D.8.4 Explain the increases potency of diamorphine (heroin) compared to morphine

Although diamorphine (heroin) and morphine share the same basic structure, heroin is more potent and thus more addictive because of its polarity due to the presence of its functional groups.

Below are the structures of heroin (on the left) and morphine (on the right)
Heroin (left) - Morphine (right)

Heroin (left) - Morphine (right)

Comparison of structures:
Identical except the functional groups on the left.
Heroin has two ester groups which are less polar.
Morphine has 2 polar hydroxyl groups

Heroin is insoluble in water because it is less polar due to the presence of ester groups that can't form hydrogen bonds with water. Heroin can therefore be transported to the less polar/lipid parts of the body like the brain and nervous systems.Heroin will reach higher concentrations in the brain (due to is less polar nature) and thus will appear more potent and produce greater analgesic effects, which in turn, makes it more addictive.
Morphine is soluble in water because the polar hydoxyl groups can form hydrogen bonds with water and will penetrate the brain less easily than heroin.