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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
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 δ
Carbon NMR Spectra
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
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 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
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 NaBH 4 can reduce ketones and aldehydes, not esters, carboxylic acids, acyl chlorides, or amides
Reducing Reagents- Lithium Aluminum Hydride LiAlH 4 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)
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