Methods of Preparation of Aldehydes and Ketones

Methods of Preparation of Aldehydes and Ketones

Edited By Shivani Poonia | Updated on Jul 02, 2025 07:46 PM IST

One enters a bakery and is greeted by the smell of newly baked bread and cookies. These are compounds with a pleasing odor known as aldehydes and ketones. Aldehydes and ketones fall under the category of such important organic compounds that find an application in foodstuffs industries, pharmaceuticals, and perfumes. From the smell of freshly cut grass—sharp and crisp, due to an aldehyde—to the sharp, sweet nail polish remover smell, a ketone, these chemicals enrich our senses.

This Story also Contains
  1. General Concept and Definitions
  2. Preparation of Aldehydes
  3. Preparation of Ketones
  4. Relevance and Applications
  5. Rosenmund Reduction
  6. Stephen Reduction
  7. Some Solved Examples
  8. Summary
Methods of Preparation of Aldehydes and Ketones
Methods of Preparation of Aldehydes and Ketones

Aldehydes and ketones do not restrict themselves to the sensory arena of pleasures; on the other hand, they turn out to be some important classes of intermediates for the synthesis of many pharmaceuticals and fine chemicals. Chemists and industrialists dealing with these versatile molecules should be conversant with how to prepare such compounds. In this paper, we will take a closer look at the methods of preparation of aldehydes and ketones and their significance or applications.

General Concept and Definitions

Aldehydes and ketones are a class of organic compounds that contain a carbonyl group,$\mathrm{C}=\mathrm{O}$. In aldehydes, the carbonyl group is bonded to at least one hydrogen atom and is always located at the end of the chain. Familiar examples include formaldehyde and benzaldehyde. In the case of the ketones, the carbonyl group is bonded to two carbon atoms, thus being part of the carbon chain. Acetone and butanone are familiar ketones. These are of great importance as compounds in organic chemistry because of their reactivity and the wide range of reactions they undergo. Knowing their methods of preparation provides a basis for their further study of chemical behavior and applications.

Preparation of Aldehydes

There are many ways to make aldehydes. All of them require a particular set of chemical circumstances to best yield the desired product. One common method is simply the oxidation of primary alcohols. For instance, ethanol can be oxidized to acetaldehyde with a mild oxidizing agent such as PCC (Pyridinium chlorochromate). Alkenes can be transformed into aldehydes by hydroformylation, where an alkene reacts with carbon monoxide and hydrogen in the presence of a catalyst. Reduction of acid chlorides by reagents like lithium tri-tert-butoxyaluminum hydride also forms aldehydes. These two techniques bring versatility to the preparation of aldehydes for various uses.

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Oxidation

Oxidation of ROH involves the cleavage of (O-H) and (C-H) bonds to form (C=O) bonds. Such reactions are also called dehydrogenation reactions because H2 is lost from an alcohol molecule. Strong oxidizing agents such as acidic $\mathrm{KMnO}_4$ or acidic $\mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7$ oxidize 1o ROH first to an aldehyde and then to carboxylic acids. Further, 2o alcohols are oxidized to ketones and 3o alcohols do not undergo oxidation, but under drastic conditions such as strong oxidizing agents(KMnO4) and at high temperatures cleavage of (C-C) bonds takes place and a mixture of RCOOH containing a lesser number of C atoms is formed. 1o ROH can be converted up to the aldehyde stage either by the use of ${CrO}_3$ in an anhydrous medium or with PCC or with Jones reagent.
Some examples include:

$\mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_2 \mathrm{OH} \xrightarrow{\mathrm{PCC}} \mathrm{CH}_3-\mathrm{CH}=\mathrm{CH}-\mathrm{CHO}$

Dehydrogenation

1o alcohols undergo dehydrogenation to give aldehydes only. 2o alcohols give ketones and 3o alcohols give alkenes. Some examples include.


