Questions: 4. Morphine is a natural alkaloid derived from the opium poppy and a commonly used analgesic for moderate to severe pain. Heroin is a semi-synthetic opioid, created in the laboratory by acylating morphine. Heroin is more potent than morphine and passes through the blood-brain barrier more readily. More lipophilic (nonpolar) compounds are known to pass through the blood-brain barrier. Examining the structures of morphine and heroin depicted below, why does heroin pass through the blood-brain barrier more easily than morphine? (Hint: Identify the difference(s) between these two structures. How does that difference affect the blood-brain barrier?)

Solution Steps

Morphine has two hydroxyl (-OH) groups, whereas heroin has two acetyl ester groups (-O-C(=O)-CH3) in place of the hydroxyl groups. This means heroin is formed by the acetylation of morphine.

The hydroxyl groups in morphine are polar and can participate in hydrogen bonding, making the molecule more hydrophilic (water-loving). The blood-brain barrier is hydrophobic (water-fearing), so these polar hydroxyl groups hinder morphine's ability to pass through.

Heroin, with its acetyl ester groups, is less polar and more lipophilic (fat-loving) than morphine. The acetyl groups mask the polar hydroxyl groups, reducing hydrogen bonding capability. This increased lipophilicity allows heroin to dissolve more readily in the lipid-rich cell membranes of the blood-brain barrier, allowing it to cross more easily and rapidly than morphine.

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