Question

Write down the products of ozonolysis of 1,2 -dimethylbenzene (o-xylene). How does the result support Kekulé structure for benzene?

Short answer

Ozonolysis of o-xylene yields glyoxal and dimethylglyoxal, supporting benzene's alternating double bond structure (Kekulé model).

Step-by-step solution

Understanding the Concept of Ozonolysis

Ozonolysis is a reaction where ozone is used to cleave carbon-carbon double bonds. It is particularly useful for alkenes and also some aromatic compounds like benzene derivatives, where the double bonds are cleaved to form carbonyl compounds.

Identify the Structure of o-Xylene

o-Xylene, also known as 1,2-dimethylbenzene, is an aromatic hydrocarbon with the formula C₆H₄(CH₃)₂. It consists of a benzene ring with two methyl groups attached at the 1st and 2nd positions.

Reaction Setup

In ozonolysis, the aromatic ring in compounds like o-xylene is decomposed by ozone to give products containing carbonyl groups (aldehydes or ketones). For o-xylene, the benzene ring is cleaved to form three carbonyl functional groups.

Products of Ozonolysis

The ozonolysis of o-xylene results in the formation of two moles of glyoxal (OHC-CHO) and one mole of dimethylglyoxal Di-aldehyde (OHC-C(CH₃)-CHO) from the split of the aromatic ring and side methyl groups.

Analysis of Result with Kekulé Structure

The reaction products, specifically the formation of three pairs of carbonyl compounds, support the alternating double bond structure of benzene as proposed in the Kekulé model, indicating the presence of three double bonds.

Key concepts

Kekulé structure

The Kekulé structure is a model of benzene proposed by the chemist August Kekulé. This structure suggests that benzene consists of a six-carbon ring with alternating single and double bonds. This special arrangement explains several unique properties of benzene, including its stability and reactivity.
Benzene has been found to be more stable than expected for a fully unsaturated hydrocarbon. The Kekulé model helps explain this stability by assuming resonance - a phenomenon where electrons are delocalized over the ring, allowing benzene to maintain stability and symmetry.
In ozonolysis, benzene’s structure breaks down into simpler molecules, which helps confirm this alternating bond structure. The result of the ozonolysis experiment with o-xylene shows products that suggest the existence of double bonds in the ring, affirming the idea proposed in Kekulé's model.

o-xylene

O-xylene, also known as 1,2-dimethylbenzene, is a type of aromatic hydrocarbon. This molecule consists of a benzene ring with two methyl groups attached at adjacent carbon atoms, specifically, the first and second positions. This arrangement gives o-xylene certain properties such as higher boiling and melting points compared to the benzene itself.
The methyl groups in o-xylene also influence the chemical behavior of the compound. During reactions like ozonolysis, these groups affect where the molecule will split and which products will form. For o-xylene, the reaction leads to the formation of a mix of aldehydes based on the position of the methyl groups and the structured ring.
This makes o-xylene an interesting subject in studying aromatic compounds, where variations in structure have a significant impact on chemical properties and reactions.

aromatic compounds

Aromatic compounds are a class of molecules characterized by stable ring structures and typically have distinct, sweet scents. They possess a conjugated pi-electron system that gives them unique chemical stability and reactivity compared to non-aromatic compounds.
Benzene is the simplest aromatic compound and serves as a parent molecule for others like o-xylene. These aromatic compounds undergo unique reactions—such as electrophilic aromatic substitution, which keeps the aromatic ring intact.
However, under specific reactions like ozonolysis, aromatic rings can be cleaved. This is important in industrial and synthetic chemistry because it allows chemists to break down stable aromatic rings into useful products. The resulting products from such reactions can provide insights into the arrangement of bonds in the original compound, as seen with o-xylene.]