Question

Explain why \(\mathrm{OH}^{\circ}\) reacts more quickly than \(\mathrm{HOO}^{\circ}\) to abstract hydrogen from other molecules.

Short answer

\(\mathrm{OH}^{\circ}\) is more reactive due to its strong tendency to stabilize its unpaired electron, the lower energy needed to break its bonds, and less steric hindrance.

Step-by-step solution

Analyze Reactivity

The reactivity of a radical depends on its ability to stabilize the extra electron. The hydroxyl radical \(\mathrm{OH}^{\circ}\) is highly reactive because it is an unpaired electron and oxygen is very electronegative, making it a strong tendency to gain electrons.

Consider Bond Strength

Hydrogen abstraction involves breaking the O-H or O-O bond in \(\mathrm{OH}^{\circ}\) and \(\mathrm{HOO}^{\circ}\) respectively. The bond energy required to break the O-H bond in \(\mathrm{OH}^{\circ}\) is less than that for O-O bond in \(\mathrm{HOO}^{\circ}\), thus \(\mathrm{OH}^{\circ}\) reacts faster.

Examine Steric Effects

\(\mathrm{OH}^{\circ}\) is a smaller molecule compared to \(\mathrm{HOO}^{\circ}\). This smaller size means there is less steric hindrance, allowing \(\mathrm{OH}^{\circ}\) to approach other molecules more easily to abstract hydrogen.

Key concepts

Hydroxyl Radical

The hydroxyl radical (\(\mathrm{OH}^{\circ}\)) is one of the most reactive species in chemistry. It features an unpaired electron that greatly contributes to its reactivity, making it highly reactive and capable of rapidly initiating chemical reactions. This radical is crucial in atmospheric and environmental chemistry, influencing processes such as pollutant degradation and the breakdown of organic compounds. The presence of an unpaired electron on a highly electronegative oxygen atom causes \(\mathrm{OH}^{\circ}\) to seek out electrons to pair with, explaining its aggressive reactivity.
This electron-seeking behavior allows it to interact vigorously with various substrates, making it one of the key species in radical chemistry.

Hydrogen Abstraction

Hydrogen abstraction is a fundamental process in radical chemistry where a radical removes a hydrogen atom from another molecule. This process essentially involves breaking a bond in one molecule and forming a new bond with the hydrogen atom. The efficiency of hydrogen abstraction depends heavily on the strength and type of bond that needs to be broken.
In the context of the hydroxyl radical, it is adept at abstracting hydrogen due to its high reactivity. The presence of its unpaired electron enables it to effectively destabilize existing bonds and form new ones with hydrogen atoms from other molecules. This mechanism is vital in many chemical reactions, especially those involved in combustion and atmospheric chemistry, where it can initiate chain reactions leading to further radical formation.

Bond Energy

Bond energy is a critical concept when understanding why some radicals like the hydroxyl radical react faster than others. It refers to the amount of energy required to break a specific chemical bond. In the case of \(\mathrm{OH}^{\circ}\) and \(\mathrm{HOO}^{\circ}\), the difference in bond energies between these radicals greatly influences their reactivity.
The bond energy required to break the O-H bond in \(\mathrm{OH}^{\circ}\) is less than the energy needed to break the O-O bond in \(\mathrm{HOO}^{\circ}\). This lower bond energy requirement enables \(\mathrm{OH}^{\circ}\) to more readily abstract hydrogen. This fundamental difference helps explain why \(\mathrm{OH}^{\circ}\) reacts more quickly in chemical reactions, initiating reactions with less energy input compared to \(\mathrm{HOO}^{\circ}\).

Steric Effects

Steric effects pertain to the influence of the physical size and spatial arrangement of atoms within a molecule on its reactivity. These effects can hinder or promote reactions, depending on how easily a molecule can approach its target.
The hydroxyl radical, being smaller than \(\mathrm{HOO}^{\circ}\), experiences fewer steric hindrances when interacting with other molecules. This smaller size allows \(\mathrm{OH}^{\circ}\) greater flexibility and freedom to get close to potential bonding sites on a substrate, facilitating hydrogen abstraction. Conversely, the larger size of \(\mathrm{HOO}^{\circ}\) presents more steric hindrance, slowing down its ability to react with substrates effectively. This spatial consideration is crucial in understanding how and why certain radicals are more reactive or faster in chemical reactions than others.