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

Intro to Organometallics & Reactions of Aldehydes and Ketones

Overview

This section introduces the fundamental concepts of organometallic chemistry and the reactivity of aldehydes and ketones, focusing on their structure, nomenclature, and the principles that govern their chemical behavior. Understanding these topics is essential for mastering nucleophilic addition reactions and the use of organometallic reagents in organic synthesis.

Structure and Properties of Carbonyl Compounds

Carbonyl Functional Group

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

• Types:

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

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

• Polarity: The oxygen atom is more electronegative than carbon, making the carbonyl carbon partially positive ($\delta^+$) and the oxygen partially negative ($\delta^-$).

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

Reactivity of Carbonyl Compounds

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

• Steric Effects: The more substituted the carbonyl carbon (i.e., the more alkyl groups attached), the less reactive it is due to steric hindrance.

• Reactivity Order: Formaldehyde (least hindered) > Aldehydes > Ketones (most hindered).

Example: Formaldehyde ($H_2C=O$) is more reactive than acetone ($CH_3COCH_3$) because it is less sterically hindered.

Nomenclature of Aldehydes and Ketones

Systematic Naming

• 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 with -one (e.g., propanone for acetone).

• Numbering: The carbonyl carbon receives the lowest possible number in the parent chain.

• Common Names: Some simple aldehydes and ketones have widely used common names (e.g., formaldehyde for methanal, acetone for propanone).

Examples of Nomenclature

• 3-formylbenzoic acid: A benzene ring with a carboxylic acid and a formyl group at the 3-position.

• 3-methylbutanal: A four-carbon aldehyde with a methyl group at the 3-position.

• 2-oxocyclohexanecarboxylic acid: A cyclohexane ring with a ketone at the 2-position and a carboxylic acid group.

• Butan-2-one: A four-carbon ketone with the carbonyl at the 2-position (also known as methyl ethyl ketone).

Structure	Common Name	IUPAC Name
H2C=O	Formaldehyde	Methanal
CH3CHO	Acetaldehyde	Ethanal
CH3COCH3	Acetone	Propanone
PhCOCH3	Acetophenone	1-Phenylethanone
PhCHO	Benzaldehyde	Benzenecarbaldehyde

Reactivity: Nucleophilic Addition to Carbonyls

General Mechanism

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

• The oxygen atom, now bearing a negative charge, is often protonated to yield an alcohol or related product.

General Equation:

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

Example: Addition of a hydride ion ($H^-$) from sodium borohydride ($NaBH_4$) to acetone produces isopropanol.

Factors Affecting Reactivity

• Electronic effects: Electron-withdrawing groups increase the electrophilicity of the carbonyl carbon.

• Steric effects: Bulky groups near the carbonyl decrease reactivity by hindering nucleophilic approach.

Summary Table: Reactivity of Carbonyl Compounds

Compound	Structure	Relative Reactivity	Reason
Formaldehyde	H2C=O	Most reactive	Least steric hindrance
Aldehyde	RCHO	Intermediate	One alkyl group
Ketone	RCOR'	Least reactive	Two alkyl groups (more steric hindrance)

Key Terms

• Carbonyl group: Functional group with a carbon-oxygen double bond.

• Aldehyde: Compound with a terminal carbonyl group ($RCHO$).

• Ketone: Compound with an internal carbonyl group ($RCOR'$).

• Nucleophile: Species that donates an electron pair to form a chemical bond.

• Electrophile: Species that accepts an electron pair.

• Steric hindrance: Decreased reactivity due to bulky groups blocking access to a reactive site.

Applications and Examples

• Biological relevance: Carbonyl compounds are found in sugars, hormones, and metabolic intermediates.

• Industrial importance: Aldehydes and ketones are used in the synthesis of plastics, pharmaceuticals, and fragrances.

Additional info: The notes also reference organometallic reagents and their reactions with carbonyls, which are covered in more detail in subsequent sections. Understanding the basic structure and reactivity of carbonyls is foundational for these advanced topics.