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

Carbonyl Compounds: Structure and Bonding

Overview of 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.

Bonding: 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 a major contributor being the double bond and a minor contributor with a positive charge on carbon and a negative charge on oxygen.

• The carbonyl bond is highly polar, with a significant dipole moment due to the difference in electronegativity between carbon and oxygen.

Nomenclature of Ketones and Aldehydes

IUPAC and Common Naming

Aldehydes and ketones are named according to IUPAC rules, but many have common names that are widely used.

• Aldehydes: Replace the terminal -e of the parent alkane with -al (e.g., ethanal for CH3CHO).

• Ketones: Replace the terminal -e of the parent alkane with -one (e.g., propanone for CH3COCH3).

• Common names to recognize: acetone, acetophenone, benzophenone, formaldehyde, acetaldehyde, benzaldehyde.

Example: The IUPAC name for CH3COCH3 is propanone (common name: acetone).

Physical Properties of Ketones and Aldehydes

Boiling Points and Hydrogen Bonding

• Aldehydes and ketones have higher boiling points than alkanes and ethers of similar molecular weight 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 compounds like water.

Example: Acetone is miscible with water due to its ability to accept hydrogen bonds from water molecules.

Spectroscopy of Ketones and Aldehydes

Infrared (IR) Spectroscopy

• The C=O stretch appears strongly around 1700 cm-1.

• Aldehyde C-H stretch appears near 2720 cm-1 (often as a pair of bands).

• Conjugation with double bonds or aromatic rings lowers the C=O stretching frequency.

NMR Spectroscopy

• Protons on carbons adjacent to the carbonyl appear downfield (2-2.5 ppm in 1H NMR).

• The aldehyde proton appears further downfield (9-10 ppm).

• The carbonyl carbon appears at 190-220 ppm in 13C NMR.

Mass Spectrometry

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

Ultraviolet (UV) Spectroscopy

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

Synthesis of Ketones and Aldehydes

Review of Synthetic Methods

• Grignard Reaction followed by Oxidation: Grignard reagents react with certain carbonyl compounds, and subsequent oxidation can yield aldehydes or ketones.

• Controlled Oxidation: Use of mild oxidizing agents (e.g., PCC) can stop oxidation at the aldehyde stage, preventing further oxidation to carboxylic acids.

• Ozonolysis: Cleavage of alkenes with ozone produces aldehydes and/or ketones.

• Friedel-Crafts Acylation: Aromatic rings react with acid chlorides in the presence of AlCl3 to form aryl ketones.

• Gatterman-Koch Reaction: Used to synthesize benzaldehyde derivatives.

• Oxymercuration or Hydroboration-Oxidation of Alkynes: Terminal alkynes yield aldehydes; internal alkynes yield ketones.

Synthesis from Carboxylic Acids

• Carboxylic acids can be converted to ketones via acid chloride intermediates and reaction with organometallic reagents.

Synthesis from Nitriles

• Grignard reagents react with nitriles to form ketones after hydrolysis.

• DIBAL-H (diisobutylaluminum hydride) can reduce nitriles to aldehydes.

Synthesis from Acid Chlorides and Esters

• Reduction of acid chlorides/esters with weak reducing agents (e.g., DIBAL-H, LiAlH(OtBu)3) yields aldehydes.

• Grignard reagents with acid chlorides yield tertiary alcohols; organocuprates (Gilman reagents) yield ketones.

Reactions of Ketones and Aldehydes: Nucleophilic Addition

General Mechanism

• Nucleophiles attack the electrophilic carbonyl carbon, forming a tetrahedral intermediate.

• Basic nucleophiles (e.g., Grignard, hydride) add directly; acidic conditions often involve protonation of the carbonyl oxygen first.

Example Mechanism (Basic Nucleophile):

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

Hydration of Ketones and Aldehydes

Hydrate Formation

• Water adds to the carbonyl group to form a geminal diol (hydrate).

• Formaldehyde is most reactive, followed by other aldehydes, then ketones.

• Mechanism involves nucleophilic addition of water, often acid- or base-catalyzed.

Formation of Cyanohydrins

• Cyanide ion adds to the carbonyl group, forming a cyanohydrin.

• Hydrolysis of the nitrile group yields a carboxylic acid.

Example: Reaction of acetone with HCN forms 2-hydroxy-2-methylpropanenitrile.

Formation of Imines and Related Compounds

Imines (Schiff Bases)

• Primary amines react with aldehydes/ketones to form imines (Schiff bases) via condensation (loss of water).

• Reversible under acidic aqueous conditions.

Condensations with Hydroxylamine and Hydrazine

• Hydroxylamine forms oximes; hydrazine forms hydrazones; semicarbazide forms semicarbazones.

Formation of Acetals, Ketals, and Hemiacetals

• Aldehydes/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.

Use of Acetals as Protecting Groups

• Acetals are stable to base and many nucleophiles, making them useful for protecting carbonyl groups during synthesis.

• They can be removed (deprotected) by acid-catalyzed hydrolysis.

The Wittig Reaction

• Converts aldehydes/ketones to alkenes using a phosphorus ylide.

• Ylides are prepared from triphenylphosphine and alkyl halides, followed by deprotonation.

• Mechanism involves nucleophilic addition and a [2+2] cycloaddition (the Wittig intermediate).

Example: $\ce{Ph_3P=CH_2 + R_2C=O \rightarrow R_2C=CH_2 + Ph_3P=O}$

Oxidation and Reduction of Aldehydes and Ketones

Oxidation of Aldehydes

• Aldehydes can be oxidized to carboxylic acids using reagents like Ag2O (Tollens' reagent), CrO3, or KMnO4.

• The Tollens test is a qualitative test for aldehydes (silver mirror formation).

• Ketones are generally resistant to oxidation under normal conditions.

Reduction of Aldehydes and Ketones

• Hydride reagents (NaBH4, LiAlH4) reduce aldehydes to primary alcohols and ketones to secondary alcohols.

• Raney nickel and catalytic hydrogenation can reduce C=O to CH2.

• Deoxygenation reactions (e.g., Clemmensen and Wolff-Kishner reductions) convert carbonyls to methylene groups.

• Clemmensen: Zn(Hg)/HCl; Wolff-Kishner: NH2NH2/KOH, heat.

Example (Wolff-Kishner):

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

Summary Table: Key Reactions of Aldehydes and Ketones

Reaction Type	Reagents	Product	Notes
Hydration	H2O (acid/base catalyzed)	Geminal diol (hydrate)	Reversible; more favored for aldehydes
Cyanohydrin formation	HCN, NaCN	Cyanohydrin	Useful for further transformations
Imine formation	Primary amine, acid	Imine (Schiff base)	Condensation reaction
Acetal formation	Alcohol, acid	Acetal/ketal	Protecting group for carbonyls
Wittig reaction	Phosphorus ylide	Alkene	Forms new C=C bond
Oxidation	Tollens, CrO3, KMnO4	Carboxylic acid (from aldehyde)	Ketones generally unreactive
Reduction	NaBH4, LiAlH4	Alcohol	Primary (from aldehyde), secondary (from ketone)
Deoxygenation	Zn(Hg)/HCl, NH2NH2/KOH	Alkane	Clemmensen, Wolff-Kishner reductions

Additional info: This study guide expands on the learning objectives by providing definitions, mechanisms, and examples for each key reaction and property of aldehydes and ketones, as would be expected in a college-level organic chemistry course.