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Organic Chemistry
Table of Contents
Unit 19 Overview: Aldehydes & Ketones: Nucleophilic Addition
19.1 Naming Aldehydes and Ketones
19.2 Preparing Aldehydes and Ketones
19.3 Oxidation of Aldehydes and Ketones
19.4 Nucleophilic Addition Reactions of Aldehydes and Ketones
19.5 Nucleophilic Addition of H2O: Hydration
19.6 Nucleophilic Addition of HCN: Cyanohydrin Formation
19.7 Nucleophilic Addition of Hydride and Grignard Reagents: Alcohol Formation
19.8 Nucleophilic Addition of Amines: Imine and Enamine Formation
19.9 Nucleophilic Addition of Hydrazine: The Wolff–Kishner Reaction
19.10 Nucleophilic Addition of Alcohols: Acetal Formation
19.11 Nucleophilic Addition of Phosphorus Ylides: The Wittig Reaction
19.12 Biological Reductions
19.13 Conjugate Nucleophilic Addition to α,β‑Unsaturated Aldehydes and Ketones
19.14 Spectroscopy of Aldehydes and Ketones

Aldehydes and ketones are crucial organic compounds with diverse synthesis methods. From oxidizing alcohols to partially reducing esters, chemists have developed various techniques to create these carbonyl-containing molecules. Understanding these processes is key to mastering organic synthesis.

Reaction mechanisms play a vital role in aldehyde and ketone synthesis. Whether it's forming chromate ester intermediates or cleaving ozonides, these step-by-step processes explain how reactants transform into products. Grasping these mechanisms helps predict and control organic reactions.

Synthesis of Aldehydes and Ketones

Synthesis methods for aldehydes

  • Oxidation of primary alcohols
    • Oxidizing agents like pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC) selectively oxidize primary alcohols to aldehydes without further oxidation to carboxylic acids
    • Mechanism involves formation of chromate ester intermediate followed by elimination of oxidizing agent
  • Partial reduction of esters
    • Reducing agents like diisobutylaluminum hydride (DIBAL-H) partially reduce esters to aldehydes at low temperatures (-78°C)
    • Bulkiness of DIBAL-H prevents further reduction of aldehyde to primary alcohol
    • Mechanism involves formation of tetrahedral intermediate followed by elimination of alkoxyaluminum species

Preparation approaches for ketones

  • Oxidation of secondary alcohols
    • Oxidizing agents (such as chromic acid ($H_2CrO_4$) prepared from sodium dichromate ($Na_2Cr_2O_7$) and sulfuric acid ($H_2SO_4$), PCC, and PDC) oxidize secondary alcohols to ketones
    • Mechanism involves formation of chromate ester intermediate followed by elimination of oxidizing agent
  • Ozonolysis of alkenes
    • Ozone ($O_3$) cleaves alkenes to form carbonyl compounds including ketones
      1. Ozonolysis performed by bubbling ozone through alkene solution in suitable solvent (dichloromethane) at low temperatures
      2. Resulting ozonide intermediate reduced using reducing agent like dimethyl sulfide (DMS) or zinc dust
    • Mechanism involves cycloaddition of ozone to alkene forming unstable molozonide which rearranges to ozonide
    • Ozonide cleaved by reducing agent yielding carbonyl compounds
      • Symmetrical alkenes yield single ketone product
      • Unsymmetrical alkenes yield mixture of ketone and aldehyde products

Aldehydes vs ketones in synthesis

  • Similarities
    • Both prepared by oxidation of alcohols
      • Primary alcohols yield aldehydes
      • Secondary alcohols yield ketones
    • Oxidation mechanisms for both involve formation of chromate ester intermediate
  • Differences
    • Aldehydes prepared by partial reduction of esters using DIBAL-H, ketones cannot be prepared using this method
    • Ketones prepared by ozonolysis of alkenes, aldehydes only prepared by ozonolysis of unsymmetrical alkenes as one of products
    • Oxidation of alcohols to aldehydes requires milder oxidizing agents (PCC, PDC) to prevent further oxidation to carboxylic acids, oxidation of alcohols to ketones can be performed using stronger oxidizing agents like chromic acid

Functional Group Interconversion and Reaction Mechanisms

  • Oxidation and reduction reactions play crucial roles in the interconversion of functional groups
    • Oxidation of alcohols to aldehydes and ketones involves the loss of hydrogen atoms
    • Reduction of aldehydes and ketones to alcohols involves the gain of hydrogen atoms
  • Reaction mechanisms for these transformations typically involve:
    • Formation of reactive intermediates
    • Electron transfer processes
    • Rearrangement of bonds within molecules

Key Terms to Review (36)

