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Reactions of Aldehydes and Ketones

Reactions of Aldehydes and Ketones

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Aldehydes and ketones are everywhere in your daily life, even in your favorite cakes and biscuits! The Mailliard reaction creates that scrumptious golden crust when the carbonyl group in sugars react with amino acids at high temperatures. Another aldehyde, called formaldehyde or methanal, is commonly used in tanning, embalming, and as a fungicide. Plus, it can be reacted with other molecules to create useful compounds like adhesives or explosives.

In this article, we're going to explore the reactions of aldehydes and ketones. First, we'll look at the C = O carbonyl group's polarity and how it makes it easily react. Then, we'll dive into some of the most common reactions and learn how to draw their mechanisms. Finally, we'll learn how to tell the difference between aldehydes, ketones, and other organic molecules. By understanding these reactions, we can better understand the world around us and how it's made!

Why do aldehydes and ketones react?

Aldehydes and ketones both have a carbonyl functional group, which is made up of a carbon atom and an oxygen atom double-bonded together. Due to the oxygen atom being more electronegative than carbon, the carbonyl bond is polar and has a partial negative and partial positive charge. This makes it attractive to nucleophiles, which are molecules that are attracted to positively charged atoms and can donate an electron pair to form a new bond.

Aldehydes and ketones are unsaturated compounds because they contain a double bond, which makes them easily susceptible to addition reactions. When they react with nucleophiles, they undergo nucleophilic addition reactions. However, they can also undergo oxidation and reduction reactions. Nucleophilic addition reactions involve the nucleophile attacking the electrophilic carbon atom in the carbonyl group, breaking the double bond and forming a new bond between the carbon and nucleophile. This results in the formation of a new molecule with a new functional group. The specific reaction mechanism depends on the nature of the nucleophile and the reaction conditions. Overall, understanding the reactions of aldehydes and ketones with nucleophiles is key to understanding their reactivity and chemical properties.

What is a nucleophile?

Nucleophiles are molecules or ions that donate an electron pair to form a new bond with a positively charged atom, such as the carbon atom in the carbonyl group. They are typically negatively charged or contain a partially negatively charged atom, and have a spare pair of electrons in their outer shell.

Examples of nucleophiles include the cyanide ion, the hydride ion, water, and ammonia. In this article, we'll focus on reactions involving the cyanide and hydride ions.

The cyanide ion, CN-, is a strong nucleophile that can react with aldehydes and ketones to form cyanohydrins. This reaction is called cyanohydrin formation and is an important synthetic tool in organic chemistry. The mechanism involves attack of the nucleophilic cyanide ion on the electrophilic carbon atom in the carbonyl group, followed by protonation of the resulting intermediate to form the cyanohydrin product.

The hydride ion, H-, is another strong nucleophile that can react with aldehydes and ketones to form alcohols. This reaction is called reduction and is an important way to convert carbonyl compounds into alcohols. The mechanism involves attack of the nucleophilic hydride ion on the electrophilic carbon atom in the carbonyl group, followed by protonation of the resulting intermediate to form the alcohol product. Overall, understanding the reactions of nucleophiles with aldehydes and ketones is key to understanding the versatility and importance of these compounds in organic chemistry.

Reaction with hydrogen cyanide

The general reaction between an aldehyde and cyanide ion (CN-) involves nucleophilic addition of the cyanide ion to the carbonyl carbon, followed by protonation of the resulting intermediate to form a hydroxynitrile or cyanohydrin. The reaction can be represented by the following equation:

RCHO + CN- → RCH(OH)CN

In this reaction, R represents the rest of the molecule attached to the carbonyl group. The hydroxynitrile product contains both a hydroxyl (-OH) and a nitrile (-CN) functional group, which makes it a versatile intermediate for further reactions.

The reaction is typically carried out using aqueous potassium or sodium cyanide, which generates cyanide ions in solution. This is a safer alternative to using hydrogen cyanide directly, which is an extremely poisonous gas.

