chapter 18: ketones and aldehydes. classes of carbonyl compounds

Chapter 18: Ketones and Aldehydes

Classes of Carbonyl Compounds

Carbonyl

• C=O bond is shorter, stronger and more polar than C=C bond in alkenes

Nomenclature: Ketone

• Number chain so the carbonyl carbon has the lowest number

• Replace “e” with “one”

Nomenclature: Cyclic Ketone

• Carbonyl carbon is #1

Nomenclature: Aldehydes

• Carbonyl carbon is #1• Replace “e” with “al”• If aldehyde is attached to ring, suffix

“carbaldehyde” is used

Nomenclature

• With higher-priority functional groups, ketone is “oxo” and an aldehyde is a “formyl” group

• Aldehydes have higher priority than ketones

Nomenclature- Common Names: Ketones

• Name alkyl groups attached to carbonyl• Use lower case Greek letters instead of numbers

Nomenclature

Boiling Points

• Ketones and aldehydes are more polar. Have higher boiling point that comparable alkanes or ethers

Solubility: Ketones and Aldehydes

• Good solvent for alcohols• Acetone and acetaldehyde are miscible in water

Formaldehyde

• Gas at room temperature

IR Spectroscopy

• Strong C=O stretch around 1710 cm-1 (ketones) or 1725 cm-1 (simple aldehydes)

• C-H stretches for aldehydes: 2710 and 2810 cm-1

IR Spectroscopy

• Conjugation lowers carbonyl frequencies to about 1685 cm-1

• Rings with ring strain have higher C=O frequencies

Proton NMR Spectra

• Aldehyde protons normally around δ9-10• Alpha carbon around δ2.1-2.4

Carbon NMR Spectra

Mass Spectrometry (MS)

Mass Spectrometry (MS)

Mass Spectrometry (MS)

McLafferty Rearrangement

• Net result: breaking of the , bond and transfer of a proton from the carbon to oxygen

Ultraviolet Spectra of Conjugated Carbonyls

• Have characteristic absorption in UV spectrum• Additional conjugate C=C increases max about

30 nm, additional alkyl groups increase about 10nm

Carbonyl Electronic Transitions

Industrial Uses

• Acetone and methyl ethyl ketone are common solvents

• Formaldehyde is used in polymers like Bakelite and other polymeric products

• Used as flavorings and additives for food

Industrial Uses

Synthesis of Aldehydes and Ketones

• The alcohol product of a Grignard reaction can be oxidized to a carbonyl

Synthesis of Aldehydes and Ketones

• Pyridinium chlorochromate (PCC) or a Swern oxidation takes primary alcohols to aldehydes

Synthesis of Aldehydes and Ketones

• Alkenes can be oxidatively cleaved by ozone, followed by reduction

Synthesis of Aldehydes and Ketones

• Friedel-Crafts Acylation

Synthesis of Aldehydes and Ketones• Hydration of Alkynes

• Involves a keto-enol tautomerization• Mixture of ketones seen with internal alkynes

Synthesis of Aldehydes and Ketones• Hydroboration-oxidation of alkyne

• Anti-Markovnikov addition

Synthesis Problem

Synthesis of Aldehydes and Ketones• Organolithium + carboxylic acid ketone (after

dehydration)

Synthesis of Aldehydes and Ketones• Grignard or organolithium reagent + nitrile

ketone (after hydrolysis)

Synthesis of Aldehydes and Ketones• Reduction of nitriles with aluminum hydrides will

afford aldehydes

Synthesis of Aldehydes and Ketones• Mild reducing agent lithium aluminum tri(t-

butoxy)hydride with acid chlorides

Synthesis of Aldehydes and Ketones• Organocuprate (Gilman reagent) + acid chloride

ketone

Nucleophilic Addition

• Aldehydes are more reactive than ketones

Wittig Reaction

• Converts the carbonyl group into a new C=C bond• Phosphorus ylide is used as the nucleophile

Wittig Reaction• Phosphorus ylides are prepared from

triphenylphosphine and an unhindered alkyl halide

• Butyllithium then abstracts a hydrogen from the carbon attached to phosphorus

Wittig Reaction- Mechanism• Betaine formation

• Oxaphosphetane formation

Wittig Reaction- Mechanism• Oxaphosphetane collapse

How would you synthesize the following molecule using a Wittig Reaction

Hydration of Ketones and Aldehydes• In aqueous solution, a ketone or aldehyde is in

equilibrium with it’s hydrate

• Ketones: equilibrium favors keto form

Hydration of Ketones and Aldehydes• Acid-Catalyzed

Hydration of Ketones and Aldehydes• Base-Catalyzed

Cyanohydrin Formation• Base-catalyzed nucleophilic addition

• HCN is highly toxic

Formation of Imines

• Imines are nitrogen analogues of ketones and aldehydes

• Optimum pH is around 4.5

Formation of Imines- Mechanism

Condensations with Amines

Acetal Formation

Hemiacetal Formation- Mechanism• Must be acid-catalyzed

Acetal Formation- Mechanism• Must be acid-catalyzed

Hydrolysis of Acetals• Acetals can be hydrolyzed by addition of dilute acid• Excess of water drives equilibrium towards

carbonyl formation

Cyclic Acetals• Addition of diol produces cyclic acetal• Reaction is reversible

• Used as a protecting group• Stable in base, hydrolyze in acid

Cyclic Acetals- Protecting Group

• Acetals are stable in base, only ketone reduces• Hydrolysis conditions protonate the alkoxide and

restore the aldehyde

Oxidation of Aldehydes

• Easily oxidized to carboxylic acids

Tollens Test• Involves a solution of silver-ammonia complex to

the unknown compound• If an aldehyde is present, its oxidation reduces

silver ion to metallic silver

Reducing Reagents- Sodium Borohydride• NaBH4 can reduce ketones and aldehydes, not

esters, carboxylic acids, acyl chlorides, or amides

Reducing Reagents- Lithium Aluminum Hydride

• LiAlH4 can reduce any carbonyl

Reducing Reagents- Catalytic Hydrogenation• Widely used in industry• Raney nickel is finely divided Ni powder saturated

with hydrogen gas• Will attack alkene first, then carbonyl

Deoxygenation of Ketones and Aldehydes• Clemmensen reduction or Wolff-Kishner reactions

can deoxygenate ketones and aldehydes

Clemmensen Reduction• Uses Zinc-Mercury amalgam in aqueous HCl

Wolff-Kishner Reduction• Forms hydrazone, then needs heat with strong

base like KOH or potassium tert-butoxide• Use high-boiling solvent (ethylene glycol,

diethylene glycol, or DMSO)