19M.3.sl.TZ1.18b
| Date | May 2019 | Marks available | [Maximum mark: 2] | Reference code | 19M.3.sl.TZ1.18b |
| Level | sl | Paper | 3 | Time zone | TZ1 |
| Command term | Discuss | Question number | b | Adapted from | N/A |
Opium and its derivatives have been used for thousands of years as strong analgesics.
Discuss how the difference in structure of two opiates, codeine and morphine, affect their ability to cross the blood–brain barrier. Use section 37 of the data booklet.
[2]
«codeine crosses blood–brain barrier more easily»
morphine has more hydroxyl/OH «groups than codeine» [✔]
codeine/ether group is less polar
OR
hydroxyl/OH «groups in morphine» H-bond to water [✔]
Note: Award [1 max] if no statement or an incorrect statement about the blood–brain barrier.
Many candidates identified the relevant structural difference from the data booklet structures and could use these to explain the difference in the ability of the drugs to reach the brain, though some were confused as to whether this was favoured by aqueous solubility. This again was more frequently discussed in terms of polarity than the ability of –OH groups to form hydrogen bonds.