The addition of dilute hydrochloric acid to the hydroxynitrile product can result in the formation of an alpha-hydroxy acid, which is an important intermediate in the synthesis of amino acids. This reaction is called hydrolysis, and it involves the cleavage of the nitrile group to form a carboxylic acid and a protonated amide intermediate, which is then hydrolyzed to form the alpha-hydroxy acid.

Overall, the reaction of aldehydes and ketones with cyanide ions is an important synthetic tool in organic chemistry, and can be used to access a variety of useful intermediates for further reactions.

The general reaction between an aldehyde and the cyanide ion
The general reaction between an aldehyde and the cyanide ion

The negative cyanide ion is attracted to the partially positively charged carbon in the C=O bond. It forms a bond with this carbon and forces the C=O double bond to break, transferring one of the bonding pairs of electrons to the oxygen atom. The oxygen atom is now a negatively charged ion with a lone pair of electrons. It reacts with a positive hydrogen ion from the solution to form a hydroxyl functional group, -OH.The end product is a hydroxynitrile.

The reaction has the following general equation:

To see how ketones react with cyanide ions, we simply replace the hydrogen atom bonded to the carbonyl group with another organic R group, as you can see in the following mechanism:

Nucleophilic addition of a ketone. 

Let's now apply that to a named aldehyde, ethanal.

Nucleophilic addition of ethanal using the cyanide ion
Nucleophilic addition of ethanal using the cyanide ion

The reaction between potassium cyanide and the ketone butanone involves nucleophilic addition of the cyanide ion to the carbonyl carbon of butanone, followed by protonation of the resulting intermediate to form a hydroxynitrile or cyanohydrin product. The reaction can be represented by the following equation:

CH3COCH2CH2COCH3 + CN- → CH3COCH2CH(OH)CN + K+

In this reaction, the cyanide ion attacks the carbonyl carbon of butanone, forming a tetrahedral intermediate. The intermediate then collapses, and a proton from the acidic medium (e.g. water) is transferred to the oxygen atom, forming the hydroxynitrile product.

The product of this reaction is named 3-hydroxy-2-methylbutanenitrile. The hydroxyl group is located on carbon 3 of the carbon chain, and the nitrile group is located on carbon 2. The carbon attached to the nitrogen atom is labeled carbon 1.

Overall, the reaction between potassium cyanide and aldehydes or ketones is an important synthetic tool in organic chemistry, and can be used to access a variety of useful intermediates for further reactions. It is important to remember to include all partial charges and lone pairs of electrons when drawing organic mechanisms, and to use proper nomenclature when naming the resulting products.

Nucleophilic addition of butanone using the cyanide ion
Nucleophilic addition of butanone using the cyanide ion

Look at the product. It still has a chain that is four carbon atoms long. But counting the carbon attached to the nitrogen as carbon 1, you can see that there is now both a methyl group and a hydroxyl group joined to carbon 2. We therefore name this molecule 2-hydroxy-2-methylbutanenitrile. 2-hydroxy-2-methylbutanenitrile. 

Isomeric products of nucleophilic addition

In the context of organic chemistry, optical isomers are important because they can have different chemical and biological properties. This is because their three-dimensional structures are different, even though their chemical formulas are the same. For example, one isomer may interact differently with a receptor or enzyme than the other isomer, leading to different biological effects.

In the case of the reaction between butanone and the cyanide ion, the two isomers produced are 3-hydroxy-2-methylbutanenitrile and its enantiomer. These molecules have the same chemical formula and functional groups, but their arrangement of atoms in space is different. Specifically, they differ in the orientation of the hydroxyl and methyl groups around the chiral center, which is the carbon atom adjacent to the nitrile group.

Because of their different three-dimensional structures, the two isomers may have different chemical and biological properties. For example, one isomer may be more soluble in a certain solvent, or may have a stronger affinity for a particular receptor or enzyme. This is why it is important to consider the stereochemistry of a molecule when studying its properties and reactions.

