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Preparation of Aliphatic Aldehydes and Ketones
Dehydrogenation and oxidation of alcohol
a) By oxidation of alcohols
Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols respectively.
Common oxidising agents are KMnO4, K2Cr2O7 and CrO3. Strong oxidising agents oxidise the aldehyde produced by the oxidation of a primary alcohol to carboxylic acid.
Low molecular weight primary alcohols may be oxidised to aldehydes if the reaction temperature is so adjusted that the aldehyde being lower boiling then the alcohol, distils out of the reaction mixture as soon as it is formed, thus escaping from further oxidation.
Reaction temperature for this reaction is maintained slightly above 349 K.
The general reaction is :
Collin's reagent (chromium trioxide - pyridine complex) is a very good oxidising agent for converting primary alcohol to aldehydes. The reagent checks the further oxidation o aldehydes to carboxylic acids Collins reagent is used in non-aqueous medium like CH2Cl2.
On mixing pyridine (C6H5N), CrO3 and HCl in dichloromethane, pyridine chloro-chromate (C5H5NH+CrO3 Cl-) abbreviated as PCC, is made
Ketones can be prepared by using similar oxidising agents from secondary alcohols.
b) By dehydrogenation
When vapours of primary or secondary alcohols are passed over copper gauze at 573 K, they get dehydrogenated to form aldehydes or ketones respectively.
Other heated metal catalysts like silver or copper may be used. This method is suitable for valuable alcohols and is of industrial application.
The dehydrogenation reaction is a better method of preparation because there is no risk of further oxidation of aldehyde.
Ozonolysis of alkenes
Alkenes react with ozone to form ozonides which on subsequent reductive cleavage with zinc dust and water or H2/Pd give aldehydes, Ketones or a mixture of both depending on the substitution pattern of the alkene.
Zinc dust removes H2O2 formed, which otherwise can further oxidise the aldehyde formed to acids.
Using a suitable alkene, the desired aldehyde or ketone can be formed.
Aldehydes are prepared from acid chlorides by reaction with H2 in the presence of palladium catalyst supported on barium sulphate. The catalyst is poisoned by addition of sulphur or quinoline. The poisoning is done to avoid the further reduction of aldehydes to alcohols. This reaction is called Rosenmund's reaction.
Ketones are obtained by reacting acid chlorides with dialkyl cadmium.
Ketones cannot be obtained by Rosen Mund's reaction.
Gem dihalides on hydrolysis give carbonyl compounds
(i) R – CHX2 ——————→ R – CHO
(ii) R – C – R' —————→ R – C – R'
This method is not used much since aldehydes are affected by alkali and dihalides are usually prepared from the carbonyl compounds.
Calalytic distillation of fatty acid
Distillaiton of salt of fatty acid
Calalyic hydration of alkynes