Question

Recognize Most useful organic molecules are made from raw materials using several steps. This is called a multistep synthesis pathway. Label the types of reaction or process taking place in each step of the multistep synthesis pathway below. petroleum \(\rightarrow\) ethane \(\rightarrow\) chloroethane \(\rightarrow\) ethene \(\rightarrow\) ethanol \(\rightarrow\) ethanoic (acetic) acid

Short answer

The multistep synthesis pathway involves the following reactions: 1. Thermal cracking: petroleum → ethane 2. Chlorination: ethane → chloroethane 3. Dehydrohalogenation: chloroethane → ethene 4. Hydration: ethene → ethanol 5. Oxidation: ethanol → ethanoic (acetic) acid

Step-by-step solution

Petroleum to Ethane

The first step in the pathway is the conversion of petroleum to ethane. This process is called "cracking," which involves breaking down large hydrocarbon molecules from petroleum into smaller ones, like ethane. The type of reaction taking place here is thermal cracking.

Ethane to Chloroethane

The second step is the conversion of ethane to chloroethane. This reaction involves the substitution of a hydrogen atom in ethane with a chlorine atom, resulting in the formation of chloroethane. This type of reaction is known as a halogenation reaction, more specifically, a chlorination reaction.

Chloroethane to Ethene

The third step involves converting chloroethane to ethene. This reaction is called dehydrohalogenation, where a hydrogen atom and a halogen atom (chlorine, in this case) are removed from adjacent carbons of an alkyl halide, like chloroethane, to produce an alkene, such as ethene.

Ethene to Ethanol

The fourth step is the conversion of ethene to ethanol. This reaction is known as hydration of alkene, where water (H₂O) is added to the double bond in ethene to produce ethanol. The type of reaction taking place here is an addition reaction.

Ethanol to Ethanoic (Acetic) Acid

In the final step, ethanol is converted to ethanoic (acetic) acid. This reaction involves the oxidation of ethanol to form ethanoic acid. A common oxidizing agent used in this process is potassium dichromate (K₂Cr₂O₇). The type of reaction occurring in this step is an oxidation reaction. Here's the multistep synthesis pathway with the labeled reactions: petroleum (thermal cracking)→ ethane (chlorination)→ chloroethane (dehydrohalogenation)→ ethene (hydration)→ ethanol (oxidation)→ ethanoic (acetic) acid

Key concepts

Multistep Synthesis Pathway

Multistep synthesis pathways are essential in organic chemistry for creating complex molecules from simpler ones. This involves several reaction steps, each designed to transform the starting material incrementally. In facilitated steps, desired transformations can be achieved.
Each step in such a pathway is a carefully orchestrated reaction that tailors the structure of the molecule closer to the target product. In the given synthesis example, we begin with petroleum and proceed through multiple reactions to eventually form ethanoic acid.
This pathway involves various types of chemical reactions such as cracking, halogenation, and oxidation, amongst others, demonstrating how different reaction types are harnessed to achieve complex molecule construction. Understanding each transformation is crucial, as each step not only contributes specific structural changes but affects the proceeding steps as well.

Cracking

Cracking is a pivotal industrial process applied in the petrochemical industry. This process involves breaking large hydrocarbon chains into smaller, more useful molecules. Thermal cracking, as discussed in the exercise, uses high temperatures to induce this breakdown.
When cracking petroleum, the goal is to produce more valuable fractions such as ethane, which are then used as feedstock for further chemical transformations.

• Converts larger hydrocarbons into smaller molecules.
• Creates compounds like ethane for further processing.

The ability to manipulate the size of hydrocarbons allows for the efficient production of a variety of chemical compounds essential for multiple applications.

Halogenation Reaction

Halogenation is a chemical reaction that involves the incorporation of halogen atoms into other molecules. In the case of the exercise, ethane undergoes halogenation to become chloroethane, specifically during a chlorination reaction.
In a halogenation reaction, a halogen such as chlorine reacts with an alkane like ethane to replace one or more hydrogen atoms with halogen atoms. This substitution process is catalyzed either thermally or photochemically to facilitate the reaction.

• Replaces hydrogen atoms with halogens (e.g., chlorine).
• Creates compounds with new chemical properties used in further synthesis.

Halogenated compounds often serve as intermediates for creating a diverse array of products, illustrating the versatility and utility of halogenation in organic synthesis pathways.

Oxidation Reaction

An oxidation reaction refers to a process where a molecule loses electrons, often resulting in an increase in oxygen content within the molecule. Ethanol's conversion to ethanoic acid, as seen in the final step of the exercise, is a classic example of such a reaction.
During this transformation, ethanol is oxidized, typically using an oxidizing agent like potassium dichromate. The reaction alters the alcohol functional group into a carboxylic acid group, which significantly changes its chemical and physical properties.

• Loss of electrons and increase in oxygen content.
• Alters functional groups within molecules, providing new characteristics.

Understanding oxidation reactions is crucial for manipulating the functional groups in organic compounds, enabling the design and synthesis of a wide range of chemical products.