In summary, the reaction between butanone and the cyanide ion can produce two optical isomers, which are mirror images of each other. These isomers may have different chemical and biological properties, and their study is an important part of organic chemistry.

They are optical isomers, known as enantiomers
They are optical isomers, known as enantiomers

For further information on isomers, check out Optical Isomerism.

Reduction of aldehydes and ketones

Yes, when you react aldehydes and ketones with a reducing agent which supplies the hydride ion, you get an alcohol. This is a type of nucleophilic addition reaction, more commonly known as reduction. Common reducing agents include sodium tetrahydridoborate (III) in an aqueous solution, also known as sodium borohydride. LiAlH4 and NaBH4 are also capable of reducing aldehydes and ketones to the corresponding alcohol. The reaction involves nucleophilic attack by the hydride anion, followed by protonation of the alkoxide intermediate.

Sodium borohydride
Sodium borohydride

The general equation for the reduction of a ketone is:

R2C=O + 2[H] → R2CH-OH

In this reaction, the hydride ion acts as a nucleophile and attacks the partially positive carbon atom in the C=O bond. The C=O bond breaks and one of the bonding pairs of electrons is transferred to the oxygen atom, forming a negative oxygen ion with a spare pair of electrons. The oxygen atom’s spare pair of electrons is attracted to a positive hydrogen ion from the solution and forms the hydroxyl functional group, -OH. The product is a secondary alcohol, with two R groups attached to the carbon atom with the hydroxyl functional group.

It is important to note that the reduction of aldehydes and ketones is a reversible reaction. This means that the alcohol product can be oxidized back to the aldehyde or ketone under certain conditions. Therefore, the reduction of aldehydes and ketones is often used as a synthetic tool to selectively prepare alcohols, which can then be further modified or functionalized as needed. 

Isomeric products of reduction

That's correct. Optical isomerism arises due to the presence of a chiral center in a molecule, which is a carbon atom bonded to four different groups. Primary alcohols do not have a chiral center, as they have at least two hydrogen atoms bonded to the carbon atom with the hydroxyl group, making it impossible for them to exhibit optical isomerism. In contrast, secondary and tertiary alcohols can exhibit optical isomerism if they have a chiral center, as they have two or three carbon atoms bonded to the carbon atom with the hydroxyl group, respectively.

Therefore, when reducing a ketone, it is important to consider the stereochemistry of the starting material and the reducing agent used, as this can influence the formation of optical isomers in the product.

Oxidation of aldehydes

That's correct. One way to distinguish between aldehydes and ketones is to use oxidizing agents, such as acidified potassium dichromate (VI), as aldehydes can be further oxidized to carboxylic acids while ketones cannot. This is because ketones lack the hydrogen atom attached to the carbonyl carbon that is required for oxidation to occur. Therefore, if a reaction with an oxidizing agent produces a carboxylic acid, the starting material must have been an aldehyde.

It is important to note that there are other methods for distinguishing between aldehydes and ketones, such as using Tollens' reagent or Fehling's solution, which are based on the ability of aldehydes to be oxidized to carboxylic acids and reduce certain metal ions, respectively. These tests can be useful in identifying unknown compounds in the lab.

Potassium dichromate (VI)

Oxidising an aldehyde with potassium dichromate (VI) turns the solution from orange to green. However, there will be no colour change with a ketone.

The reaction of acidifed potassium dichromate (VI) with a ketone, left, and an aldehyde, right
The reaction of acidifed potassium dichromate (VI) with a ketone, left, and an aldehyde, right

Tollens' reagent

That's correct! Tollens' reagent is a commonly used test for distinguishing between aldehydes and ketones. It is a solution of ammoniacal silver nitrate, which contains the diaminesilver (I) ion, [Ag(NH3)2]+. When an aldehyde is added to Tollens' reagent, it is oxidized to a carboxylic acid, while the silver ion is reduced to metallic silver, which forms a characteristic silver mirror on the inner surface of the test tube. However, ketones do not react with Tollens' reagent under normal conditions and the solution remains colorless.

