Introduction to Organometallics & Reactions of Aldehydes and Ketones

Overview

This study guide covers the fundamental concepts of organometallic chemistry and the reactions of aldehydes and ketones, focusing on their structure, nomenclature, reactivity, and key synthetic transformations. These topics are central to organic chemistry and are essential for understanding carbonyl chemistry and advanced synthesis.

Carbonyl Group Structure and Reactivity

Basic Information

• Carbonyl Group: A functional group consisting of a carbon atom double bonded to an oxygen atom (C=O).

• Aldehyde: Contains a terminal carbonyl group (at the end of a carbon chain).

• Ketone: Contains an internal carbonyl group (within the carbon chain).

• Electronegativity: The oxygen atom is highly electronegative, making the carbonyl carbon electron-deficient and electrophilic.

• Geometry: The carbonyl carbon is sp2 hybridized, resulting in a trigonal planar structure (~120° bond angles).

• Reactivity: Carbonyl compounds are highly reactive towards nucleophiles due to the partial positive charge on the carbonyl carbon.

• Steric Effects: Increased substitution around the carbonyl carbon decreases reactivity due to steric hindrance.

Example: Aldehydes are generally more reactive than ketones because they have fewer substituents around the carbonyl carbon.

Nomenclature of Aldehydes and Ketones

IUPAC and Common Names

• Aldehydes: Named by replacing the terminal -e of the parent alkane with -al (e.g., ethanal for acetaldehyde).

• Ketones: Named by replacing the terminal -e of the parent alkane with -one (e.g., propanone for acetone).

• Common Names: Frequently used for simple aldehydes and ketones (e.g., formaldehyde for methanal, acetone for propanone).

Carbonyl Producing Reactions

Preparation of Aldehydes

• Oxidation of Primary Alcohols: Mild oxidizing agents such as PCC convert primary alcohols to aldehydes.

• Reduction of Esters and Acid Chlorides: DIBAL-H and LiAlH(OtBu)3 are selective reducing agents for converting esters and acid chlorides to aldehydes.

• Hydration of Alkynes: Hydroboration-oxidation of terminal alkynes yields aldehydes.

Preparation of Ketones

• Oxidation of Secondary Alcohols: Strong oxidizing agents (CrO3, Na2Cr2O7, PCC) convert secondary alcohols to ketones.

• Friedel-Crafts Acylation: Aromatic rings react with acid chlorides in the presence of AlCl3 to form aryl ketones.

• Hydration of Alkynes: Acid-catalyzed hydration of internal alkynes yields ketones.

• Oxidative Cleavage of Alkenes: Ozonolysis or other oxidative methods can produce both ketones and aldehydes.

Reactivity and Mechanisms of Aldehydes and Ketones

Nucleophilic Addition

• General Mechanism: Nucleophile attacks the electrophilic carbonyl carbon, forming a tetrahedral intermediate.

• Protonation: The intermediate is typically protonated to yield the final product.

• Reactivity Order: Aldehydes > Ketones (due to less steric hindrance and greater electrophilicity).

Example: Addition of Grignard reagents to aldehydes and ketones to form alcohols.

Oxidation and Reduction of Carbonyl Compounds

Oxidation

• Primary Alcohols: Oxidized to aldehydes (PCC) or further to carboxylic acids (stronger oxidants).

• Secondary Alcohols: Oxidized to ketones.

• Chromium Reagents: Common oxidants include CrO3, Na2Cr2O7, PCC.

Reduction

• Reducing Agents: LiAlH4 (lithium aluminum hydride) reduces most carbonyls to alcohols; NaBH4 (sodium borohydride) is more selective for aldehydes and ketones.

• Mechanism: Hydride transfer to the carbonyl carbon, followed by protonation.

Equation:

Organometallic Reagents

Types and Preparation

• Organolithium Reagents: Prepared by reaction of alkyl halides with lithium metal.

• Grignard Reagents: Prepared by reaction of alkyl halides with magnesium in dry ether.

• Organocuprates (Gilman Reagents): Prepared by reaction of organolithium compounds with copper(I) iodide.

Reactions with Carbonyl Compounds

• Grignard and Organolithium: Add to aldehydes and ketones to form alcohols.

• Organocuprates: React with acid chlorides to form ketones; less reactive towards aldehydes and ketones.

• Reaction with CO2: Grignard reagents react with carbon dioxide to form carboxylic acids.

Equation:

Protecting Groups

Purpose and Application

• Protecting Groups: Used to temporarily mask reactive functional groups during multi-step synthesis.

• Common Protecting Groups for Carbonyls: Acetals and cyclic acetals (formed from diols).

• Deprotection: Protecting groups can be removed under acidic conditions after the desired reaction is complete.

Example: Formation of a cyclic acetal from a carbonyl and ethylene glycol.

Wittig Reaction

Mechanism and Application

• Wittig Reaction: Converts carbonyl compounds (aldehydes or ketones) to alkenes using a phosphonium ylide.

• Ylide: An organophosphorus reagent with adjacent positive and negative charges.

• Mechanism: Nucleophilic addition of the ylide to the carbonyl, followed by elimination to form the alkene.

Equation:

Summary Table: Key Reactions of Aldehydes and Ketones

Additional info: Some mechanistic details and reagent properties have been expanded for clarity and completeness.