Intro to Redox Reactions: Videos & Practice Problems

Redox reactions, central to both general and organic chemistry, involve the transfer of electrons between reactants, leading to changes in oxidation states. In organic chemistry, these reactions are characterized by alterations in the number of carbon-oxygen and carbon-hydrogen bonds within a molecule. Oxidation is marked by an increase in carbon-oxygen bonds, transitioning from alcohols to aldehydes or ketones, and ultimately to carboxylic acids. Reduction, conversely, increases carbon-hydrogen bonds, moving from carboxylic acids to aldehydes or ketones, and then to alcohols. The focus in organic chemistry is on these intermediate stages, rather than the extremes of hydrocarbons or carbon dioxide, highlighting the transformation of functional groups like alcohols, aldehydes, ketones, and carboxylic acids. Understanding these processes is crucial for grasping the mechanisms of organic transformations and the synthesis of different organic compounds.

Identifying Redox Reactions

Redox reactions, or oxidation-reduction reactions, are fundamental processes in both general and organic chemistry, characterized by the transfer of electrons between reactants. In general chemistry, redox reactions are defined by this electron transfer, while in organic chemistry, the focus shifts to the changes in the amounts of oxygen and hydrogen in molecules.

Oxidation in organic chemistry is defined as the process of increasing the number of carbon-oxygen bonds within a molecule. Conversely, reduction involves increasing the number of carbon-hydrogen bonds. For instance, when considering the transformation of a hydrocarbon like methane to carbon dioxide, the focus is primarily on the intermediate stages involving alcohols, aldehydes, and carboxylic acids.

Starting with an alcohol, the first oxidation step introduces a carbon-oxygen bond, leading to the formation of an aldehyde or ketone, which contains two carbon-oxygen bonds. Further oxidation results in the formation of a carboxylic acid, characterized by three carbon-oxygen bonds. If the oxidation process continues, carbon dioxide is produced, which has four carbon-oxygen bonds, although this stage is not typically relevant in organic chemistry discussions.

On the other hand, reduction begins with a carboxylic acid, which has three carbon-oxygen bonds. The reduction process converts it to an aldehyde or ketone, reducing the carbon-oxygen bonds to two while simultaneously increasing the carbon-hydrogen bonds. Continuing this reduction leads to the formation of an alcohol, which has three carbon-hydrogen bonds. This process highlights the importance of the transformations between alcohols, aldehydes, ketones, and carboxylic acids, while the extremes of hydrocarbons and carbon dioxide are generally excluded from organic chemistry considerations.

Intro To Redox Reactions Example

In the context of chemical reactions, understanding whether a reaction is an oxidation or reduction process is crucial. In this particular reaction, we focus on a carbon atom that undergoes a transformation. Initially, this carbon is bonded to one oxygen atom and one hydrogen atom, represented as a carbon-oxygen bond and a carbon-hydrogen bond, respectively. The presence of these bonds is significant in determining the nature of the reaction.

As the reaction progresses, we observe a change in the bonding of this carbon atom. It loses its carbon-hydrogen bond, resulting in zero carbon-hydrogen bonds, while simultaneously increasing its carbon-oxygen bonds from one to two. This increase in the number of carbon-oxygen bonds indicates that the carbon atom is being oxidized. In oxidation reactions, there is a loss of electrons or an increase in oxidation state, which is consistent with the observed changes in bonding.

In summary, the reaction in question represents an oxidation process, where an alcohol is converted into a ketone. This transformation highlights the fundamental principles of oxidation, characterized by the increase in bonds to more electronegative atoms, such as oxygen, and the loss of hydrogen bonds.

Determine if the following reaction represents an oxidation, reduction or neither.

• A. Oxidation
• B. Reduction
• C. Neither

Answer: (B)

Determine if the following reaction represents an oxidation, reduction or neither.

• A. Oxidation
• B. Reduction
• C. Neither

Answer: (C)

Here’s what students ask on this topic:

What has to happen to the carbon center for an organic redox reaction to occur?

For an organic redox reaction to occur involving a carbon center, there must be a change in the oxidation state of that carbon atom. This change is typically facilitated by either the gain or loss of electrons. In a reduction reaction, the carbon center gains electrons, which often corresponds to the addition of hydrogen atoms or the removal of oxygen atoms. Conversely, in an oxidation reaction, the carbon center loses electrons, which is commonly associated with the loss of hydrogen atoms or the addition of oxygen atoms or other electronegative groups.

The specific mechanisms and reagents that facilitate these electron transfers vary widely, depending on the particular organic redox reaction. For instance, a common oxidizing agent might be potassium permanganate (_KMnO4), while a reducing agent could be lithium aluminum hydride (_LiAlH4). The actual electron transfer can occur through a variety of pathways, including single electron transfers, hydride shifts, or polar bond formation and cleavage.

What is the organic product of this redox reaction where ethanol is added at the end to reduce excess Cr(VI) remaining in the solution to Cr(III)?

When ethanol is used to reduce excess chromium(VI) in a solution to chromium(III), the organic product formed as a result of this redox reaction is acetaldehyde. This reaction occurs because ethanol (C₂H₅OH) acts as a reducing agent, donating electrons to the chromium(VI) species.

The overall reaction can be summarized as follows:

3 CH₃CH₂OH + 2 CrO₃ + 16 H⁺ → 3 CH₃CHO + 4 H₂O + 2 Cr³⁺

In this reaction, chromium(VI) is typically present in the form of chromic acid (H₂CrO₄) or dichromate ions (Cr₂O₇^2-), and the ethanol is oxidized to acetaldehyde (CH₃CHO). The chromium(VI) is reduced to chromium(III) ions (Cr³⁺), which are less toxic and more environmentally benign than the hexavalent form. The reaction also produces water (H₂O) and requires an acidic environment, indicated by the presence of hydrogen ions (H⁺).