Problem 65 - Chemistry The Central Science

Question

Hydroxyl radicals can pluck hydrogen atoms from molecules ("hydrogen abstraction"), and hydroxide ions can pluck protons from molecules ("deprotonation"). Write the reaction equations and Lewis dot structures for the hydrogen abstraction and deprotonation reactions for the generic carboxylic acid \(\mathrm{R}-\mathrm{COOH}\) with hydroxyl radical and hydroxide ion, respectively. Why is hydroxyl radical more toxic to living systems than hydroxide ion?

Step-by-Step Solution

Answer

In the hydrogen abstraction reaction with a hydroxyl radical (•OH), R-COOH reacts with •OH to form R-CO• and H₂O. In the deprotonation reaction with a hydroxide ion (OH⁻), R-COOH reacts with OH⁻ to form R-COO⁻ and H₂O. The hydroxyl radical is more toxic to living systems due to its high reactivity and ability to disrupt essential biomolecules like DNA, proteins, lipids, and carbohydrates, leading to cell injury and death. In contrast, the hydroxide ion is less reactive and more selective, causing less destructive effects on biomolecules and lower toxicity.

Step 1: Write the balanced chemical equations for both reactions

For the hydrogen abstraction reaction with hydroxyl radical (•OH): \[ \mathrm{R-COOH} + \mathrm{•OH} \rightarrow \mathrm{R-CO•} + \mathrm{H_2O} \] For the deprotonation reaction with hydroxide ion (OH⁻): \[ \mathrm{R-COOH} + \mathrm{OH^-} \rightarrow \mathrm{R-COO^-} + \mathrm{H_2O} \]

Step 2: Draw the Lewis dot structures for these reactions

For the hydrogen abstraction reaction with hydroxyl radical (•OH), we have: - R-COOH: O=C(R)-OH - •OH: O• with one lone pair of electrons - R-CO•: O=C(R)• with two lone pairs of electrons on the oxygen atom - H₂O: O-H-H with two lone pairs of electrons on the oxygen atom For the deprotonation reaction with hydroxide ion (OH⁻), we have: - R-COOH: O=C(R)-OH - OH⁻: O-H with three lone pairs of electrons and a negative charge on the oxygen atom - R-COO⁻: O=C(R)-O⁻ with three lone pairs of electrons and a negative charge on the oxygen atom - H₂O: O-H-H with two lone pairs of electrons on the oxygen atom

Step 3: Explain why the hydroxyl radical is more toxic to living systems than the hydroxide ion

The hydroxyl radical (•OH) is more reactive and less selective than the hydroxide ion (OH⁻) because it has an unpaired electron, which makes it a highly reactive species. Due to this high reactivity, it can abstract a hydrogen atom from nearly any molecule, leading to disruption of biomolecules such as DNA, proteins, lipids, and carbohydrates in living organisms. This disruption of essential biomolecules can eventually cause cell injury and death, ultimately leading to toxicity in living systems. On the other hand, the hydroxide ion (OH⁻) is a weaker nucleophile, making it less reactive and more selective in its reactions. It can deprotonate molecules but at a slower rate and with less destructive effects on biomolecules compared to the hydroxyl radical. Hence, the hydroxide ion is less toxic than the hydroxyl radical to living systems.