Preparation of Ketones

Methods of preparation of ketones are mainly from the oxidation of secondary alcohols. For example, isopropanol gets oxidized to acetone using an oxidizing agent like chromic acid or potassium permanganate. The other way is through Friedel-Crafts acylation in which an aromatic compound is caused to react with an acyl chloride under a Lewis acid catalyst to produce a ketone. Ketones can also be prepared by hydration of alkynes, essentially meaning that an alkyne reacts with water in the presence of a catalyst, such as acid. Each one permits tailoring of the synthesis process according to specific needs and conditions.

Relevance and Applications

The preparation of aldehydes and ketones is not an exercise in methodology; it has a deep practical impact. Many drugs are aldehydes or ketones, and effective routes for their synthesis are in demand from the pharmaceutical industry. One example is the anti-inflammatory pain reliever ibuprofen, whose synthesis involves ketone intermediates. In foods, aldehydes give the flavors of vanilla, cinnamon, and many other essential oils that add richness to food experiences. In perfumery, 5%-25% concentrations of aldehydes and ketones in perfumes give them their appeal and lasting qualities. With their diversified methods of preparation, better processes and products can be designed that have impacts on several aspects of everyday life.

Rosenmund Reduction

Partial hydrogenation of benzoyl chloride with finely divided Pd as a catalyst in the presence of $\mathrm{BaSO}_4$ and S or quinoline in boiling xylene as solvent gives benzaldehyde. This reaction is called Rosenmund reduction. The catalyst under the above condition is called Lindlar's catalyst or poisoned Pd. The Lindlar's catalyst also reduces $(\mathrm{C}=\mathrm{C})$ to $(\mathrm{C}=\mathrm{C})$in syn-addition.
It is BaSO4 that prevents the aldehyde from being further reduced to alcohol and acts as a poison to the Pd catalyst. A small amount of sulfur and quinoline is very effective in poisoning the catalyst in aldehyde reduction. Moreover, S and quinoline react with a small amount of $\mathrm{H}_2$ to give $\mathrm{H}_2 \mathrm{~S}$ gas and hydroquinoline, thereby limiting H2 for further reduction of aldehyde to alcohol.
The reaction occurs as follows:

Stephen Reduction

Nitriles are partially reduced to corresponding imine with SnCl2 in the presence of HCl, which on hydrolysis gives the corresponding aldehyde. It does not reduce$(\mathrm{C}=\mathrm{C})$ or $(\mathrm{C}=\mathrm{C})$. This reaction is known as Stephen reduction. The reaction occurs as follows.

From Acyl chlorides

Formyl chloride gives an aldehyde and all other halides five ketones. The reaction occurs as follows.

From Nitriles

Grignard reagents give aldehydes with hydrogen cyanide (HCN) and ketones with alkyl cyanides (RCN). The reaction occurs as follows:

Recommended topic video on(Methods of Preparation of Aldehydes and Ketones)

Some Solved Examples

Example 1
Question:

Ethanol upon treatment with which one of the following will give ethanal?
$
\begin{aligned}
& \text { 1) } \mathrm{KMnO}_4 / \mathrm{H}_2 \mathrm{SO}_4 \\
& \text { 2) } \mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7 / \mathrm{H}_2 \mathrm{SO}_4
\end{aligned}
$

3) PCC

$
\text { 4) } \mathrm{KMnO}_4 / \mathrm{NH}_4 \mathrm{Cl}
$
Solution:

As we learned

PCC is a mild oxidizing agent and it is used to convert primary or secondary alcohols into aldehydes and ketones respectively.

Therefore, Option(3) is correct

Example 2
Question:

Propan-2-ol upon treatment with which reagent will give acetone?

$\begin{aligned} & \mathrm{KMnO}_4 / \mathrm{H}_2 \mathrm{SO}_4 \\ & \mathrm{PDC} \\ & \mathrm{K}_2 \mathrm{Cr}_2 \mathrm{O}_7 / \mathrm{H}_2 \mathrm{SO}_4 \\ & \mathrm{KMnO}_4 / \mathrm{NH}_4 \mathrm{Cl}\end{aligned}$

Solution:

Therefore, Option(2) is correct

Example 3
Question:

$\mathrm{R}-\mathrm{CN} \xrightarrow{\mathrm{SnCl}_2 / \mathrm{HCl}} A \xrightarrow{\mathrm{H}_2 \mathrm{O}} B+C$

B is :

1)RCONH2

2)RCHO

3)RNC

4)RCOOH

Solution:

As we have learned

Stephen's reduction -

By Stephen's reduction cyanides are converted into imines which on hydrolysis convert into aldehydes or ketones.

