10.3.4. Oxidation and Reduction Reactions

A. Oxidation of Organic Compounds

Definition (organic context): Increase in oxidation state of carbon; often accomplished by increasing the number of carbon–oxygen bonds or reducing the number of C–H bonds.

Common oxidizing agents:

1. Potassium permanganate (KMnO4) in acidic, neutral, or basic medium.
2. Potassium dichromate (K2Cr2O7) in acid (H2SO4) (orange solution turns green).
3. Jones reagent (CrO3 in H2SO4).
4. PCC (pyridinium chlorochromate) for mild oxidation, stopping at aldehyde stage.
5. Ozone (O3) for oxidative cleavage of alkenes.

This chunk defines oxidation in the context of organic chemistry. In this context, oxidation refers to the increase in the oxidation state of carbon, which typically occurs when carbon forms more bonds with oxygen (like in carbonyl compounds) or reduces the number of hydrogen bonds it has. Oxidizing agents like potassium permanganate and potassium dichromate are commonly used in laboratory settings to achieve these oxidations. For example, potassium dichromate changes color when it reacts with compounds, indicating a chemical change. Different reagents are used depending on the degree of oxidation required and the specific organic compound being oxidized.

Consider how a piece of apple browns when exposed to air. This browning is an oxidation reaction; the apple's compounds react with oxygen in the air, changing their structure and making the apple look different. In the lab, we use chemicals like potassium permanganate to oxidize organic compounds in a controlled way, similar to how nature uses oxygen to oxidize fruits over time.

Oxidation of Alcohols

1. Primary alcohol → aldehyde → carboxylic acid:

   • Mild oxidant (e.g., PCC) yields aldehyde selectively (no further oxidation to acid).
   • Strong oxidant (KMnO4, K2Cr2O7/H2SO4) with heat yields carboxylic acid.
   • Mechanism: Protonate –OH, form water leaving group, generate carbocation, deprotonate to form aldehyde; aldehyde is further oxidized by formation of geminal diol intermediate then acid.

2. Secondary alcohol → ketone:

   • Reagents such as KMnO4, K2Cr2O7, PCC will oxidize secondary alcohol to ketone (e.g., 2-propanol to acetone).

3. Tertiary alcohols:

   • Generally resist oxidation except under very strong conditions that cleave C–C bonds, yielding mixtures of smaller fragments.

This chunk discusses the oxidation process for different types of alcohols. Primary alcohols can first be oxidized to aldehydes (with mild oxidants) and then further oxidized to carboxylic acids (with strong oxidants). The mechanism described highlights how the –OH group is turned into a leaving group, allowing the carbon to undergo oxidation. Secondary alcohols are oxidized to ketones, while tertiary alcohols do not generally undergo oxidation under normal conditions because they lack hydrogen atoms on the carbon bonded to the –OH group, making their oxidation more complex. Instead, they may break apart under extreme conditions, generating smaller molecules.

Think of the oxidation of alcohols like the aging of wood. Just as wood can change in appearance and properties when exposed to air over time (like turning gray), primary alcohols can gradually convert to other chemical forms (like aldehydes and acids) when oxidized. Tertiary alcohols, on the other hand, are like sturdy old trees that resist change; they don't easily alter until they're forced to break down completely under significant stress.

Oxidation of Aldehydes and Ketones

1. Aldehydes are easily oxidized to carboxylic acids by mild oxidizing agents (Tollens’ reagent [Ag(NH3)2]+, Fehling’s solution, Benedict’s solution, or mild dichromate).
2. Ketones resist oxidation unless strong conditions break C–C bonds, yielding carboxylic acids or other fragments.

This section outlines the oxidization behavior of aldehydes and ketones. Aldehydes are quite reactive to oxidation and can be easily converted to carboxylic acids using milder oxidizing agents. Ketones, however, are more stable and generally resist oxidation unless extreme conditions are applied, leading to the breaking of their C–C bonds, which can yield smaller molecules including carboxylic acids.

Imagine how a fresh apple (an aldehyde) will change if left out in the open air; it begins to oxidize and can spoil, turning into something like apple cider vinegar (a carboxylic acid). On the other hand, consider a sealed jar of mayonnaise (similar to a ketone). It doesn't spoil or oxidize easily without the right conditions, maintaining its form until exposed to significant changes like heat or contamination.