Organometallics and Reactions of Aldehydes & Ketones: Structure, Reactivity, and Nomenclature

Intro to Organometallics & Reactions of Aldehydes and Ketones

Overview

This section introduces the chemistry of organometallic compounds and the fundamental reactions of aldehydes and ketones, focusing on their structure, reactivity, and nomenclature. These topics are essential for understanding nucleophilic addition mechanisms and the role of carbonyl compounds in organic synthesis.

Structure and Reactivity of Carbonyl Compounds

Carbonyl Group Characteristics

• Definition: A carbonyl group consists of a carbon atom double-bonded to an oxygen atom ($C=O$).

• Types:

  • Terminal carbonyl: Found in aldehydes (carbonyl at the end of a chain).

  • Internal carbonyl: Found in ketones (carbonyl within the chain).

• Polarity: The oxygen is highly electronegative, making the carbonyl carbon partially positive ($ ext{O}^{ ext{δ}-}$ and $ ext{C}^{ ext{δ}+}$).

• Geometry: Carbonyl carbons are sp2 hybridized, resulting in a trigonal planar structure with bond angles of approximately 120°.

Reactivity Trends

• Nucleophilic Attack: The partial positive charge on the carbonyl carbon makes it susceptible to attack by nucleophiles.

• Steric Hindrance: The more substituted the carbonyl carbon, the less reactive it is due to steric hindrance. Formaldehyde (least hindered) is most reactive, while ketones (more hindered) are less reactive.

Example: Reactivity Order

• Formaldehyde > Aldehyde > Ketone

Nomenclature of Aldehydes and Ketones

Systematic Naming Rules

• Aldehydes: Suffix -al (e.g., butanal).

• Ketones: Suffix -one (e.g., butan-2-one).

• Carboxylic Acids: Suffix -oic acid (e.g., pentanoic acid).

• Formyl Group: Prefix formyl- for a branch containing a carbonyl.

Common Names and Structures

Compound	Structure	Common Name
Formaldehyde	$H_2C=O$	Methanal
Acetaldehyde	$CH_3CHO$	Ethanal
Acetone	$CH_3COCH_3$	Propanone
Acetophenone	$C_6H_5COCH_3$	Acetophenone

Example: Naming a Branched Aldehyde

• 3-formylbenzoic acid: Benzoic acid with a formyl group at position 3.

• 3-methylbutanal: Butanal with a methyl group at position 3.

Mechanism of Nucleophilic Addition to Carbonyls

General Mechanism

• Step 1: Nucleophile attacks the partially positive carbonyl carbon.

• Step 2: The oxygen atom becomes negatively charged and may be protonated to form an alcohol.

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

Example: Cyclopentanecarbaldehyde

• Shows nucleophilic attack on a cyclic aldehyde.

Special Notes on Aldehydes

Structural Constraints

• Aldehydes can only be at the end of a carbon chain (terminal carbonyl).

Summary Table: Reactivity and Nomenclature

Type	Suffix	Example	Reactivity
Aldehyde	-al	Butanal	High
Ketone	-one	Butan-2-one	Moderate
Formaldehyde	-al	Formaldehyde	Highest

Key Takeaways

• Carbonyl compounds are highly reactive due to their polar nature and planar geometry.

• Nucleophilic addition is a central reaction for aldehydes and ketones.

• Nomenclature follows systematic rules based on the position and type of carbonyl group.

Additional info: These notes are based on lecture slides and handwritten annotations, with academic context added for clarity and completeness.