Question

What products would you expect to be formed in the ozonization (with reductive workup) of the following substances (consider carefully which bonds are likely to be most reactive)? (a) o-xylene (b) naphthalene

Short answer

In reductive ozonolysis of o-xylene and naphthalene, no products are formed as the double bonds within their aromatic ring structures are not susceptible to ozonolysis. Therefore, their structures remain unchanged during the reaction.

Step-by-step solution

ozonolysis of o-xylene

o-xylene has two methyl groups attached to the benzene ring. Since the double bonds in benzene ring are not reactive towards ozone, there won't be any cleavage of bonds here. Thus, the structure of o-xylene remains the same after ozonolysis.

ozonolysis of naphthalene

Naphthalene consists of two benzene rings fused together. Just like the case for o-xylene, the double bonds in the benzene rings are not susceptible to ozone. As such, no cleavage of bonds will occur here either, leaving the structure of naphthalene the same after ozonolysis. In conclusion, applying reductive ozonolysis to these substances would not result in any change to their structures because the types of bonds present in these substances are not reactive with ozone. This prediction assumes that the conditions of the ozonolysis do not break basic chemical rules, such as the stability of aromatic rings.

Key concepts

Ozonization of Aromatic Compounds

Understanding the interaction between ozone and aromatic compounds such as benzene rings is pivotal in organic chemistry. Typically, ozonolysis is used to break down carbon-carbon double bonds by adding ozone to the substance, forming ozonides. In alkenes, this process causes the cleavage of the double bond, leading to the formation of two carbonyl compounds upon further reductive workup.
The reactivity of ozone with aromatic compounds like benzene, xylene, and naphthalene, however, is significantly reduced.

Aromatic Rings and Ozone

Aromatic rings contain conjugated systems—overlapping p-orbitals that allow for delocalized electrons, providing unique stability to the compound. This stability makes the double bonds in aromatic systems less reactive towards ozone, preventing the ozonolysis reaction that is typically observed with non-aromatic alkenes.
For instance, when o-xylene—an aromatic compound with two methyl groups—is exposed to ozone, you would not expect any bond cleavage or formation of ozonides. This is because the ozone does not add to the aromatic ring due to its innate stability. The same principle applies to naphthalene, which consists of fused benzene rings, further reinforcing the notion that these rings resist reaction with ozone and thus maintain their structural integrity.

Reactivity of Benzene Ring

The benzene ring is a hallmark of chemical stability and reluctance to undergo certain types of reactions, which is the cornerstone of its unique reactivity profile.

Chemical Stability of Benzene

The six carbon atoms within a benzene ring are sp2 hybridized and form a planar hexagonal ring. The electrons from the p-orbitals are delocalized over the entire ring structure, creating a cloud of electron density above and below the plane of the carbon atoms. This delocalization is crucial, as it distributes the electron density evenly across the ring, leading to a significant stabilization of the molecule. This same delocalization, however, makes the benzene ring less reactive towards electrophiles, including ozone, because the electrons are not concentrated in one area, but rather spread out over the entire ring.

Electrophilic Aromatic Substitution

Instead, the benzene ring is more prone to electrophilic aromatic substitution reactions, where an electrophile replaces one of the hydrogen atoms. Substitution over addition maintains the aromaticity of the compound, which is a driving force behind the chemistry of benzene. The reluctance of benzene to undergo ozonolysis can be attributed to its need to preserve this aromatic state, highlighting the unique reactivity profile of the benzene ring.

Stability of Aromatic Compounds

The concept of aromatic stability is not just a theoretical notion but an empirical truth reflected in the chemical behavior of benzene and its derivatives. Aromatic compounds, characterized by their benzene rings, are incredibly resistant to reactions that would disrupt their aromatic character, including reductive ozonolysis.
A measure of this stability can be found in the molecule's resonance energy, which is the energy difference between the actual molecule and the most stable hypothetical contributing structure without delocalization.

Resonance and Aromatic Stability

Benzene, for instance, has a high resonance energy, which equates to a large degree of stability. This stability, arising from the electron delocalization and resonance, leads to reluctance in the participation in addition reactions that would break the aromaticity, such as ozonolysis.
In summary, the structural integrity and stability of aromatic compounds are maintained, not just due to the spread of electron density, but also due to the high resonance energy which underlies the strength of the aromaticity. These features make them less susceptible to the typical cleavage reactions induced by ozone, protecting their aromatic nature even in the presence of potentially reactive substances like ozone.