Alkyl group: An alkyl group is a type of hydrocarbon chain that branches off from the main molecular structure in organic compounds, typically derived by removing one hydrogen atom from an alkane, giving it the general formula CnH2n+1. These groups are not stable on their own but can form strong covalent bonds with other atoms or groups, influencing the compound's physical and chemical properties.
β Diketone: A β-diketone is an organic compound containing two ketone groups separated by a carbon atom, which is the beta (β) position relative to each ketone group. These molecules are characterized by the presence of hydrogen atoms on the carbon between the two carbonyl (C=O) groups, making them acidic and prone to enolate ion formation.
Ketone: A ketone is a functional group in organic chemistry that consists of a carbonyl group (a carbon-oxygen double bond) bonded to two alkyl or aryl groups. Ketones are widely encountered in various organic chemistry topics, including the hydration of alkynes, oxidative cleavage of alkynes, organic synthesis, oxidation and reduction reactions, and the chemistry of aldehydes and ketones.
Aldehyde: An aldehyde is a class of organic compounds containing a carbonyl group (C=O) where the carbon atom is bonded to one hydrogen atom and one alkyl or aryl group. Aldehydes are important functional groups in organic chemistry and are involved in various reactions and synthesis pathways.
Reaction Mechanisms: Reaction mechanisms describe the step-by-step process by which a chemical reaction occurs, including the rearrangement of atoms, the formation and breaking of chemical bonds, and the movement of electrons. Understanding reaction mechanisms is crucial for predicting the products of a reaction, explaining experimental observations, and designing new synthetic routes in organic chemistry.
Reduction: Reduction is a chemical process that involves the gain of electrons by a molecule or atom, resulting in a decrease in its oxidation state. This term is particularly important in the context of various organic chemistry reactions and transformations.
Oxidation: Oxidation is a fundamental chemical process in which a substance loses electrons, resulting in an increase in its oxidation state. This term is central to understanding various reactions and transformations in organic chemistry, from the hydration of alkenes to the oxidation of alcohols and aldehydes.
Molozonide: A molozonide is an unstable intermediate formed during the ozonolysis reaction, which is a method of cleaving alkenes and alkynes to produce carbonyl compounds. The molozonide is a key step in the overall mechanism of these oxidative cleavage reactions.
Ozonide: An ozonide is a chemical compound formed by the reaction of ozone (O3) with an alkene or alkyne. Ozonides are important intermediates in the oxidative cleavage of carbon-carbon double and triple bonds, leading to the formation of carbonyl compounds.
Ozone: Ozone is a highly reactive allotrope of oxygen that is formed by the interaction of oxygen molecules with ultraviolet radiation. It plays a crucial role in the context of organic chemistry, particularly in the oxidation of alkenes, the cleavage of alkynes, and the preparation of aldehydes and ketones.
Dimethyl Sulfide: Dimethyl sulfide is a colorless, flammable, and volatile organic compound with a strong, unpleasant odor. It is an important chemical species that is commonly encountered in various organic chemistry contexts, including the oxidation of alkenes, the oxidative cleavage of alkynes, and the preparation of aldehydes and ketones.
Ozonolysis: Ozonolysis is a chemical reaction in which an alkene is cleaved by ozone (O3) to form two carbonyl compounds, such as aldehydes and/or ketones. This reaction is a powerful tool for the analysis and synthesis of organic compounds, particularly in the context of understanding the oxidation of alkenes and the preparation of aldehydes and ketones.
PCC: PCC, or Pyridinium Chlorochromate, is a versatile oxidizing agent used in organic chemistry for the selective oxidation of alcohols to aldehydes and ketones. This powerful reagent is widely employed in various reactions across multiple topics, including the hydration of alkynes, the oxidation of alcohols, and the preparation and oxidation of aldehydes and ketones.
Oxidizing Agents: Oxidizing agents, also known as oxidants, are substances that have the ability to oxidize other substances by removing electrons from them. They play a crucial role in various organic chemistry reactions, including the oxidative cleavage of alkynes, the oxidation of aromatic compounds, the oxidation of alcohols, the preparation of aldehydes and ketones, and the preparation of carboxylic acids.
Swern Oxidation: The Swern oxidation is a chemical reaction used to convert primary and secondary alcohols into aldehydes and ketones, respectively. It is a mild and selective method for the oxidation of alcohols without over-oxidation to carboxylic acids.
Chromic Acid: Chromic acid, also known as chromium(VI) oxide, is a strong oxidizing agent commonly used in organic chemistry. It is a bright orange-red crystalline compound with the chemical formula CrO3, which can be used to oxidize various organic compounds.
Alkyl Group: An alkyl group is a hydrocarbon substituent derived from an alkane by the removal of a single hydrogen atom. Alkyl groups are commonly found in various organic chemistry topics, including substituent effects in electrophilic substitutions, the properties of alcohols and phenols, and the preparation of aldehydes and ketones.
