Aldehydes and Ketones: Structure, Properties, Synthesis, and Reactions (Chapter 18 Study Guide)

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

18.1 Carbonyl Compounds

Introduction to Carbonyl Groups

The carbonyl group is a functional group characterized by a carbon atom double-bonded to an oxygen atom (C=O). It is a key feature in many organic compounds, including aldehydes and ketones.

Aldehydes: The carbonyl carbon is bonded to at least one hydrogen atom.
Ketones: The carbonyl carbon is bonded to two other carbon atoms.

Example: Formaldehyde (an aldehyde), Acetone (a ketone)

18.2 Structure of the Carbonyl Group

Bonding and Geometry

The carbonyl carbon is sp2 hybridized, resulting in a trigonal planar geometry with bond angles of approximately 120°.
The C=O bond is shorter and stronger than a C=C bond due to the higher electronegativity of oxygen.
Resonance: The carbonyl group exhibits resonance, with the major contributor being the neutral form and a minor contributor with a positive charge on carbon and negative on oxygen.
Dipole Moment: The C=O bond is highly polar, with a partial positive charge on carbon and partial negative on oxygen.

Resonance Structure Example:

$ \ce{R2C=O <-> R2C^{+}-O^{-}} $

18.3 Nomenclature of Ketones and Aldehydes

IUPAC and Common Names

Aldehydes: Named by replacing the -e of the parent alkane with -al (e.g., ethanal).
Ketones: Named by replacing the -e of the parent alkane with -one (e.g., propanone).
Common Names: Some ketones and aldehydes have widely used common names (e.g., acetone, formaldehyde, benzaldehyde).

Example Table:

Common Name	Structure
Acetone	CH3COCH3
Acetophenone	C6H5COCH3
Benzophenone	(C6H5)2CO
Formaldehyde	HCHO
Acetaldehyde	CH3CHO
Benzaldehyde	C6H5CHO

18.4 Physical Properties of Ketones and Aldehydes

Boiling Points and Hydrogen Bonding

Aldehydes and ketones have higher boiling points than alkanes and ethers due to the polar C=O bond, but lower than alcohols (which can hydrogen bond with themselves).
They cannot form hydrogen bonds with themselves but can act as hydrogen bond acceptors with water, making small aldehydes and ketones water-soluble.

Example: Acetone is miscible with water due to hydrogen bonding with water molecules.

18.5 Spectroscopy of Ketones and Aldehydes

Infrared (IR) Spectroscopy

C=O Stretch: Strong absorption near 1710 cm-1.
Aldehyde C-H Stretch: Two weak absorptions near 2720 and 2820 cm-1.
Conjugation: Lowers the C=O stretching frequency by about 20-30 cm-1.

NMR Spectroscopy

1H NMR: Aldehyde proton appears at 9-10 ppm; protons on carbons adjacent to C=O appear at 2-2.5 ppm.
13C NMR: Carbonyl carbon appears at 190-220 ppm.

Mass Spectrometry

Common cleavage patterns include alpha cleavage and the McLafferty rearrangement (especially in ketones with gamma hydrogens).

Ultraviolet (UV) Spectroscopy

n → π* and π → π* transitions are observed; conjugation increases absorption wavelength.

18.7 Reviewing Syntheses of Ketones and Aldehydes

Key Synthetic Methods

Grignard Reaction: Addition to esters or nitriles followed by hydrolysis yields ketones or aldehydes.
Controlled Oxidation: Primary alcohols can be oxidized to aldehydes (e.g., PCC as oxidant) without further oxidation to acids.
Ozonolysis: Cleavage of alkenes with ozone yields aldehydes and/or ketones.
Friedel-Crafts Acylation: Synthesis of aryl ketones from aromatic rings and acid chlorides.
Gatterman-Koch Reaction: Synthesis of benzaldehyde derivatives.
Oxymercuration/Hydroboration-Oxidation of Alkynes: Terminal alkynes yield aldehydes; internal alkynes yield ketones.

18.8 Synthesis of Ketones from Carboxylic Acids

Conversion Sequence

Carboxylic acids can be converted to ketones via formation of acid chlorides, followed by reaction with organometallic reagents (e.g., organocuprates).

18.9 Synthesis of Ketones and Aldehydes from Nitriles

Grignard Addition and DIBAL-H Reduction

Grignard Reagents: Addition to nitriles followed by hydrolysis yields ketones.
DIBAL-H: Reduces nitriles to aldehydes under controlled conditions.

