8.2.2 - Preparation of Aldehydes
Enroll to start learning
You’ve not yet enrolled in this course. Please enroll for free to listen to audio lessons, classroom podcasts and take practice test.
Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Introduction to Aldehyde Preparation
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Today, we'll discuss how we can prepare aldehydes. One common method involves the oxidation of primary alcohols. Can anyone tell me what happens to the alcohol in this reaction?
The primary alcohol is converted into an aldehyde.
Exactly! What kind of reagents do you think we might use for this oxidation?
We can use mild oxidizing agents like PCC or strong ones like KMnO4.
Great! Remember: we use different oxidizing agents based on how selective we want our reaction to be. Now, oxidation also generates ketones from secondary alcohols. Can anyone give an example of this?
Yes! If we oxidize isopropanol, we get acetone, which is a ketone.
Correct! Let's summarize this. Oxidation is key for creating both aldehydes and ketones aptly from alcohols.
Industrial Methods of Aldehyde Preparation
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Now let's shift gears to the industrial methods of preparing aldehydes. One common method is dehydrogenation. Who can explain how this works?
Alcohol vapors pass over a catalyst, causing them to lose hydrogen and form aldehydes.
Exactly! We typically use silver or copper as catalysts. It's efficient for producing large amounts of aldehydes. Another interesting reaction is ozonolysis of alkenes. How does that help us prepare aldehydes?
Ozonolysis cleaves alkene bonds. After treatment with zinc and water, it produces aldehydes or ketones.
Well done! This shows how we can create aldehydes through various synthetic routes. Let's solidify this knowledge by discussing the specific reactions that form aldehydes from aromatic compounds.
Reactions Involving Acyl Chlorides
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Lastly, we'll discuss methods like Rosenmund reduction, which is a versatile reaction for preparing aldehydes. Can anyone describe it?
In Rosenmund reduction, we hydrogenate acyl chlorides over palladium on barium sulfate.
Correct! This reaction is crucial because it selectively gives us aldehydes from acyl chlorides. Now, does anyone remember the Stephen reaction?
Yes! It involves nitriles that are reduced to imines before hydrolysis yields aldehydes.
Exactly! You all are doing great. Remember these methods, as they greatly impact synthetic organic chemistry and industry.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
The preparation of aldehydes and ketones can be achieved through several methods including oxidation, reduction, and reactions involving acyl chlorides and nitriles. Each method has specific reagents and conditions that can yield desired products, highlighting the versatility of organic synthesis.
Detailed
Preparation of Aldehydes
In this section, we explore critical methods for preparing aldehydes and ketones, essential organic compounds used in various applications. Aldehydes can typically be synthesized through the following methods:
1. By Oxidation of Alcohols
- Primary alcohols are oxidized to form aldehydes. This is a fundamental reaction often covered in introductory organic chemistry.
- Secondary alcohols yield ketones under oxidation reactions.
2. By Dehydrogenation of Alcohols
- A more indirect method suitable for industrial applications involves the dehydrogenation of volatile alcohols. This can be conducted using catalysts such as silver or copper.
3. From Hydrocarbons
- Ozonolysis of alkenes results in the formation of aldehydes or ketones depending on the substitution pattern.
- Hydration of alkynes adds water across the triple bond, converting it into aldehydes or ketones, facilitated by acids and mercuric salts.
4. Specific Preparation Methods for Aldehydes:
- Rosenmund Reduction: Hydrogens acyl chlorides over palladium on barium sulfate to yield aldehydes.
- Stephen Reaction: The reduction of nitriles using stannous chloride in hydrochloric acid to yield aldehydes after hydrolysis.
- Gatterman-Koch Reaction: Treating benzene with carbon monoxide and hydrogen chloride in the presence of aluminum chloride yields aromatic aldehydes.
These methods underscore the essential techniques in organic synthesis, making it easier for chemists to access aldehydes and ketones efficiently.
Youtube Videos
Audio Book
Dive deep into the subject with an immersive audiobook experience.
Hydrogenation of Acyl Chlorides
Chapter 1 of 7
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
- From acyl chloride (acid chloride)
Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called Rosenmund reduction.
