Question

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

Short answer

(a) Hexanoic acid with \( \mathrm{CrO}_3 \), hexanal with DMP. (b) Hexan-2-one with both reagents. (c) Hexanoic acid with \( \mathrm{CrO}_3 \), no reaction with DMP.

Step-by-step solution

Understand the Reactants and Reagents

We need to predict the oxidation products of different alcohols and aldehydes when treated with two different reagents: \( \mathrm{CrO}_3 \) in aqueous acid and the Dess-Martin periodinane (DMP). \( \mathrm{CrO}_3 \) in aqueous acid is a strong oxidizing agent typically used to oxidize primary alcohols to carboxylic acids and secondary alcohols to ketones. The DMP is a milder reagent usually used to convert primary alcohols to aldehydes and secondary alcohols to ketones.

Oxidation of Hexan-1-ol

Hexan-1-ol is a primary alcohol. When oxidized with \( \mathrm{CrO}_3 \) in aqueous acid, it is expected to form hexanoic acid due to complete oxidation to the acid. When oxidized with DMP, it is expected to form hexanal, stopping at the aldehyde stage due to the milder nature of DMP.

Oxidation of Hexan-2-ol

Hexan-2-ol is a secondary alcohol. Both \( \mathrm{CrO}_3 \) in aqueous acid and DMP oxidize secondary alcohols to ketones. Therefore, the expected product for both oxidizing agents is hexan-2-one.

Oxidation of Hexanal

Hexanal is an aldehyde. When oxidized by \( \mathrm{CrO}_3 \) in aqueous acid, which is a strong oxidizing agent, hexanal is further oxidized to hexanoic acid. However, DMP does not typically oxidize aldehydes further, so no reaction is expected with DMP.

Key concepts

Oxidation Reactions

In organic chemistry, oxidation reactions are transformations where the oxidation state of a molecule, atom, or ion changes, generally by losing electrons. This is often accompanied by either an increase in oxygen or a decrease in hydrogen in the molecule.
Oxidation reactions are crucial for converting functional groups within compounds, especially in metabolic pathways and synthetic chemistry.
For example:

• Primary alcohols can be oxidized to aldehydes and further to carboxylic acids.
• Secondary alcohols are usually oxidized to ketones.
• Aldehydes may be oxidized to carboxylic acids with stronger oxidizing agents.

Understanding how these transformations happen is vital as it allows chemists to design routes for the synthesis of complex molecules, optimizing steps to create the desired products efficiently.

Alcohol Oxidation

Alcohol oxidation is a process of transforming alcohols into carbonyl compounds such as aldehydes, ketones, or acids. The degree of oxidation depends on whether the alcohol is primary, secondary, or tertiary.
Here's a closer look at alcohol oxidation:

• Primary alcohols: These can be oxidized to aldehydes and further to carboxylic acids. The fate depends largely on the oxidizing agent used. Strong agents like chromium trioxide in aqueous acid can take the oxidation all the way to a carboxylic acid, as was the case with hexan-1-ol to hexanoic acid.
• Secondary alcohols: These typically oxidize to ketones and do not go further in oxidation. For instance, hexan-2-ol reduces to hexan-2-one with both strong and mild oxidizing agents.
• Tertiary alcohols: Not oxidized as there's no hydrogen atom on the carbon with the -OH group to remove.

This nuanced understanding helps predict the outcomes of reactions based on the type of alcohol and reagents applied.

Reagents in Organic Chemistry

Reagents are substances used to cause a chemical reaction, and in organic chemistry, selecting the right reagent is crucial for controlling the reaction pathway.
The two reagents discussed in the exercise are:

• Chromium trioxide (\( \mathrm{CrO}_3 \)): This is a potent oxidizing agent, often used in aqueous acid. Its strength lies in converting primary alcohols all the way to carboxylic acids and secondary alcohols to ketones, making it ideal for complete oxidations.
• Dess-Martin periodinane (DMP): A popular reagent for gentle oxidation, DMP stops short at the aldehyde stage in primary alcohol oxidations, ideal when one wants to avoid over-oxidation to acids. It converts secondary alcohols into ketones and generally doesn't further oxidize aldehydes.

Choosing the appropriate reagent is linked to the desired outcome of the reaction, letting chemists fine-tune oxidative processes to synthesize complex molecules or study metabolic pathways.