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?

Short answer

The reaction equations and the Lewis dot structures for hydrogen abstraction and deprotonation of a generic carboxylic acid with hydroxyl radical and hydroxide ion can be represented as: Hydrogen Abstraction: \( \mathrm{HO} \cdot + \mathrm{R}-\mathrm{COOH} \rightarrow \mathrm{H_2O} + \mathrm{R}-\mathrm{COO} \cdot \) Lewis dot structures: \(\mathrm{HO} \cdot\), \(\mathrm{R} \equiv \stackrel{\displaystyle \mathrm{O}}{\mathrm{C}}-\stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\), \(\mathrm{H}\stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\), \(\mathrm{R}\equiv \stackrel{\displaystyle \mathrm{O}}{\mathrm{C}}-\mathrm{O} \cdot\) Deprotonation: \( \mathrm{OH}^- + \mathrm{R}-\mathrm{COOH} \rightarrow \mathrm{H_2O} + \mathrm{R}-\mathrm{COO}^-\) Lewis dot structures: \(\mathrm{\stackrel{\displaystyle O}{H}}^-\), \(\mathrm{R} \equiv \stackrel{\displaystyle \mathrm{O}}{\mathrm{C}}-\stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\), \(\mathrm{H}\stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\), \(\mathrm{R} \equiv \stackrel{\displaystyle \mathrm{O}}{\mathrm{C}}-\mathrm{O}^- \equiv\) Hydroxyl radicals are more toxic to living systems compared to hydroxide ions due to their high reactivity caused by the presence of an unpaired electron, making them strong oxidizing agents capable of causing widespread cellular damage. Meanwhile, hydroxide ions are less reactive and participate in controlled reactions, hence less toxic to living systems.

Step-by-step solution

Write the reaction equation for hydrogen abstraction

A hydroxyl radical is represented as \(\mathrm{HO} \cdot\) and it interacts with the carboxylic acid molecule (\(\mathrm{R}-\mathrm{COOH}\)) by abstracting a hydrogen atom from it. The reaction equation can be written as follows: \(\mathrm{HO} \cdot + \mathrm{R}-\mathrm{COOH} \rightarrow \mathrm{H_2O} + \mathrm{R}-\mathrm{COO} \cdot\)

Write the reaction equation for deprotonation

A hydroxide ion is represented as \(\mathrm{OH}^-\) and it interacts with the carboxylic acid molecule (\(\mathrm{R}-\mathrm{COOH}\)) by taking a proton (H\(^+\)) from it. The reaction equation can be written as follows: \(\mathrm{OH}^- + \mathrm{R}-\mathrm{COOH} \rightarrow \mathrm{H_2O} + \mathrm{R}-\mathrm{COO}^-\)

Draw Lewis dot structures for hydrogen abstraction

Lewis dot structures represent the valence electrons of atoms and molecules involved in the reactions. For hydrogen abstraction, the following Lewis dot structures can be drawn: Hydroxyl radical: \(\:\:\:\: \: \cdot \stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\) Carboxylic acid: \(\mathrm{R} \equiv \stackrel{\displaystyle \mathrm{O}}{\mathrm{C}}-\stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\) After the hydrogen abstraction reaction, we get: Water: \(\mathrm{H}\stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\) Alkoxyl radical: \(\mathrm{R}\equiv \stackrel{\displaystyle \mathrm{O}}{\mathrm{C}}-\mathrm{O} \cdot\)

Draw Lewis dot structures for deprotonation

For deprotonation, we consider the following Lewis dot structures: Hydroxide ion: \(\:\:\:\: \: \mathrm{\stackrel{\displaystyle O}{H}}^-\) Carboxylic acid: \(\mathrm{R} \equiv \stackrel{\displaystyle \mathrm{O}}{\mathrm{C}}-\stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\) After the deprotonation reaction, we get: Water: \(\mathrm{H}\stackrel{\displaystyle \mathrm{O}}{\mathrm{H}}\) Carboxylate ion: \(\mathrm{R} \equiv \stackrel{\displaystyle \mathrm{O}}{\mathrm{C}}-\mathrm{O}^- \equiv\)

Explain why hydroxyl radical is more toxic than hydroxide ion

Hydroxyl radicals are highly reactive due to the presence of the unpaired electron, which makes them strong oxidizing agents. They can easily abstract hydrogen atoms from molecules, resulting in the formation of other reactive species like alkoxyl radicals. In living systems, hydroxyl radicals can unspecifically attack various cellular components such as lipids, proteins, and DNA, and cause oxidative damage. This can lead to cell dysfunction, inflammation, and even cell death. On the other hand, hydroxide ions are less reactive and selective in their interactions with molecules. They often act as weak nucleophiles, attacking electrophilic sites, and perform deprotonation, generally in a controlled manner. As a result, hydroxyl radicals are more toxic and hazardous to living systems, causing widespread cellular damage, while hydroxide ions are less toxic and participate in controlled reactions.

Key concepts

Hydrogen Abstraction

Hydrogen abstraction is a process where a hydrogen atom is removed from a molecule by a radical, such as a hydroxyl radical (\(\mathrm{HO}\cdot\)). This radical features an unpaired electron, making it highly reactive and ready to bond with the hydrogen atom. When the hydroxyl radical interacts with a carboxylic acid\(\mathrm{(R-COOH)}\), it takes away a hydrogen atom, leading to the formation of water\(\mathrm{(H_2O)}\) and a carbon-centered radical, an alkoxyl radical\(\mathrm{(R-COO\cdot)}\).

Hydrogen abstraction is significant due to its role in free radical chain reactions, leading to further chemical transformations. This process can initiate polymerization or lead to the degradation of certain materials. In biological systems, this reactivity can damage essential molecules, including DNA, posing risks to cell health and function.

Deprotonation

Deprotonation involves the removal of a proton (\(H^+\)) from a molecule. It is often facilitated by a base, such as a hydroxide ion (\(\mathrm{OH}^-\)), interacting with an acidic hydrogen in carboxylic acids. In the deprotonation process of a generic carboxylic acid\(\mathrm{(R-COOH)}\), the hydroxide ion abstracts the proton, resulting in the formation of water \(\mathrm{(H_2O)}\) and a carboxylate ion\(\mathrm{(R-COO^-)}\).

This reaction is a fundamental concept in acid-base chemistry, reflecting the behavior of acids and bases in various environments, including biological ones. Deprotonation is also crucial in maintaining pH balance in biological systems, regulating enzyme activity, and facilitating molecular interactions.

• It is generally a controlled process.
• Deprotonation tends to be less destructive compared to radical reactions.

Lewis Dot Structures

Lewis dot structures are visual representations of the valence electrons in atoms and molecules. These structures help depict chemical reactions, providing clarity on how electrons are shared or transferred between atoms. For a hydroxyl radical (\(\mathrm{HO}\cdot\)), the Lewis dot structure highlights its unpaired electron. In contrast, the hydroxide ion (\(\mathrm{OH}^-\)) has a full valence electron shell, represented by complete pairs of electrons around the oxygen atom.

When representing carboxylic acid (\(\mathrm{R-COOH}\)), the Lewis structure includes shared electron pairs between carbon, oxygen, and hydrogen atoms. This depiction aids in understanding the molecular transformations during reactions like hydrogen abstraction and deprotonation.

For students and chemists, utilizing Lewis structures bridges the gap between theoretical chemical equations and practical molecular interactions, improving comprehension of complex chemical behaviors.