Question

Aldehydes and ketones can be prepared by ______ of the corresponding alcohol.

Short answer

Aldehydes and ketones can be prepared by the oxidation of the corresponding alcohol using suitable oxidizing agents like PCC, DMP, KMnO4, or K2Cr2O7, depending on the desired product.

Step-by-step solution

Understand the reaction

The preparation of aldehydes and ketones involves converting alcohols (with the -OH functional group) to aldehydes and ketones (with the C=O functional group).

Identify the reaction method

To prepare aldehydes and ketones, chemists use a process called "oxidation" of the corresponding alcohol. Oxidation refers to the loss of electrons or an increase in the oxidation state of an atom, ion, or molecule.

Identify the agents used in oxidation

To facilitate the oxidation of alcohols to aldehydes and ketones, one must use an oxidizing agent. Common oxidizing agents include KMnO4 (potassium permanganate), K2Cr2O7 (potassium dichromate), and PCC (pyridinium chlorochromate). The choice of the oxidizing agent and reaction conditions depends on whether we want to produce an aldehyde or a ketone.

Reaction conditions for forming aldehydes and ketones

For creating aldehydes, a gentle oxidizing agent like PCC (pyridinium chlorochromate) or DMP (Dess-Martin periodinane) is chosen to prevent over-oxidation to carboxylic acids. The overall reaction can be represented as: Alcohol + PCC/DMP \( \rightarrow \) Aldehyde + byproducts The oxidation of secondary alcohols to form ketones requires stronger oxidizing agents like potassium permanganate (KMnO4) or potassium dichromate (K2Cr2O7). The reaction is: Secondary Alcohol + KMnO4/K2Cr2O7 \( \rightarrow \) Ketone + byproducts

Conclusion

Aldehydes and ketones can be prepared by the oxidation of the corresponding alcohol using suitable oxidizing agents like PCC, DMP, KMnO4, or K2Cr2O7, depending on the desired product.

Key concepts

Oxidation of Alcohols

Oxidation of alcohols is a fascinating chemical transformation where alcohols are converted into more oxidized compounds like aldehydes and ketones. This process involves the loss of hydrogen atoms or the addition of oxygen atoms.
It's like helping the molecule gain oxygen or lose out hydrogen . The reaction requires specific conditions and a suitable oxidizing agent.
Oxidation is often represented in chemical equations by an increase in the oxidation state of the molecule. For instance, ethanol (a primary alcohol) is oxidized to form acetaldehyde, while isopropanol (a secondary alcohol) is oxidized to produce acetone.

Oxidizing Agents

To oxidize alcohols, chemists use robust chemicals known as oxidizing agents. These are substances that bring about oxidation by accepting electrons.
Some common oxidizing agents include:

• PCC (Pyridinium chlorochromate): Often used for mild oxidation to avoid further oxidation to carboxylic acids, especially when forming aldehydes.
• DMP (Dess-Martin periodinane): Another gentle oxidizing agent preferred for converting alcohols to aldehydes without progressing to acids.
• KMnO4 (Potassium permanganate): A strong oxidizer capable of converting secondary alcohols to ketones.
• K2Cr2O7 (Potassium dichromate): Another powerful agent useful for forming ketones from secondary alcohols under harsher conditions.

Choosing the right oxidizing agent is crucial because it determines the reaction's direction and the resulting products.

Aldehyde Formation

Aldehydes are formed primarily from the oxidation of primary alcohols. These alcohols have their hydroxyl group (-OH) attached to a carbon atom, which is connected to at least two hydrogen atoms.
Mild oxidizing agents like PCC or DMP are ideal for this process to prevent further oxidation, which can turn aldehydes into carboxylic acids.
The general reaction can be portrayed as follows:Primary Alcohol + PCC/DMP \( \rightarrow \) Aldehyde + byproducts.
For example, if you take ethanol and oxidize it gently using PCC, it will form acetaldehyde, stopping the reaction before the acid stage. This selective oxidation is key to maintaining the integrity of the desired product.

Ketone Formation

Ketones are produced from the oxidation of secondary alcohols. These alcohols have their hydroxyl group connected to a carbon atom bound to two other carbon atoms.
The oxidation usually requires stronger oxidizing agents, like KMnO4 or K2Cr2O7, due to the stability of the carbon framework.
The reaction is generally represented as:Secondary Alcohol + KMnO4/K2Cr2O7 \( \rightarrow \) Ketone + byproducts.
Take isopropanol, for example; when oxidized with potassium dichromate, it forms acetone. This shows how the carbon structure plays a pivotal role in the outcome of the oxidation, specifically directing the formation towards ketones when secondary alcohols are involved.