Mechanism Example:

$ \ce{R-MgBr + R'CN \xrightarrow{H_2O} R-COR'} $

18.10 Synthesis from Acid Chlorides and Esters

Reduction and Organometallic Reactions

Reduction: Acid chlorides and esters can be reduced to aldehydes using weak reducing agents (e.g., DIBAL-H, LiAlH(OtBu)3).
Grignard Reagents: React with acid chlorides to give tertiary alcohols (unless controlled for ketone formation).
Organocuprates (Gilman Reagents): React with acid chlorides to give ketones.

18.11 Reactions of Ketones and Aldehydes: Nucleophilic Addition

General Mechanism

Nucleophiles attack the electrophilic carbonyl carbon, forming a tetrahedral intermediate.
Mechanism varies under basic (direct attack) or acidic (protonation first) conditions.

General Equation:

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

18.12 Hydration of Ketones and Aldehydes

Formation of Geminal Diols (Hydrates)

Water adds to the carbonyl group to form a hydrate (geminal diol).
Formaldehyde is most reactive; ketones are least reactive in hydration.

Mechanism: Nucleophilic addition of water, followed by proton transfer.

18.13 Formation of Cyanohydrins

Reaction with Cyanide

Cyanide ion adds to the carbonyl group, forming a cyanohydrin.
Cyanohydrins can be hydrolyzed to carboxylic acids.

General Equation:

$ \ce{R_2C=O + HCN \rightarrow R_2C(OH)CN} $

18.14 Formation of Imines

Condensation with Primary Amines

Primary amines react with aldehydes or ketones to form imines (Schiff bases).
Reaction is a condensation (water is eliminated).
Reversible under acidic conditions.

General Equation:

$ \ce{R_2C=O + R'NH_2 \rightarrow R_2C=NR' + H_2O} $

18.15 Condensations with Hydroxylamine and Hydrazine

Formation of Oximes, Hydrazones, and Semicarbazones

Oximes: Formed with hydroxylamine (NH2OH).
Hydrazones: Formed with hydrazine (NH2NH2).
Semicarbazones: Formed with semicarbazide.

Example Table:

Reagent	Product
NH2OH	Oxime
NH2NH2	Hydrazone
NH2NHCONH2	Semicarbazone

18.16 Formation of Acetals

Acetals, Ketals, and Hemiacetals

Aldehydes and ketones react with alcohols (in acid) to form hemiacetals, then acetals (or ketals for ketones).
Acetal formation is reversible and does not occur under basic conditions.
Cyclic acetals can form when diols are used.
Carbohydrates often exist as hemiacetals or acetals.

General Equation:

$ \ce{R_2C=O + 2 R'OH \xrightarrow{H^+} R_2C(OR')_2 + H_2O} $

18.17 Use of Acetals as Protecting Groups

Protecting the Carbonyl Group

Acetals are stable to base and nucleophiles, making them useful as protecting groups for carbonyls during multi-step synthesis.
They can be removed (deprotected) by acid hydrolysis.

18.18 The Wittig Reaction

Alkene Synthesis via Phosphorus Ylides

The Wittig reaction forms alkenes from aldehydes or ketones and phosphorus ylides.
Ylide: A compound with adjacent positive and negative charges (e.g., Ph3P=CH2).
Mechanism involves nucleophilic attack and formation of a four-membered ring intermediate.

General Equation:

$ \ce{R_2C=O + Ph_3P=CHR' \rightarrow R_2C=CHR' + Ph_3P=O} $

18.19 Oxidation of Aldehydes

Conversion to Carboxylic Acids

Aldehydes are easily oxidized to carboxylic acids using oxidizing agents (e.g., Ag2O, Tollens' reagent).
Tollens' Test: Used to distinguish aldehydes (positive test) from ketones (negative test).
Ketones are generally resistant to oxidation under normal conditions.

18.20 Reduction of Ketones and Aldehydes

Hydride and Catalytic Reductions

Hydride Reagents: NaBH4 (mild), LiAlH4 (strong) reduce aldehydes to primary alcohols and ketones to secondary alcohols.
Raney Nickel: Catalytic hydrogenation reduces C=O to CH2.
Deoxygenation: Complete removal of oxygen from the carbonyl group (e.g., Clemmensen and Wolff-Kishner reductions).
Clemmensen Reduction: Zn(Hg)/HCl conditions; reduces carbonyl to methylene.
Wolff-Kishner Reduction: NH2NH2/KOH; involves hydrazone intermediate and loss of N2.

General Equation (Wolff-Kishner):

$ \ce{R_2C=O \xrightarrow{NH_2NH_2, KOH} R_2CH_2 + N_2} $

Additional info: This guide expands on the learning objectives by providing definitions, mechanisms, and examples for each key reaction and property of aldehydes and ketones. For detailed mechanisms and further examples, refer to the main textbook (Wade, 9th edition, Chapter 18).