Question

A third radical animation is "Benzylic oxidation." This reaction is shown using oxygen \(\left(\mathrm{O}_{2}\right)\) in the initiation step. Why can oxygen remove a hydrogen from isopropylbenzene? Is the benzylic hydrogen acidic? Is oxygen basic? What is the hybridization of the benzylic carbon in the intermediate? Is the hybridization of the benzylic carbon fixed or does it change?

Short answer

Oxygen removes hydrogen because it forms a radical. The benzylic hydrogen is not highly acidic. Oxygen in this case is not acting as a base. The benzylic carbon's hybridization remains sp3.

Step-by-step solution

Introduction to the Reaction

The reaction in question is the benzylic oxidation of isopropylbenzene using oxygen \( \left(\mathrm{O}_{2}\right) \), typically in the initiation step of a radical reaction.

Reason for Hydrogen Removal by Oxygen

Oxygen can remove a hydrogen from isopropylbenzene because it forms a radical when it splits into two oxygen atoms. These oxygen radicals are highly reactive and can abstract hydrogen atoms from hydrocarbons like isopropylbenzene.

Assessing the Acidity of Benzylic Hydrogen

The benzylic hydrogen in isopropylbenzene is not particularly acidic. It is relatively more acidic compared to other sp3 hybridized C-H bonds due to the resonance stabilization of the resulting benzylic radical.

Basic Nature of Oxygen

In this context, oxygen acts as a radical species rather than a basic species. Its reactivity is driven by the formation of radicals rather than basicity.

Hybridization of Benzylic Carbon

In the intermediate state after the hydrogen abstraction, the benzylic carbon of isopropylbenzene remains sp3 hybridized as it forms a benzyl radical.

Hybridization Change

The hybridization of the benzylic carbon does not change throughout the reaction. It remains sp3 hybridized.

Key concepts

Radical Reactions

Radical reactions involve the creation and reaction of radicals, which are molecules or atoms with an unpaired electron. These processes often involve initiation, propagation, and termination steps.

In the case of benzylic oxidation, oxygen \(\text{\text{O}_{2}}\text{\text{}}\) plays a crucial role in the initiation step. Here, oxygen splits into two highly reactive oxygen radicals. These radicals can abstract (remove) a hydrogen atom from isopropylbenzene. This creates a new radical on the benzylic carbon.

Oxygen radicals are high-energy species. This makes them very reactive, enabling them to participate in hydrogen abstraction.

Understanding radical reactions helps explain why oxygen can interact with isopropylbenzene in such a specific manner.

Resonance Stabilization

Resonance stabilization is a concept where a molecule or ion can be represented by two or more structures. These structures, called resonance forms, describe the distribution of electrons within the molecule.

In benzylic oxidation, resonance stabilization plays a vital role. When the benzylic hydrogen is removed, a benzylic radical forms. This radical is stabilized by resonance because the unpaired electron can delocalize into the aromatic ring.

The delocalization allows the electron density to spread out over several atoms, lowering the energy of the molecule. This makes the benzylic radical more stable compared to radicals without such stabilization.

It's this resonance stabilization that makes the benzylic hydrogen slightly more acidic and easier to abstract.

sp3 Hybridization

Hybridization is a concept in chemistry where atomic orbitals mix to form new hybrid orbitals. These hybrids explain the bonding in molecules.

In isopropylbenzene, the benzylic carbon is sp3 hybridized. This means it has one s orbital and three p orbitals mixed to form four sp3 hybrid orbitals. The sp3 hybridization allows the carbon to form up to four sigma bonds with other atoms.

During the benzylic oxidation reaction, even after hydrogen abstraction takes place, the hybridization of the benzylic carbon does not change. It remains sp3 hybridized throughout the reaction.

Understanding hybridization helps in predicting the geometry and reactivity of molecules involved in chemical reactions.

Hydrogen Abstraction

Hydrogen abstraction is a process where a hydrogen atom is removed from a molecule, leaving behind a radical.

In the benzylic oxidation of isopropylbenzene, the oxygen radicals generated in the initiation step are highly reactive. They can abstract a hydrogen atom from the benzylic position of isopropylbenzene.

This process creates a benzylic radical, which is resonance-stabilized. This means the electron deficiency caused by the removal of hydrogen is shared across the aromatic ring, stabilizing the intermediate.

The ability of oxygen to perform hydrogen abstraction stems from its high reactivity and the lower activation energy needed due to the resonance stabilization of the resulting radical.