PREPARATION OF ALDEHYDES AND KETONES /CLASS 12

Summary

In this chemistry session, Anita Raj teaches the preparation of aldehydes and ketones through oxidation processes. She explains how primary alcohols like ethyl alcohol can be oxidized to form aldehydes using acidified potassium dichromate, with controlled conditions to prevent further oxidation. Secondary alcohols are converted to ketones in a similar manner. The video also covers the production of aldehydes and ketones from alkenes through reductive ozonolysis, hydration of alkynes, catalytic hydrogenation of acid chlorides, and dry distillation of calcium salts of carboxylic acids. Additionally, aldehydes can be produced from nitrites via the Stetten reduction process.

Takeaways

• 🧪 Aldehydes and ketones can be prepared by controlled oxidation processes.
• 🌡 Primary alcohols are oxidized to aldehydes using acidified potassium dichromate in a controlled manner to prevent further oxidation to carboxylic acids.
• 🔄 Ethyl alcohol specifically yields acetaldehyde when oxidized with acidified potassium dichromate.
• 🔄 Secondary alcohols, like isopropyl alcohol, are oxidized to ketones under similar conditions, resulting in compounds like acetone.
• ⚖️ Unsymmetrical alkenes undergo reductive ozonolysis to produce two different aldehydes, while symmetrical alkenes yield only one type.
• 🌐 Aldehydes can be obtained from alkenes without branching at the carbon atoms involved in the double bond.
• 🔥 Alkynes, like acetylene, can be hydrated to produce aldehydes, with an intermediate unstable vinyl alcohol forming and then transforming into acetaldehyde.
• 🌀 Ketones are produced from alkenes with branching at the carbon atoms involved in the double bond through reductive ozonolysis.
• ⚛️ Acid chlorides can be reduced to aldehydes and ketones using different methods, including catalytic hydrogenation and reactions with dimethylcadmium.
• 🔬 Aldehydes and ketones can also be derived from carboxylic acids through the dry distillation of their calcium salts.
• 🧪 Stiff's reduction involves the partial reduction of nitriles to aldehydes using zinc and hydrochloric acid at specific temperatures.

Q & A

What is the controlled oxidation process for preparing aldehydes?

The controlled oxidation process for preparing aldehydes involves the use of acidified potassium dichromate, which is potassium dichromate in the presence of sulfuric acid. This process oxidizes primary alcohols, such as ethyl alcohol, to aldehydes like acetaldehyde (CH3CHO), with the removal of hydrogen as water.

Why is the oxidation of primary alcohols to aldehydes a controlled process?

The oxidation of primary alcohols to aldehydes must be controlled because if it proceeds without control, the aldehyde can further oxidize to a carboxylic acid. It's important to remove the aldehyde from the reaction mixture as soon as it forms to prevent further oxidation.

How are ketones produced from secondary alcohols?

Ketones are produced from secondary alcohols through oxidation using acidified potassium dichromate. For example, isopropyl alcohol undergoes oxidation, leading to the removal of hydrogen as water and the formation of a ketone such as acetone (CH3COCH3).

What is reductive ozonolysis and how does it produce aldehydes?

Reductive ozonolysis is a process where alkenes react with ozone to form an ozonide intermediate, which then reacts with zinc dust in the presence of water to produce aldehydes. This method is used for alkenes without branching at the carbon atoms involved in the double bond.

How does the symmetry of an alkene affect the type of aldehydes produced in reductive ozonolysis?

The symmetry of an alkene determines the types of aldehydes produced in reductive ozonolysis. Unsymmetrical alkenes yield two different aldehydes, while symmetrical alkenes yield the same type of aldehyde.

What is the role of zinc in the reductive ozonolysis process?

In the reductive ozonolysis process, zinc reacts with the oxygen from the ozonide intermediate, forming zinc oxide and reducing the ozonide to an aldehyde.

How are ketones produced from alkenes with branching?

Ketones are produced from alkenes with branching at the carbon atoms involved in the double bond through reductive ozonolysis. The branching leads to the formation of ketones rather than aldehydes upon ozonolysis and reduction.

What is the hydration of alkynes and how does it lead to the formation of aldehydes?

The hydration of alkynes involves passing alkynes, such as acetylene, through dilute sulfuric acid in the presence of mercuric sulfate, leading to the formation of aldehydes like acetaldehyde (CH3CHO). This process involves the addition of water across the triple bond.

How are aldehydes and ketones produced from acid chlorides?

Aldehydes can be produced from acid chlorides by catalytic hydrogenation in the presence of a palladium catalyst supported by barium sulfate. For ketones, acid chlorides are treated with dimethyl cadmium in dry ether to produce ketones like acetone (CH3COCH3).

What is the significance of using a poisoned catalyst in the hydrogenation of acid chlorides?

Using a poisoned catalyst, such as one partially poisoned with sulfur, prevents the over-reduction of aldehydes to alcohols, ensuring that the desired aldehyde product is obtained and not further reduced.

How are aldehydes and ketones produced from the dry distillation of calcium salts of carboxylic acids?

Aldehydes and ketones can be produced by dry distillation of calcium salts of carboxylic acids. For example, calcium acetate undergoes dry distillation to produce acetaldehyde (CH3CHO), while calcium salts of other carboxylic acids can produce ketones.

What is the Stiff reduction and how is it used to produce aldehydes?

The Stiff reduction is a process where nitriles are partially reduced to aldehydes using zinc and hydrochloric acid at specific temperatures. This method involves the conversion of nitriles to imine hydrochlorides, which upon boiling with water yield aldehydes.