Question

Suppose propene \(\left(\mathrm{CH}_{2}=\mathrm{CH}-\mathrm{CH}_{3}\right)\) is the hydrocarbon that reacts with the hydroxyl radical - OH. Write the set of chemical reactions which ultimately produce an aldehyde. What is this final aldehyde?

Short answer

The final aldehyde is acetaldehyde ( \(\text{CH}_3\text{CHO}\) ).

Step-by-step solution

Identify Initial Reactants

The reaction involves propene (\(\text{CH}_2=\text{CH}-\text{CH}_3\)) and hydroxyl radical (OH). The hydroxyl radical will attack the \(\text{C=C}\) double bond of propene.

Formation of Radical Intermediate

The hydroxyl radical adds to the double bond, forming a radical intermediate. The reaction can be represented as:\[\text{CH}_2=\text{CH}-\text{CH}_3 + \cdot\text{OH} \rightarrow \text{CH}_2(\cdot\text{OH})-\text{CH}-\text{CH}_3\]

Rearrangement and Oxygen Addition

The radical on the second carbon can react with oxygen \((\text{O}_2)\), introducing an additional radical site:\[\text{CH}_2(\cdot\text{OH})-\text{CH}-\text{CH}_3 + \text{O}_2 \rightarrow \text{CH}_2(\cdot\text{OOH})-\text{CH}_2-\text{CH}_3\]

Radical Chain Propagation

The peroxy radical can undergo reaction with another radical or decompose to form a carbonyl group. This can lead to the following reaction:\[\text{CH}_2(\cdot\text{OOH})-\text{CH}_2-\text{CH}_3 \rightarrow \text{CH}_3-\text{CHO} + \text{H}_2\text{O}_2\]This forms the aldehyde, acetaldehyde (\(\text{CH}_3\text{CHO}\)).

Identify Final Product

The final aldehyde produced in this sequence of reactions is acetaldehyde (\(\text{CH}_3\text{CHO}\)). The reaction primarily involves free radical reactions and radical stability.

Key concepts

Free Radical Mechanism

Free radical reactions play an important role in organic chemistry. They involve species with unpaired electrons, these are radicals, which react in a specific sequence to form stable products. In the example of propene and the hydroxyl radical (OH), the process begins with the generation of a radical intermediate.

• The hydroxyl radical attacks the double bond in propene, forming a new radical intermediate.


• This new radical can further react with other molecules, leading to a cascade of reactions known as a radical chain reaction.

These reactions often feature initiation, propagation, and termination steps, where radicals are continuously generated, reacted, and eventually quenched.

Chemical Reaction Pathways

Chemical reaction pathways refer to the series of steps a chemical reaction follows, leading to the formation of products. In the propene reaction, the pathway involves several critical steps. Initially, the hydroxyl radical adds to propene to form a radical intermediate.
Following this, the intermediate reacts with oxygen to form a peroxy radical. This is a key transition that leads towards the formation of the desired product.

• Rearrangement of atoms and introduction of reaction partners (like oxygen) is a hallmark of these pathways.


• The sequence of chemical reactions determines the efficiency and outcome of the transformation.

This specific pathway results in the formation of acetaldehyde, illustrating the complexity and stepwise nature of organic reactions.

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds and their reactions. It includes understanding mechanisms like free radical reactions, which are central to many organic transformations. In our example, propene, a hydrocarbon, reacts with radicals to produce new compounds.

• The focus is often on the transformation of organic molecules through different reaction mechanisms.


• Free radical mechanisms are particularly vital, as they can proceed under mild conditions and involve common intermediates.

These reactions are foundational within organic chemistry and help illustrate how complex organic molecules can be synthesized from simpler ones. Understanding these processes is crucial for mastering the field and applying it to real-world problems.