Question

What products would you expect from oxidation of the following compounds with \(\mathrm{Cr} \mathrm{O}_{3}\) in aqueous acid? With the Dess-Martin periodinane? (a) Hexan-1-ol (b) Hexan-2-ol (c) Hexanal

Short answer

(a) Chromic acid: Hexanoic acid, Dess-Martin: Hexanal. (b) Both: Hexan-2-one. (c) Chromic acid: Hexanoic acid, Dess-Martin: No change.

Step-by-step solution

Understanding Oxidation Agents

We have two oxidation agents: (i) Chromic acid, which strongly oxidizes alcohols, and (ii) Dess-Martin periodinane, a more selective oxidizing agent. Chromic acid will oxidize primary alcohols to carboxylic acids and secondary alcohols to ketones. Dess-Martin periodinane, however, will oxidize primary alcohols to aldehydes.

Oxidation of Hexan-1-ol

Hexan-1-ol, a primary alcohol, will be oxidized by chromic acid to hexanoic acid. With Dess-Martin periodinane, it will be oxidized to hexanal, an aldehyde.

Oxidation of Hexan-2-ol

Hexan-2-ol is a secondary alcohol. Upon oxidation with either chromic acid or Dess-Martin periodinane, it will be converted to hexan-2-one, which is a ketone.

Oxidation of Hexanal

Hexanal is an aldehyde. When oxidized with chromic acid, it will be further oxidized to hexanoic acid. However, Dess-Martin periodinane will not further oxidize an aldehyde, so it remains unchanged.

Key concepts

Chromic Acid Oxidation

Chromic acid is a powerful oxidizing agent commonly used in organic chemistry. It is particularly effective in oxidizing alcohols due to its strong oxidizing potential, which allows it to donate oxygen to other compounds.
When chromic acid oxidizes primary alcohols, it converts them into carboxylic acids. This occurs because the oxidation process involves the release of two hydrogen atoms and the addition of an oxygen atom, resulting in the formation of the carboxylic group. Secondary alcohols, on the other hand, are transformed into ketones because they undergo oxidation without breaking the carbon chain.
This characteristic makes chromic acid a valuable tool for converting:

• Primary alcohols to carboxylic acids
• Secondary alcohols to ketones

This process is carried out in an acidic aqueous environment, facilitating the efficient transfer of electrons and ensuring a complete reaction.

Alcohol Oxidation

The oxidation of alcohols is a key reaction in organic chemistry, allowing for the transformation of simple alcohol molecules into more functionalized organic compounds.
Primary alcohols, such as hexan-1-ol, could undergo different oxidation paths:

• With aggressive oxidants like chromic acid, they readily form carboxylic acids.
• With selective oxidants like Dess-Martin periodinane, they form aldehydes.

Secondary alcohols like hexan-2-ol typically yield ketones upon oxidation. This process involves the removal of hydrogen atoms from the alcohol group, leading to the formation of a carbonyl group.
Understanding these pathways is crucial as they have various applications in synthesis, where precision in the type of oxidizing agent used can dictate the reaction outcome.

Dess-Martin Periodinane

Dess-Martin periodinane is a mild and selective oxidizing agent used in organic chemistry to precisely convert alcohols into other functional groups without over-oxidation.
This reagent is particularly useful for converting primary alcohols into aldehydes. Unlike chromic acid, it stops the oxidation at the aldehyde stage, preventing further conversion into carboxylic acids. For secondary alcohols, it will oxidize them to ketones, similar to chromic acid, but with greater control and less risk of undesired overreaction.
Highlights of Dess-Martin periodinane include:

• Selective oxidation of primary alcohols to aldehydes
• Transformation of secondary alcohols into ketones
• High efficiency and ease of handling in organic synthesis

Its gentle nature makes it a preferred choice in lab reactions where precision is needed, especially for fragile compounds.

Aldehyde Formation

Aldehydes are important organic compounds that can be synthesized through the targeted oxidation of primary alcohols. The choice of oxidizing agent is crucial in controlling the reaction, as it can determine whether a primary alcohol is converted into an aldehyde or further into a carboxylic acid.
Using Dess-Martin periodinane allows chemists to stop at the aldehyde stage, as seen when oxidizing hexan-1-ol to hexanal. This selectivity is beneficial in synthesis since aldehydes are valuable intermediates in organic reactions and can undergo further transformations to create more complex compounds.
Additionally, chromic acid does not typically yield aldehydes from primary alcohols due to its stronger oxidizing properties. It usually proceeds directly to form carboxylic acids. Understanding these differences enables better planning in synthetic routes and can impact the yield and purity of the desired product. Therefore, controlling the oxidation step is key to successful aldehyde production.