- wherein

$\mathrm{R}-\mathrm{C} \equiv \mathrm{N} \underset{2[\mathrm{H}}{\mathrm{l}} \mathrm{SnCl}_2 / \mathrm{HClRCH}=\mathrm{NH} \cdot \mathrm{HCl} \xrightarrow{\mathrm{H}_2 \mathrm{O}} \mathrm{RCHO}+\mathrm{NH}_4 \mathrm{Cl}$

Summary

There is a summary of the major points regarding aldehydes and ketones, important organic compounds applied in various aspects. We took into consideration their definitions and the general properties of their carbonyl groups. Common ways of preparing aldehydes include the oxidation of primary alcohols, hydroformylation of alkenes, and reduction of acid chlorides. On the other hand, some ways ketones are prepared include oxidation of secondary alcohols, Friedel-Crafts acylation, and hydration of alkynes.

Frequently Asked Questions (FAQs)

1. What is the significance of the Cannizzaro reaction in aldehyde chemistry?
The Cannizzaro reaction is a disproportionation reaction where an aldehyde lacking an α-hydrogen is converted into an alcohol and a carboxylic acid in the presence of a strong base. While not directly a method of aldehyde synthesis, this reaction is significant because it can be used to convert certain aldehydes into useful products, and understanding it is crucial for predicting the behavior of aldehydes under basic conditions.
2. What is the role of oxidative cleavage in aldehyde and ketone synthesis?
Oxidative cleavage reactions, such as periodate cleavage of 1,2-diols or ozonolysis of alkenes, are important methods for synthesizing aldehydes and ketones. These reactions break carbon-carbon bonds, often in cyclic compounds, to form two carbonyl groups. Understanding these reactions is crucial for planning synthetic routes, especially when starting from complex natural products or when targeting specific carbonyl compound structures.
3. How does the Wolff-Kishner reduction relate to aldehyde and ketone synthesis?
The Wolff-Kishner reduction converts aldehydes and ketones to alkanes. While not a direct method of carbonyl synthesis, understanding this reaction is important for several reasons: it helps in planning synthetic routes, it can be used to remove unwanted carbonyl groups, and the reverse reaction concept can be applied in retrosynthetic analysis to identify potential carbonyl precursors.
4. How does the Baeyer-Villiger oxidation relate to ketone chemistry?
The Baeyer-Villiger oxidation converts ketones to esters by inserting an oxygen atom between the carbonyl carbon and one of its adjacent carbons. While not directly a method of ketone synthesis, this reaction is crucial in ketone chemistry for several reasons: it can be used to protect ketones, to synthesize certain esters, and in retrosynthetic analysis to identify potential ketone precursors for target molecules.
5. How does the Dakin reaction contribute to the interconversion of aldehydes and phenols?
The Dakin reaction converts an ortho- or para-hydroxybenzaldehyde to a dihydroxybenzene (catechol or hydroquinone) using hydrogen peroxide in basic conditions. While not directly a method of aldehyde synthesis, understanding this reaction is important for several reasons: it provides a way to remove certain aldehydes, it can be used in retrosynthetic analysis, and the reverse reaction concept can be applied to identify potential routes to certain aromatic aldehydes.
6. What are the main methods for preparing aldehydes?
The main methods for preparing aldehydes include oxidation of primary alcohols, reduction of carboxylic acids or their derivatives, and ozonolysis of alkenes. Each method involves different reagents and reaction conditions, allowing for the synthesis of various aldehydes depending on the starting material and desired product.
7. Why is the oxidation of primary alcohols to aldehydes considered a delicate process?
The oxidation of primary alcohols to aldehydes is delicate because aldehydes can be easily over-oxidized to carboxylic acids. This process requires careful control of reaction conditions, such as temperature and oxidizing agent concentration, to stop the oxidation at the aldehyde stage and prevent further oxidation to the carboxylic acid.
8. How does the Kornblum oxidation differ from other oxidation methods for aldehyde synthesis?
The Kornblum oxidation is a unique method for converting primary alkyl halides to aldehydes using dimethyl sulfoxide (DMSO) as the oxidizing agent. Unlike many other oxidation methods, it doesn't require metal-based oxidants or harsh conditions. This makes it particularly useful for sensitive substrates that might not tolerate other oxidation conditions.
