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 reactions 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 carbon partially positive (δ+) and the oxygen partially negative (δ-).

• Geometry: Carbonyl carbons are sp2 hybridized, resulting in a trigonal planar structure (~120° bond angles).

Reactivity Trends

• Susceptibility to Nucleophilic Attack: The partial positive charge on the carbon makes it reactive toward nucleophiles.

• Steric Effects:

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

  • Formaldehyde (least hindered) is most reactive; acetone (more hindered) is less reactive.

Example: Nucleophilic Addition Mechanism

• A nucleophile attacks the carbonyl carbon, forming a tetrahedral intermediate.

• Protonation of the oxygen atom may occur to stabilize the intermediate.

Nomenclature of Aldehydes and Ketones

IUPAC and Common Naming

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

• Ketones: Named by replacing “-e” with “-one” (e.g., propanone for acetone).

• Common Names: Frequently used for simple compounds (e.g., formaldehyde, acetone).

Functional Group Location and Naming

• Formyl group (-CHO): Used for aldehyde branches.

• Oxo group (=O): Used for ketone branches.

• Carboxylic acids: Named with “-oic acid” suffix.

Example Table: Common Aldehydes and Ketones

Compound	Structure	Common Name	IUPAC Name
HCHO	O=CH2	Formaldehyde	Methanal
CH3CHO	O=CHCH3	Acetaldehyde	Ethanal
CH3COCH3	O=CH(CH3)2	Acetone	Propanone
C6H5COCH3	O=CHC6H5CH3	Acetophenone	1-Phenylethanone

Key Concepts in Carbonyl Chemistry

Nucleophilic Addition to Carbonyls

• Mechanism:

  1. Nucleophile attacks the electrophilic carbonyl carbon.

  2. Tetrahedral intermediate forms.

  3. Protonation of the oxygen atom yields the final product.

• Equation: $ ext{R}_2C=O + Nu^- ightarrow ext{R}_2C(OH)Nu $

Reactivity Comparison

• Aldehydes are generally more reactive than ketones due to less steric hindrance and greater partial positive charge on the carbonyl carbon.

• Formaldehyde is the most reactive carbonyl compound.

Applications

• Synthesis: Carbonyl compounds are key intermediates in organic synthesis, including the formation of alcohols, acids, and other functional groups.

• Biological relevance: Many metabolic pathways involve carbonyl chemistry (e.g., glucose metabolism).

Additional info:

• Some notes reference organometallic reactions and protecting groups, which are covered in later sections of the course.

• Mechanistic details and advanced reactivity (e.g., conjugated systems, enol formation) are important for deeper study.