Intro to Organometallics & Reactions of Aldehydes and Ketones

Intro 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, reactivity, nomenclature, and mechanisms. These topics are essential for understanding organic synthesis and the behavior of carbonyl compounds in chemical reactions.

Carbonyl Compounds: Structure and Reactivity

Basic Structure of Carbonyls

• Carbonyl Group: Consists of a carbon atom double-bonded to an oxygen atom (C=O).

• Terminal Carbonyl: The carbonyl group is at the end of a carbon chain (e.g., aldehydes).

• Internal Carbonyl: The carbonyl group is within the carbon chain (e.g., ketones).

Electronegativity: Oxygen is highly electronegative, making the carbonyl carbon partially positive (δ+) and the oxygen partially negative (δ-).

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

• Reactivity: The partial positive charge on the carbon makes it susceptible to nucleophilic attack.

• Steric Hindrance: The more substituted the carbonyl carbon, the less reactive it is due to steric hindrance.

Example: Formaldehyde (least hindered, most reactive) vs. acetone (more hindered, less reactive).

Nomenclature of Aldehydes and Ketones

Systematic and Common Names

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

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

• Branched Chains: Use locants and prefixes (e.g., 3-formylbenzoic acid, 3-methylbutanal).

Compound	Common Name	Systematic Name
H2C=O	Formaldehyde	Methanal
CH3CHO	Acetaldehyde	Ethanal
CH3COCH3	Acetone	Propanone
PhCOCH3	Acetophenone	1-Phenylethanone
3-formylbenzoic acid	—	3-formylbenzoic acid
3-methylbutanal	—	3-methylbutanal
2-oxocyclohexanecarboxylic acid	—	2-oxocyclohexanecarboxylic acid
butan-2-one	—	butan-2-one

Example: The carbonyl group in cyclopentanecarbaldehyde is always at the end (aldehyde).

Reactivity of Carbonyl Compounds

Nucleophilic Addition Mechanism

• Nucleophile Attack: Nucleophiles attack the partially positive carbonyl carbon.

• Intermediate Formation: The addition of a nucleophile forms a tetrahedral intermediate.

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

General Mechanism:

• Nucleophile (Nu-) attacks carbonyl carbon.

• Oxygen becomes negatively charged and is protonated by acid (H+).

Equation:

$\ce{R_2C=O + Nu^- -> R_2C(OH)Nu}$

Organometallic Compounds

Definition and Types

• Organometallic Compounds: Molecules containing a carbon-metal bond, which makes the carbon nucleophilic.

• Common Types:

  • Organolithium reagents (R-Li)

  • Grignard reagents (R-MgX)

  • Organocuprates (Gilman reagents, R2CuLi)

Preparation: Typically prepared by reacting alkyl halides with metals (e.g., Li, Mg, Cu).

Example:

• $\ce{CH_3Br + 2Li -> CH_3Li + LiBr}$ (methyl lithium)

• $\ce{RBr + Mg -> RMgBr}$ (Grignard reagent)

• $\ce{2RLi + CuI -> R_2CuLi + LiI}$ (Gilman reagent)

Reactions of Organometallics with Carbonyls

General Mechanism

• Organometallic reagents act as strong nucleophiles, attacking the carbonyl carbon.

• After nucleophilic addition, the oxygen is protonated to yield an alcohol.

Equation:

$\ce{R_2C=O + RMgX -> R_2C(OMgX)R ->[H_2O] R_2C(OH)R}$

Applications: Synthesis of alcohols from aldehydes and ketones.

Reactivity Trends and Steric Effects

Factors Affecting Reactivity

• Electronic Effects: More electron-withdrawing groups increase reactivity.

• Steric Effects: Bulky substituents decrease reactivity due to steric hindrance.

• Order of Reactivity: Formaldehyde > Aldehydes > Ketones

Compound	Reactivity
Formaldehyde	Most reactive
Aldehyde	Intermediate
Ketone	Least reactive

Summary of Key Concepts

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

• Nucleophilic addition is the primary reaction mechanism for aldehydes and ketones.

• Organometallic reagents are powerful nucleophiles used to form new carbon-carbon bonds.

• Nomenclature follows systematic rules based on the parent chain and functional group location.

Example Application: Grignard addition to acetone yields a tertiary alcohol after protonation.

Additional info: These notes expand on brief points and diagrams from the original slides and handwritten notes, providing full academic context and self-contained explanations suitable for exam preparation.