chem 2/4/26 wed notes

Oxidation of Aldehydes and Ketones

  • Oxidizing Agents

    • Presence of strong oxidizing agents can lead to carbon bond changes.

    • Aldehydes can form an additional oxygen bond

    • Aldehydes can be oxidized to carboxylic acids.

  • Aldehyde vs. Ketone

    • Aldehydes can be oxidized while ketones cannot.

    • Rationale:

    • In aldehydes, there is room at the carbon atom for another carbon-oxygen bond without disrupting existing carbon-carbon bonds.

    • In ketones, no such room exists, leading to no reaction upon oxidation.

  • Differentiating Aldehydes and Ketones

    • While they have similar physical properties (e.g., boiling points, solubilities), oxidation is a key reaction that distinguishes them.

Reactions of Alcohols and aldehydes

  • Reactions in Lab

    • Planned oxidation tests will be conducted to distinguish between aldehydes and ketones.

    • Substances being tested include ethanoic acid (acetic acid) and acetone, both colorless liquids.

  • Visualization of Reactions

    • Potassium permanganate as an oxidizing agent creates a visible change in color that indicates a reaction has taken place.

    • This purple solution will result in a brown precipitate (MnO₂) upon reduction, identifying oxidizable substances.

    • However, potassium permanganate is too strong and will oxidize substances other than aldehydes, like alcohols.

Specific Oxidation Reactions

  • Collins and Tollens Tests

    • Tollens Test: Using silver ions to react with aldehydes.

    • A positive reaction leads to the formation of solid silver, helping to visually identify aldehydes.

    • Historical Note:

      • The reaction is linked to the manufacturing of mirrors, which includes silver as a reflective layer.

    • Benedict’s Test: Utilizes copper(II) ions to test for reducing sugars (aldehydes) in urine.

    • The observable color change of the blue solution to a brick-red precipitate (Cu₂O) confirms the presence of sugars like glucose.

    • This test was once common for diagnosing conditions like diabetes.

Clinical Relevance of Tests

  • Understanding Glucosuria

    • Importance of glucose in urine indicating excessive levels in the bloodstream and its relevance to diabetes.

    • It is easier to conduct urine tests than blood tests which can help in early detection of diabetes through glucose levels.

Limitations of Subtests

  • False Positives in Benedict's Test

    • Other reducing sugars could yield similar results, complicating diagnosis.

    • Alcohol structures close to aldehydes can also impact results due to potential oxidation.

Reduction Reactions

  • Reduction of Aldehydes and Ketones

    • Aldehydes can be reduced to primary alcohols, while ketones can become secondary alcohols.

    • Oxidation process can be viewed as a reverse reaction leading to reduction.

    • General rule:

    • Reduction decreases the number of carbon-oxygen bonds in organic compounds.

Addition Reactions

  • Addition of Alcohols

    • Alcohols can react with aldehydes and ketones to form acetals or ketals.

    • An acetal arises from a reaction with an aldehyde, while a ketal forms from a ketone.

    • Synthesis of complex carbohydrates relies on these addition reactions.

    • Terminology:

    • Hemiacetal/Hemiketal: Formed from adding one alcohol whereas acetal/ketal involves two alcohol additions.

  • Visualizing the Reaction

    • Important to identify products by recognizing the structures formed during reactions:

    • Ketals resembling diethers structurally, differing in bonding.

  • Practical Application

    • Addition reactions are crucial for creating complex molecules in biochemistry and can impact energy storage and metabolism.