Concept 01 / 07Aldehydes, Ketones and Carboxylic Acids
Aldehydes and Ketones: Structure, Nomenclature and Preparation
Structure of the carbonyl group and its polarity, common and IUPAC names, and preparation of aldehydes and ketones by oxidation and dehydrogenation of alcohols, ozonolysis, hydration of alkynes, Rosenmund and Stephen reductions, DIBAL-H reduction of nitriles and esters, Etard reaction, side-chain chlorination, Gattermann-Koch reaction, Grignard reagents with nitriles, dialkylcadmium and Friedel-Crafts acylation.
Aldehydes (RCHO) and ketones (RCOR') contain the carbonyl group, .
Structure. The carbonyl carbon is hybridised, forming three bonds in a plane () and a bond with oxygen. Oxygen is more electronegative, so the bond is polar: , with resonance contributors . The electrophilic carbon is attacked by nucleophiles; the oxygen is a Lewis base.
Nomenclature. Aldehydes: suffix -al (methanal HCHO, ethanal ; common names formaldehyde, acetaldehyde; benzaldehyde). Ketones: suffix -one (propanone, acetone; butan-2-one; acetophenone ; benzophenone). Positions in common names use Greek letters ( is the carbon next to C=O).
Preparation (see figure).
By oxidation of alcohols: primary to aldehydes (PCC, or distil the aldehyde out), secondary to ketones (, ).
By dehydrogenation of alcohols: vapours over heated Cu or Ag at .
From hydrocarbons: ozonolysis of alkenes; hydration of alkynes (, dilute ): ethyne gives ethanal, other alkynes give ketones.
From acyl chlorides: Rosenmund reduction, (the catalyst is poisoned to stop further reduction). Ketones from acyl chlorides and dialkylcadmium: .
From nitriles: Stephen reaction, ; selective reduction with diisobutylaluminium hydride (DIBAL-H), which also reduces esters to aldehydes at low temperature. Ketones from nitriles and Grignard reagents: imine salt .
Aromatic aldehydes:
Etard reaction: toluene with chromyl chloride () forms a chromium complex that hydrolyses to benzaldehyde; alternatively in acetic anhydride gives benzylidene diacetate, hydrolysed to benzaldehyde.
Side-chain chlorination: toluene with in light gives benzal chloride (), hydrolysed at to benzaldehyde.
Gattermann-Koch reaction: benzene with CO and HCl in presence of anhydrous or CuCl gives benzaldehyde.
Aromatic ketones: Friedel-Crafts acylation, .
Key Takeaways:
- Carbonyl carbon , planar, electrophilic.
- Rosenmund (RCOCl), Stephen and DIBAL-H (RCN) give aldehydes.
- Etard, side-chain chlorination and Gattermann-Koch give benzaldehyde.
- Ketones: + acyl chloride, nitrile + RMgX, Friedel-Crafts acylation.
- Pitfall: Rosenmund's palladium catalyst is deliberately poisoned () so that the aldehyde is not reduced further to an alcohol.
Brain-Teaser 1: How would you convert ethanenitrile into (a) ethanal and (b) propanone?
Solution: (a) Stephen reduction (/HCl, then hydrolysis) or DIBAL-H followed by water gives ethanal. (b) Methylmagnesium bromide adds to the nitrile to give an imine salt, which on acid hydrolysis gives propanone.
Brain-Teaser 2: Name the reagents to make benzaldehyde from (a) toluene, (b) benzene, (c) benzoyl chloride.
Solution: (a) Chromyl chloride (Etard), or /light then hydrolysis. (b) CO and HCl with anhydrous /CuCl (Gattermann-Koch). (c) over Pd on (Rosenmund).
Brain-Teaser 3: Which alkyne gives butan-2-one on hydration with ?
Solution: But-1-yne or but-2-yne. But-1-yne (Markovnikov addition) gives ; but-2-yne, being symmetrical, also gives butan-2-one.
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