9. What is the role of chromium-based reagents in aldehyde and ketone synthesis?
Chromium-based reagents, such as pyridinium chlorochromate (PCC) and chromium trioxide, are powerful oxidizing agents used to convert primary and secondary alcohols to aldehydes and ketones, respectively. These reagents are known for their efficiency and reliability, but they also have drawbacks such as toxicity and environmental concerns. Understanding their role helps in choosing the most appropriate oxidation method for a given situation.
10. How does the Dess-Martin oxidation compare to other methods for converting alcohols to carbonyl compounds?
The Dess-Martin oxidation uses the Dess-Martin periodinane to convert primary and secondary alcohols to aldehydes and ketones, respectively. It's known for its mild conditions, high selectivity, and tolerance of sensitive functional groups. Compared to other oxidation methods, it often provides higher yields and cleaner reactions, making it particularly useful for complex molecules in late-stage synthesis.
11. How do ketones differ from aldehydes in their preparation methods?
Ketones are typically prepared by oxidation of secondary alcohols, whereas aldehydes are prepared from primary alcohols. Additionally, ketones can be synthesized through Friedel-Crafts acylation of aromatic compounds, which is not applicable to aldehydes. Both can be prepared by ozonolysis of alkenes, but the starting alkene structure determines whether an aldehyde or ketone is formed.
12. How does the Oppenauer oxidation differ from other oxidation methods for preparing ketones?
The Oppenauer oxidation is a unique method for oxidizing secondary alcohols to ketones using aluminum alkoxides and a ketone as the oxidizing agent. Unlike other oxidation methods, it's a transfer hydrogenation process where the alcohol is oxidized while simultaneously reducing the ketone oxidizing agent. This method is particularly useful for sensitive compounds that might decompose under harsher oxidizing conditions.
13. Why is the Grignard reaction important in ketone synthesis?
The Grignard reaction is crucial in ketone synthesis because it allows for the formation of new carbon-carbon bonds. When a Grignard reagent (an organomagnesium compound) reacts with an ester or a nitrile, it can form a ketone after workup. This reaction is versatile and can be used to synthesize a wide range of ketones, including those that might be difficult to prepare by other means.
14. How does the Nef reaction contribute to the synthesis of aldehydes and ketones?
The Nef reaction converts primary or secondary nitro compounds into aldehydes or ketones, respectively. It involves the hydrolysis of a nitronate ion under acidic conditions. This reaction is valuable because it provides a way to synthesize carbonyl compounds from nitro compounds, which are often easily prepared, thus expanding the range of possible synthetic routes to aldehydes and ketones.
15. What is the significance of the Wacker process in industrial ketone synthesis?
The Wacker process is an industrial method for converting ethene to acetaldehyde using a palladium(II) chloride catalyst and copper(II) chloride as a co-catalyst. This process is significant because it provides an efficient way to produce acetaldehyde on a large scale from readily available ethene. The reaction can also be applied to other alkenes to produce various aldehydes and ketones, making it a versatile industrial process.
16. What is the significance of the Swern oxidation in aldehyde synthesis?
The Swern oxidation is significant because it's a mild and selective method for converting primary and secondary alcohols to aldehydes and ketones, respectively. It uses dimethyl sulfoxide (DMSO) and oxalyl chloride, followed by a base, allowing for the oxidation of sensitive compounds that might not tolerate harsher oxidizing conditions.
17. Why is the Friedel-Crafts acylation important in ketone synthesis?
Friedel-Crafts acylation is crucial in ketone synthesis because it allows for the direct introduction of an acyl group onto an aromatic ring, forming an aromatic ketone. This reaction is particularly valuable in organic synthesis as it creates a new carbon-carbon bond and introduces a ketone functional group in a single step, which is often challenging to achieve through other methods.
