Question

which of the following amphetamine derivatives is more likely to cross the fat/oil - like blood - brain barrier and therefore have greater potency and potential for abuse?
oh
ho
nh2
ho
h
n - ch3
ch3
nh2
ch3
nh2

Explanation:

The blood - brain barrier is lipid - soluble. Amphetamine derivatives with fewer polar functional groups are more lipid - soluble and more likely to cross it. The first structure has multiple hydroxyl ($-OH$) groups which are polar. The second structure (methylamphetamine) has a methyl group and an amino group, and is more lipid - soluble compared to the one with multiple $-OH$ groups. The third (amphetamine) and fourth (phenethylamine) are also less complex than the first but methylamphetamine has an extra methyl group on the nitrogen which can enhance its lipid - solubility and ability to cross the blood - brain barrier, increasing potency and abuse potential.

Answer:

The second option (methylamphetamine structure: with a benzene ring, a methyl - substituted carbon adjacent to the nitrogen, and a methyl - substituted nitrogen)