Detailed Explanation
In this method, acyl chlorides, which are a type of carbonyl compound, can be converted into aldehydes by reacting them with hydrogen gas in the presence of a catalyst known as palladium on barium sulfate. This process is referred to as the Rosenmund reduction. The importance of this method lies in its ability to specifically reduce acyl chlorides to aldehydes without over-reducing them to alcohols.
Examples & Analogies
Imagine making a recipe where you need to caramelize just the edges of an apple pie instead of burning it completely. The palladium catalyst acts like a precise chef, ensuring that only the acyl chloride is reduced to an aldehyde instead of completely transforming it into something else.
Reduction of Nitriles and Esters
Chapter 2 of 7
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
- From nitriles and esters
Nitriles are reduced to corresponding imine with stannous chloride in the presence of hydrochloric acid, which on hydrolysis give corresponding aldehyde.
This reaction is called Stephen reaction.
Alternatively, nitriles are selectively reduced by diisobutylaluminium hydride, (DIBAL-H) to imines followed by hydrolysis to aldehydes:
Similarly, esters are also reduced to aldehydes with DIBAL-H.
Detailed Explanation
Nitriles can be transformed into aldehydes through reduction. First, they convert into imines with stannous chloride (a reducing agent) and hydrochloric acid. When these imines are hydrolyzed, they yield aldehydes; this process is known as the Stephen reaction. Another method employs diisobutylaluminium hydride (DIBAL-H), which can selectively reduce nitriles and esters to aldehydes. This is particularly useful for compounds where over-reduction is a concern.
Examples & Analogies
Think of DIBAL-H as a gentle rain that waters plants. Just like the rain provides just the right amount of moisture without flooding them, DIBAL-H selectively reduces nitriles and esters to aldehydes without overdoing it.
Oxidation of Aromatic Hydrocarbons
Chapter 3 of 7
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
- From hydrocarbons
Aromatic aldehydes (benzaldehyde and its derivatives) are prepared from aromatic hydrocarbons by the following methods:
(i) By oxidation of methylbenzene Strong oxidising agents oxidise toluene and its derivatives to benzoic acids. However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents that convert the methyl group to an intermediate that is difficult to oxidise further. The following methods are used for this purpose.
Detailed Explanation
Aromatic aldehydes can be derived from aromatic hydrocarbons, like toluene. When strong oxidizing agents act on toluene, they typically oxidize it all the way to benzoic acid. However, chemists can use specific reagents to halt the reaction earlier, converting the methyl group into a structure that’s less prone to oxidation. This selective oxidation allows the formation of benzaldehyde instead of over-oxidizing it to carboxylic acid.
Examples & Analogies
Imagine you have a delicate fabric dye that could get ruined if too much bleach is applied. The oxidizing agents act like bleach, but with precise maneuvering, chemists can keep just the right balance to produce benzaldehyde without damaging the integrity of the fabric.
Use of Chromyl Chloride
Chapter 4 of 7
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
(a) Use of chromyl chloride (CrO2Cl2): Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde.
This reaction is called Etard reaction.
Detailed Explanation
The Etard reaction utilizes chromyl chloride, which can oxidize the methyl group of toluene to form a complex with chromium. When this complex undergoes hydrolysis (i.e., it reacts with water), benzaldehyde is produced. This method is significant as it provides a controlled way of forming aldehydes from aromatic substrates without further reactions occurring.
Examples & Analogies
Consider a painter who wants only certain areas of a canvas highlighted. Using chromyl chloride is like the painter applying a wash that brilliantly colors the edges of the area he wants to accentuate, allowing for the careful creation of benzaldehyde without overdoing it.
Use of Chromic Oxide
Chapter 5 of 7
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
(b) Use of chromic oxide (CrO3): Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde with aqueous acid.
Detailed Explanation
In this method, treating toluene with chromic oxide in a solvent called acetic anhydride leads to the formation of benzylidene diacetate. This product can then be hydrolyzed (reacted with water) to yield benzaldehyde. This route is effective for producing aromatic aldehydes while managing the reaction conditions precisely.