Primary Alcohol: A primary alcohol is an organic compound containing a hydroxyl (-OH) functional group attached to a saturated carbon atom that is bonded to only one other carbon atom. Primary alcohols are an important class of organic compounds with diverse applications and are a key focus in the study of organic chemistry.
Secondary Alcohol: A secondary alcohol is an organic compound in which a hydroxyl group (-OH) is attached to a carbon atom that is bonded to two other carbon atoms. This structural feature distinguishes secondary alcohols from primary alcohols, where the hydroxyl group is attached to a carbon atom bonded to only one other carbon, and tertiary alcohols, where the hydroxyl group is attached to a carbon atom bonded to three other carbons.
Sodium Dichromate: Sodium dichromate, also known as sodium bichromate, is an inorganic compound with the chemical formula Na2Cr2O7. It is a powerful oxidizing agent that has various applications in organic chemistry, particularly in the oxidation of alcohols, the preparation of aldehydes and ketones, and the preparation of carboxylic acids.
Pyridinium Chlorochromate: Pyridinium chlorochromate (PCC) is an oxidizing agent commonly used in organic chemistry for the selective oxidation of alcohols to aldehydes and ketones. It is a versatile reagent that plays a key role in the preparation and oxidation of carbonyl compounds.
Alkanone: An alkanone, also known as a ketone, is a class of organic compounds containing a carbonyl group (C=O) bonded to two alkyl or aryl groups. Alkanones are important functional groups in organic chemistry, particularly in the context of preparing aldehydes and ketones.
Chromium Trioxide: Chromium trioxide, also known as chromic acid, is an inorganic compound with the chemical formula CrO3. It is a strong oxidizing agent and is commonly used in various industrial and laboratory applications, particularly in the context of preparing aldehydes and ketones.
Distillation: Distillation is a separation technique used to purify liquids by exploiting differences in their boiling points. It involves vaporizing a liquid mixture and then condensing the vapor to obtain the desired pure components.
Chromate Ester: A chromate ester is a type of organic compound that contains a chromium-oxygen bond, typically formed by the reaction of an alcohol with chromic acid or a chromium-containing oxidizing agent. These esters are important intermediates in the preparation of aldehydes and ketones, as they can be selectively oxidized to form the desired carbonyl compounds.
Alkanal: An alkanal, also known as an aldehyde, is a class of organic compounds containing a carbonyl group (C=O) with a hydrogen atom attached to the carbon. Alkanals are important intermediates in organic synthesis and have various applications in the chemical industry.
Carbon-Oxygen Double Bond: The carbon-oxygen double bond is a covalent bond where a carbon atom shares two pairs of electrons with an oxygen atom, forming a rigid, planar structure. This bond is a key feature in the structure and reactivity of carbonyl compounds, which are fundamental to organic chemistry.
Functional Group Interconversion: Functional group interconversion refers to the process of converting one type of functional group into another through a series of chemical reactions. This is a crucial concept in organic chemistry, particularly in the context of preparing aldehydes and ketones.
Swern: The Swern oxidation, also known as the Swern-Moffatt oxidation, is a chemical reaction used to convert alcohols into aldehydes or ketones. It is a powerful and versatile method for the oxidation of alcohols, particularly those that are sterically hindered or sensitive to basic conditions.
Diisobutylaluminum Hydride: Diisobutylaluminum hydride (DIBAL-H) is a reducing agent commonly used in organic chemistry for the selective reduction of esters, nitriles, and halides to aldehydes or alcohols. It is a powerful, yet mild reducing agent that can be used to transform various functional groups without affecting others.
Reflux: Reflux refers to the backward flow of a liquid, such as a chemical solution, back into the vessel from which it originated. In the context of organic chemistry, reflux is a common technique used in the preparation of aldehydes and ketones.
Jones: The Jones reagent, also known as the Jones oxidation, is a chemical reagent used in organic chemistry for the selective oxidation of primary alcohols to aldehydes and secondary alcohols to ketones. It is a crucial tool in the preparation of aldehydes and ketones, which are important functional groups in many organic compounds.
DIBAL-H: DIBAL-H, or diisobutylaluminum hydride, is a powerful reducing agent used in organic chemistry to selectively reduce certain functional groups, such as esters and nitriles, to aldehydes and alcohols, respectively. It is a valuable tool in the preparation of aldehydes and ketones, as well as in the chemistry of esters.
Acyl Chloride: An acyl chloride is a highly reactive organic compound derived from a carboxylic acid, where the hydroxyl (-OH) group has been replaced by a chlorine atom (-Cl). These compounds are widely used in organic synthesis as versatile intermediates for the preparation of various functional groups.
Pyridinium Dichromate: Pyridinium dichromate is an oxidizing agent used in organic chemistry, particularly in the preparation of aldehydes and ketones from alcohols. It is a versatile reagent that can selectively oxidize primary alcohols to aldehydes and secondary alcohols to ketones.
Carbonyl group: A carbonyl group is a functional group characterized by a carbon atom double-bonded to an oxygen atom, represented as C=O. This group is pivotal in organic chemistry as it forms the backbone of various important classes of compounds, influencing their chemical properties and reactivity.