18. What is the importance of protecting groups in aldehyde and ketone synthesis?
Protecting groups are crucial in aldehyde and ketone synthesis because they allow for the selective manipulation of functional groups in complex molecules. For example, an aldehyde might be protected as an acetal to prevent unwanted reactions during other synthetic steps. After the desired transformations are complete, the protecting group can be removed to reveal the aldehyde. This strategy is essential for synthesizing complex molecules with multiple functional groups.
19. What role does ozonolysis play in the preparation of both aldehydes and ketones?
Ozonolysis is a powerful method for cleaving carbon-carbon double bonds in alkenes to form carbonyl compounds. Depending on the structure of the starting alkene, ozonolysis can produce aldehydes, ketones, or both. This method is particularly useful for synthesizing carbonyl compounds that might be difficult to prepare through other means.
20. How does the Weinreb ketone synthesis differ from other methods of ketone preparation?
The Weinreb ketone synthesis involves the reaction of a Weinreb amide (N-methoxy-N-methylamide) with an organometallic reagent to form a ketone. This method is unique because it allows for the controlled addition of the organometallic reagent without the risk of over-addition, which can occur with other carbonyl compounds. This makes it particularly useful for synthesizing ketones that are difficult to prepare by other means.
21. How does the Rosenmund reduction contribute to aldehyde synthesis?
The Rosenmund reduction is a method for synthesizing aldehydes from acyl chlorides. It involves the catalytic hydrogenation of an acyl chloride using a poisoned palladium catalyst. This method is valuable because it allows for the selective reduction of the acyl chloride to an aldehyde without further reduction to an alcohol.
22. How does the Luche reduction relate to ketone synthesis?
While not directly a method of ketone synthesis, the Luche reduction is important in ketone chemistry as it allows for the selective reduction of α,β-unsaturated ketones to allylic alcohols. This selectivity is crucial when manipulating ketones in complex molecules, as it allows for the reduction of the carbonyl group without affecting the carbon-carbon double bond. Understanding this reaction is important for planning synthetic routes involving ketones.
23. How does the Vilsmeier-Haack reaction contribute to aldehyde and ketone synthesis?
The Vilsmeier-Haack reaction is a method for introducing a formyl group (CHO) onto an aromatic ring or an activated alkene, forming an aldehyde or a α,β-unsaturated aldehyde. It uses a combination of phosphorus oxychloride (POCl3) and dimethylformamide (DMF) to generate an electrophilic "Vilsmeier reagent". This reaction is valuable for synthesizing aromatic and α,β-unsaturated aldehydes that might be challenging to prepare by other means.
24. How does the Meerwein-Ponndorf-Verley reduction relate to ketone synthesis?
The Meerwein-Ponndorf-Verley (MPV) reduction is a method for reducing aldehydes and ketones to alcohols using aluminum isopropoxide and isopropanol. While primarily a reduction method, understanding this reaction is important in ketone synthesis for several reasons: it's reversible (the reverse is known as the Oppenauer oxidation), it can be used to protect ketones as alcohols, and it helps in planning synthetic routes involving interconversion between alcohols and carbonyl compounds.
25. What is the importance of the Stetter reaction in aldehyde and ketone synthesis?
The Stetter reaction is a method for forming 1,4-dicarbonyl compounds by adding an aldehyde to an α,β-unsaturated carbonyl compound. This reaction is significant because it allows for the formation of new carbon-carbon bonds and introduces additional carbonyl functionality. It's particularly useful for synthesizing 1,4-diketones and related compounds that might be challenging to prepare by other means.
26. What is the Stephen reaction, and how is it used in aldehyde synthesis?
The Stephen reaction is a method for synthesizing aldehydes from nitriles (cyanides). It involves the reduction of a nitrile using stannous chloride in the presence of hydrochloric acid, followed by hydrolysis. This reaction is valuable because it provides a way to convert nitriles, which are easily synthesized, into aldehydes, expanding the range of possible starting materials for aldehyde synthesis.