This reaction is often used as a qualitative test for the presence of aldehydes in a sample. However, it is important to note that some ketones can react with Tollens' reagent under certain conditions, such as when they are heated with the reagent or when a catalyst is added. Therefore, it is always important to confirm the results of any qualitative tests with other analytical methods to ensure accuracy and reliability.

Fehling's test

Fehling's solution is another commonly used reagent for distinguishing between aldehydes and ketones. It is made up of two separate solutions, Fehling's A (a solution of copper (II) sulfate) and Fehling's B (a solution of sodium hydroxide and potassium sodium tartrate). When an aldehyde is added to Fehling's solution and heated, it is oxidized to a carboxylic acid while the copper (II) ion is reduced to copper (I) oxide, which forms a brick red precipitate. In contrast, ketones do not react with Fehling's solution under normal conditions and no color change is observed.

Both Tollens' reagent and Fehling's solution are commonly used qualitative tests for the presence of aldehydes in a sample. However, as with any qualitative test, it is important to confirm the results with other analytical methods to ensure accuracy and reliability.

File:Brady's-test-mechanism-2D-skeletal.png - Wikimedia Commons
Testing for aldehydes and ketones

Distinguishing between molecules

Potassium dichromate (VI) is a strong oxidizing agent that can be used to distinguish between alcohols, aldehydes, and ketones. When an unknown compound is added to acidified potassium dichromate (VI) and gently warmed, the solution will turn green if it is an alcohol or an aldehyde, but will remain orange if it is a ketone.

Further testing with Tollens' reagent or Fehling's solution can then be used to distinguish between an aldehyde and an alcohol. If the unknown compound is an aldehyde, Tollens' reagent will form a silver mirror on the sides of the test tube and Fehling's solution will turn from blue to brick red. However, if the unknown compound is an alcohol, no color change will be observed with either reagent.

Overall, the reactions of aldehydes and ketones are characterized by the presence of the carbonyl group, which undergoes nucleophilic addition reactions and can be oxidized or reduced under certain conditions. Differentiating between aldehydes and ketones can be done using a variety of qualitative tests, such as acidified potassium dichromate (VI), Tollens' reagent, and Fehling's solution.

Reactions of Aldehydes and Ketones

What are the reactions involving aldehydes and ketones?

Aldehydes and ketones react in a variety of reactions. For examples, they react with hydrogen cyanide to form hydroxynitriles. They can also be reduced to give primary and secondary alcohols. In addition, you can oxidise aldehydes to give carboxylic acids. However, you can't oxidise ketones.

What are the reactions of ketones?

Ketones react with hydrogen cyanide to form hydroxynitriles and can be reduced using sodium tetrahydridoborate to form secondary alcohols.

Which reaction will distinguish between ketones and aldehydes? 

There are several different reactions you can use to distinguish between aldehydes and ketones. You can use potassium dichromate (VI), for example. Aldehydes turn the solution from orange to green, but ketones give no visible reaction. Aldehydes also react with Fehling's solution, forming a brick red precipitate. However, with ketones the solution remains blue. Similarly, aldehydes react with colourless Tollens' reagent to form a metallic silver deposit on the sides of the test tube but ketones produce no visible change.

What is the name for the general reaction of aldehydes and ketones? 

There isn't a general name for the reactions of aldehydes and ketones. They can react in a variety of different ways. For example, they can both be reduced. This article will take a look at some of their different reactions.

How do you distinguish between aldehydes and ketones in one test? 

You can distinguish between aldehydes and ketones in a few different ways. Aldehydes turn potassium dichromate (VI) from orange to green but ketones produce no visible reaction. Aldehydes will also react with blue Fehling's solution, forming a brick red precipitate, but ketones won't react at all. Likewise, aldehydes react with colourless Tollens' reagent to form a silver mirror deposit on the sides of the test tube, but ketones again produce no visible reaction.

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