Intro to Redox Reactions - Online Tutor, Practice Problems & Exam Prep

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

In this video we're going to talk about redox reactions, first from a Gen. camp perspective and then from an organic chemistry perspective. Now recall that redox reactions, also known as oxidation reduction reactions, involve transference of an electron or electrons between reactants. Now this is our general chemistry definition of a redox reaction. In organic chemistry it's a little bit different. Here, redox reactions involve increasing or decreasing the oxygen or hydrogen amount on a molecule.

Now, when we say oxidation, oxidation involves increasing the number of carbon oxygen bonds on the molecule. And if we're talking about reduction, well, that involves increasing the number of carbon hydrogen bonds on the molecule. If we take a look here at oxidation versus reduction, we're going to see that on the far left side we have a hydrocarbon in the form of methane. And then on the far right side we have carbon dioxide. Here, notice that they're both grayed out. That's because we don't really deal with these two types of compounds.

When we're talking about oxidation versus reduction, we're going to focus on the middle portion, where we're going between an alcohol to what looks like an aldehyde to a carbacillic acid. That's the portion that we care about in terms of organic chemistry. Now here we can oxidize A hydrocarbon to our alcohol and notice that we had no carbon oxygen bonds initially. But our first oxidation step creates our first carbon oxygen bond. So this would be for alcohols. If we continue with our oxidation process we have one carbon oxygen bond. Now we have 2 because we're double bonded to that oxygen.

Now here I'm showing an aldehyde, but this section here stands for aldehydes and ketones. Now if we continue with oxidizing this, we get a carbolic acid and we go from being 2 carbon dioxide monsonal 3. And if we continue further, we get carbon dioxide which has four carbon oxygen bonds. But again, we don't concern ourselves with that portion because that doesn't really represent an organic molecule for us. Now going the opposite way, we're reducing, we're starting out with the carpacillic acid. We then move over to the aldehyde or ketone section here.

Notice that we go from three carbon oxygen bonds to now just two. And not just that we're increasing the number of carbon hydrogen bonds, because here we only had one carbon hydrogen bond and now we're back to two. Continue onward again. Now instead of having two carbon hydrogens, we have three and we're at the alcohol phase. And if we continued on, we'd have 4 carbon hydrogen bonds. But again, we're not concerned with the extremes in terms of this image. We're not concerned ourselves with hydrocarbons or carbon dioxide. We're operating in the highlighted section in the middle between alcohols, aldehydes and ketones and carbicolic acids.

Intro To Redox Reactions Example

Determine if the following reaction represents an oxidation or reduction reaction. If we take a look here, we're going to say that we have, as far as we can see, one carbon oxygen bond and we have one carbon hydrogen bond here. Now here, this is just a regular carbon carbon bond and this is just a regular carbon carbon bond, so we don't look at those.

Now we're focusing on this carbon because we can see that it undergoes a change here. So that's we're focusing on it. So we're going to say here we're starting out with one carbon oxygen and one carbon hydrogen. If we come over here, what's happened? Well, we can see that this carbon, which is still the same carbon here, it has lost its carbon hydrogen bond. So it has zero of those now.

And now instead of being connected to oxygen just once, it's now connected to it twice. So we have increased the number of carbon oxygen bonds, and we know when we do that, that's a sign of oxidation. So here we'd say that this reaction represents an oxidation reaction. We have an alcohol that's been oxidized into a ketone product.

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

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

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

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

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⁺).