27. What is the significance of the Noyori asymmetric hydrogenation in ketone chemistry?
The Noyori asymmetric hydrogenation is a method for reducing ketones to alcohols with high enantioselectivity using a ruthenium-BINAP catalyst. While primarily a reduction method, understanding this reaction is crucial in ketone synthesis for several reasons: it allows for the selective preparation of chiral alcohols from ketones, it can be used to protect ketones stereoselectively, and the reverse reaction concept can be applied in planning synthetic routes to specific ketones.
28. What is the significance of the Corey-Seebach reaction in aldehyde synthesis?
The Corey-Seebach reaction, also known as the umpolung reaction, allows for the reversal of the normal polarity of a carbonyl group. It involves converting an aldehyde to a 1,3-dithiane, which can then be alkylated and subsequently hydrolyzed back to an aldehyde. This reaction is significant because it enables the synthesis of aldehydes with substitution patterns that would be difficult to achieve through conventional methods.
29. What is the importance of the Wittig reaction in relation to aldehyde and ketone synthesis?
While the Wittig reaction is primarily used to convert aldehydes and ketones into alkenes, it's also important in planning synthetic routes to carbonyl compounds. By working backwards, chemists can use the Wittig reaction to determine suitable precursors for target aldehydes or ketones. Additionally, the reverse Wittig reaction can be used to synthesize carbonyl compounds from certain alkenes.
30. How does the Kulinkovich reaction relate to ketone synthesis?
The Kulinkovich reaction converts esters into cyclopropanols using a titanium-based reagent and a Grignard reagent. While not directly producing ketones, these cyclopropanols can be easily converted to homologated ketones through ring-opening reactions. This reaction is significant because it provides a unique way to form carbon-carbon bonds and access ketones with specific substitution patterns.
31. What is the significance of the Pauson-Khand reaction in ketone synthesis?
The Pauson-Khand reaction is a cycloaddition reaction between an alkyne, an alkene, and carbon monoxide, catalyzed by a cobalt complex. It forms a cyclopentenone, which is a type of α,β-unsaturated ketone. This reaction is significant because it allows for the rapid construction of complex cyclic ketones from relatively simple starting materials, often with high stereoselectivity.
32. What is the role of the Horner-Wadsworth-Emmons reaction in relation to aldehyde and ketone synthesis?
The Horner-Wadsworth-Emmons (HWE) reaction is a modification of the Wittig reaction that uses phosphonate-stabilized carbanions to react with aldehydes or ketones, forming alkenes. While primarily used to convert carbonyls to alkenes, understanding this reaction is crucial in carbonyl synthesis for retrosynthetic analysis. It allows chemists to work backwards from complex alkenes to identify potential aldehyde or ketone precursors.
33. How does the Ritter reaction contribute to ketone synthesis?
The Ritter reaction is a method for synthesizing amides from alkenes or alcohols using concentrated sulfuric acid and a nitrile. While not directly producing ketones, this reaction can be used to form α-amino ketones when applied to certain alkenes. Understanding this reaction expands the toolkit for synthesizing functionalized ketones and related compounds.
34. How does the Reformatsky reaction relate to ketone synthesis?
The Reformatsky reaction is a method for forming β-hydroxy esters by reacting an α-halo ester with an aldehyde or ketone in the presence of zinc. While not directly producing ketones, this reaction is significant in ketone synthesis because the resulting β-hydroxy esters can be easily converted to β-keto esters, which are valuable intermediates in the synthesis of more complex ketones.
35. What is the importance of the Dieckmann condensation in cyclic ketone synthesis?
The Dieckmann condensation is an intramolecular version of the Claisen condensation, used to form cyclic β-keto esters from diesters. This reaction is crucial in the synthesis of cyclic ketones, particularly five- and six-membered rings. Understanding this reaction is important for planning synthetic routes to complex cyclic ketones and related compounds.
36. How does the Paal-Knorr synthesis contribute to the preparation of furan and pyrrole derivatives?
The P

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