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Introduction to Synthetic Preparation of Aldehydes and Ketones

Ketones and Aldehydes are achieved as results from various reactions discussed in earlier sections of this text. The diagram summarizes the most significant of these.

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These techniques do not get incresed the complexity or size of molecules, with the exception of Friedel-Crafts acylation. In the following sections of this chapter we shall discover that one of the most helpful feature of aldehydes and ketones is their reactivity to carbon nucleophiles and the resulting elaboration of molecular structure that results. In short, aldehydes and ketones are significant intermediates for the synthesis or assembly of complex organic molecules.

Synthesis

There are various techniques for preparing aldehydes,but the dominant method is hydroformylation. Descriptive is the generation of butyraldehyde by hydroformylation of propene:

H2 + CO + CH3CH=CH2 → CH3CH2CH2CHO

Oxidative routes

Aldehydes are generally generated by alcohol oxidation. In industry, formaldehyde is generated on a large scale by oxidation of methanol. Oxygen is the reagent of choice being cheap and "green". In the laboratory more particular oxidizing agents are employed, but chromium(VI) reagents are well-liked. Oxidation can be accomplished by heating the alcohol by an acidified solution ofpotassium dichromate. In this example, excess dichromate will further oxidize the aldehyde to a carboxylic acid, so either aldehyde is distilled out as it forms (if volatile) or milder reagents like PCC are employed.

[O] + CH3(CH2)9OH → CH3(CH2)8CHO + H2O

Oxidation of major alcohols to form aldehydes and can be get under milder, chromium-free conditions by using techniques or reagents like IBX acid, Dess-Martin periodinane, Swern oxidation, TEMPO, or the Oppenauer oxidation.

Other oxidation route important in industry is the Wacker process, whereby ethylene is oxidized to acetaldehyde in the existence of copper and palladium catalysts (acetaldehyde is also generated on a large scale by the hydration of acetylene).

Specialty methods

Reaction Name

Substrate

Comment

Ozonolysis

Alkene

Ozonolysis of non-fully-substituted alkenes yield Aldehydes upon reductive work-up.

Organic Reduction

Ester

Reduction of an Ester with diisobutylaluminium hydride (DIBAL-H) or sodium aluminium hydride

Rosenmund Reaction

Acid Chloride

Or using lithium tri-1-butoxyaluminium hydride (LiAlH(OtBu)3)

Witting Reaction

Ketone

Reagent methoxymethylenetriphenylphosphine in a modified Witting Reaction

Formylation Reaction

Nucleophilic Arenes

Various reactions for example the Vilsmeier-Haack Reaction

Nerf Reaction

Nitro Compound

 

Zincke Reaction

Pyridines

Zincke cldehydes form in a variation

Stephen aldehyde synthesis

Nitriles

Reagents tin(II) chloride and hydrochloric acid

Meyers synthesis

Oxazine

Oxazine hydrolysis

McFadyen Stevens reaction

Hydrazide

Is a base-catalyzed thermal decomposition of acylsulfonylhydrazides

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