Examples & Analogies
Imagine a sculptor who uses a specific mold to shape a piece of clay into a precise form. The chromic oxide and acetic anhydride work together like this mold, ensuring that the product, benzaldehyde, comes out finely shaped and without damage.
Chlorination and Hydrolysis
Chapter 6 of 7
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
(ii) By side chain chlorination followed by hydrolysis
Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde.
Detailed Explanation
This method begins with the chlorination of toluene to form benzal chloride. This step involves substituting one of the hydrogen atoms in toluene with a chlorine atom. When benzal chloride undergoes hydrolysis (reaction with water), it transforms into benzaldehyde. This is a straightforward and commercially used method for producing benzaldehyde.
Examples & Analogies
Think of this method as peeling a banana. The chlorination is like removing the outer peel of the banana, while hydrolysis is akin to enjoying the fruit inside. Together, they effectively reveal the sweet benzaldehyde.
Gatterman-Koch Reaction
Chapter 7 of 7
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
(iii) By Gatterman – Koch reaction
When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.
This reaction is known as Gatterman-Koch reaction.
Detailed Explanation
The Gatterman-Koch reaction involves treating benzene (or its derivatives) with carbon monoxide and hydrogen chloride, using anhydrous aluminium chloride or cuprous chloride as a catalyst. This reaction facilitates the formation of benzaldehyde or substituted benzaldehydes from the benzene compound. It’s a useful method as it directly introduces the carbonyl group in the desired positions.
Examples & Analogies
Picture a chef who wants to perfectly season a dish. The Gatterman-Koch reaction is like adding spices (carbon monoxide and hydrogen chloride) at just the right moment while using a skilled assistant (the catalyst) to create the perfect blend of flavors, resulting in the aromatic delight of benzaldehyde.
Key Concepts
-
Oxidation of Alcohols: A primary alcohol oxidizes to an aldehyde and a secondary alcohol oxidizes to a ketone.
-
Dehydrogenation: Industrial process using heavy metal catalysts to produce aldehydes from alcohols.
-
Ozonolysis: A reaction that cleaves alkenes to form aldehydes and ketones.
-
Rosenmund Reduction: A method of converting acyl chlorides to aldehydes using palladium catalysts.
-
Stephen Reaction: A reaction in which nitriles are reduced to aldehydes through hydrolysis.
Examples & Applications
Oxidation of ethanol (C2H5OH) produces ethanal (acetaldehyde).
Dehydrogenation of isopropanol (C3H8O) yields acetone.
Ozonolysis of 1-hexene results in the formation of hexanal.
Rosenmund reduction converts acetyl chloride (CH3COCl) to ethanal (acetaldehyde).
Stephen reaction reduces benzonitrile (C6H5CN) to benzaldehyde (C6H5CHO).
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
Oxidation turns alcohols bright, Into aldehydes they take flight.
Stories
Imagine a primary alcohol setting sail on a river. As it flows gently, an oxidizing agent helps it transform into a lovely aldehyde, resembling a flower blooming on the banks.
Memory Tools
Remember: ODA - Oxidation, Dehydrogenation, Acyl chlorides are key methods!
Acronyms
R.O.S.E. - Rosenmund, Ozonolysis, Stephen reaction, and Edges of alcohols.
Flash Cards
Glossary
- Aldehyde
Organic compounds containing the functional group -CHO, characterized by a carbonyl group bonded to at least one hydrogen.
- Oxidation
A chemical reaction that involves the loss of electrons or an increase in oxidation state, often leading to the formation of aldehydes or ketones from alcohols.
- Dehydrogenation
A chemical reaction that involves the removal of hydrogen from a molecule, converting it into an aldehyde or ketone.
- Ozonolysis
A reaction where alkenes are cleaved by ozone, leading to the formation of aldehydes and ketones depending on the substituents attached.
- Rosenmund Reduction
A specific reaction where acyl chlorides are reduced to aldehydes using hydrogen over a palladium catalyst.
- Stephen Reaction
A method of reducing nitriles to aldehydes using stannous chloride in the presence of hydrochloric acid.
Reference links
Supplementary resources to enhance your learning experience.