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. These topics are central to understanding carbonyl chemistry and its applications in organic synthesis.

Structure and Reactivity of Carbonyl Compounds

Carbonyl Group Characteristics

• 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 (as in aldehydes).

• Internal carbonyl: The carbonyl group is within the carbon chain (as in ketones).

• Oxygen is highly electronegative, making the carbonyl carbon partially positive ($ ext{O}^{ ext{δ}-}= ext{C}^{ ext{δ}+}$), and thus susceptible to nucleophilic attack.

• The carbonyl carbon is sp2 hybridized, resulting in a planar structure and bond angles of approximately 120°.

Reactivity Trends

• Less substituted carbonyls (e.g., formaldehyde) are more reactive due to less steric hindrance.

• More substituted carbonyls (e.g., ketones) are less reactive due to increased steric hindrance.

• Reactivity order: Formaldehyde > Aldehyde > Ketone

Example: Formaldehyde is highly reactive and easily attacked by nucleophiles, while acetone (a ketone) is less reactive.

Nucleophilic Addition to Carbonyls

General Mechanism

• Nucleophile attacks the electrophilic carbonyl carbon.

• Results in the formation of a tetrahedral intermediate.

• Protonation of the oxygen atom yields the final addition product.

Equation:

$ ext{R}_2 ext{C}=O + ext{Nu}^- ightarrow ext{R}_2 ext{C}(Nu)O^-$

$ ext{R}_2 ext{C}(Nu)O^- + ext{H}^+ ightarrow ext{R}_2 ext{C}(Nu)OH$

Example: Addition of hydride ($ ext{H}^-$) from NaBH4 to acetone forms isopropanol.

Nomenclature of Aldehydes and Ketones

Systematic Naming Rules

• Aldehydes: Suffix -al (e.g., ethanal for acetaldehyde).

• Ketones: Suffix -one (e.g., propanone for acetone).

• Numbering starts from the carbonyl carbon for aldehydes; for ketones, the carbonyl gets the lowest possible number.

• Common names are often used for simple structures (e.g., formaldehyde, acetone).

Examples of Nomenclature

Structure	IUPAC Name	Common Name
HCHO	Methanal	Formaldehyde
CH3CHO	Ethanal	Acetaldehyde
CH3COCH3	Propanone	Acetone
PhCOCH3	1-Phenylethanone	Acetophenone

Additional info: For branched chains, use prefixes like "formyl-" for aldehyde branches and "oxo-" for ketone branches.

Key Concepts in Carbonyl Chemistry

Electrophilicity and Nucleophilic Attack

• The partial positive charge on the carbonyl carbon makes it a target for nucleophiles.

• Electron-withdrawing groups increase reactivity; electron-donating groups decrease reactivity.

Example: Cyclopentanecarbaldehyde is an example of a cyclic aldehyde, which can only be an end group.

Summary Table: Reactivity of Carbonyl Compounds

Compound	Structure	Reactivity
Formaldehyde	HCHO	Most reactive
Aldehyde	RCHO	Moderately reactive
Ketone	RCOR'	Least reactive

Applications and Further Study

• Understanding carbonyl reactivity is essential for organic synthesis, including the formation of alcohols, acids, and other derivatives.

• Organometallic reagents (e.g., Grignard, organolithium) are powerful tools for carbon-carbon bond formation via nucleophilic addition to carbonyls.

Additional info: The notes reference upcoming topics such as organometallic reactions and mechanisms, which are foundational for advanced organic chemistry.