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Chapter 21: Problem 28

Which kinds of reactions are common to aldehydes and ketones? List an example of each.

Short Answer

Expert verified
Common reactions to aldehydes and ketones include nucleophilic addition, nucleophilic addition-elimination (condensation), oxidation (specific to aldehydes), and reduction. Examples include the addition of HCN to ethanal, condensation of acetone with ethanol, oxidation of benzaldehyde to benzoic acid, and reduction of propanone to propan-2-ol.

Step by step solution

01

Identifying Common Reactions

Determine the common types of reactions that both aldehydes and ketones undergo. These include nucleophilic addition reactions, due to the presence of a carbonyl group, making the carbon atom an electrophile.
02

Listing Reactions and Examples

List the reactions and provide an example for each:1. Nucleophilic Addition: In aldehydes, a common reaction is the addition of hydrogen cyanide (HCN) to form cyanohydrins. For instance, the reaction of ethanal (an aldehyde) with HCN.2. Nucleophilic Addition-Elimination (Condensation): Both aldehydes and ketones can undergo condensation reactions with compounds having active hydrogen atoms like alcohols (forming acetals or ketals) and amines (forming imines or Schiff bases). An example is the reaction between acetone (a ketone) and ethanol to form an acetal.3. Oxidation: Aldehydes can be oxidized to carboxylic acids, while ketones are generally resistant to oxidation. A classic example is the oxidation of benzaldehyde to benzoic acid using an oxidizing agent like potassium permanganate (KMnO4).4. Reduction: Aldehydes and ketones can be reduced to alcohols using reducing agents like sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4). An example is the reduction of propanone (a ketone) to propan-2-ol.
03

Summarizing the Reaction Types

Conclude by summarizing the reaction types, ensuring that the examples given clearly illustrate the reactions typical of aldehydes and ketones.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Nucleophilic Addition
Understanding the key reactions of aldehydes and ketones begins with a fundamental process known as nucleophilic addition. This reaction is paramount because the carbonyl group in these compounds features a partially positive carbon atom, an electrophile, that is susceptible to attack by nucleophiles. Nucleophiles are species that donate an electron pair to form a chemical bond.

The classic example of a nucleophilic addition is the reaction of aldehydes, like ethanal, with hydrogen cyanide (HCN) to create cyanohydrins. This process involves the nucleophile (HCN) attacking the electrophilic carbonyl carbon, followed by the addition of the cyanide ion, resulting in the formation of a new carbon-carbon bond and hydroxyl group. The overall result transforms a simple aldehyde into a more complex molecule containing a nitrile and an alcohol functional group.
Condensation Reactions
Another significant type of reaction shared by aldehydes and ketones is the condensation reaction. This occurs when two molecules come together with the loss of a small molecule, typically water. In the case of aldehydes and ketones, condensation reactions frequently involve compounds with active hydrogen atoms, such as alcohols and amines.

For instance, when acetone (a ketone) reacts with ethanol (an alcohol), a condensation reaction occurs, leading to the formation of an acetal. This reaction is facilitated by an acid catalyst and results in the joining of the carbonyl compound with an alcohol while a water molecule is released. Condensation reactions play a crucial role in synthesizing various organic compounds and are fundamental to understanding the chemistry of carbonyl-containing compounds.
Oxidation of Aldehydes
Oxidation reactions entail the loss of electrons from a substance. Aldehydes, due to their unique structure, are particularly prone to oxidation. Upon oxidation, an aldehyde can be transformed into a carboxylic acid, which contains an additional oxygen atom bonded to the former aldehyde carbon.

A well-known example of this reaction is the oxidation of benzaldehyde to benzoic acid. This reaction typically involves an oxidizing agent, such as potassium permanganate (KMnO4), that facilitates the loss of electrons from the aldehyde. The chemical change is significant because it extends the range of products that can be synthesized from aldehydes, offering practical implications in various industries including pharmaceuticals and materials science.
Reduction of Carbonyl Compounds
Reduction, the chemical opposite of oxidation, is a reaction where a compound gains electrons. In the context of carbonyl compounds, such as aldehydes and ketones, reduction refers to the addition of hydrogen, converting them into alcohols. Utilizing reducing agents like sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4), a carbonyl compound can be effectively transformed.

Take the reduction of propanone (a ketone) to propan-2-ol as a case in point. The reducing agent donates hydride ions to the carbonyl carbon, effectively 'reducing' it to a secondary alcohol. This reaction highlights the versatility of carbonyl compounds and the ability to interconvert between functional group families, thereby playing an indispensable role in synthetic chemistry workflows.

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