Aldehydes Ketones Acid Chlorides

Aldehydes are prepared from acid chlorides by reaction with H₂ in the presence of palladium catalyst supported on barium sulphate. The catalyst is partially deactivated (poisoned) by addition of BaSO₄ or sulphur or quinoline. The partial poisoning is done in order 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 Rosenmund’s reaction. Another point that must be kept in mind is that ketones cannot be obtained by the reaction of acid chlorides with Grignard’s reagent. It is because the ketone obtained would further react with Grignard’s reagent to form tertiary alcohol.

              

Preparation of aldehydes and ketones from alcohols

Since aldehydes and ketones are first oxidation products of alcohols, hence, they can be prepared from alcohols by oxidation or dehydrogenation methods.
    
(a) By direct oxidation: Aldehydes are prepared by oxidation of primary alcohols. Acidified potassium permanganate or potassium dichromate in generally used as oxidising agents. Aldehydes are very susceptible to further oxidation to carboxylic acids. The reaction is controlled by distilling out aldehydes as soon as it is formed.


    
(i) Chromic acid (CrO₃ in acetic acid) or H₂CrO₄ in aqueous acetone (Jone’s reagent) can oxidize 1˚ alcohols to aldehydes stage only.
    
(ii) Chromic acid in acetone oxidizes allylic alcohols to vinyl ketones.



Ketones can be prepared by using similar oxidising agents from secondary alcohols.



Secondary alcohols can also be conveniently oxidized to ketones by opppenauer oxidation. The reaction involves refluxing a secondary alcohol with aluminium tert-butoxide in excess of acetone or cyclohexanone when 2˚ alcohol oxidizes to ketone and acetone reduces to isopropyl alcohol.



In this method also the ketone cannot get oxidized further to an acid.

Similarly, cyclohexanol can be converted to cyclohexanone by an identical method.
    
(b) By Catalytic 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.



The dehydrogenation reaction is a better method of preparation because there is no risk further oxidation of aldehydes.

Preparation of aldehydes and ketones from fatty acids
    
(a) By heating calcium salt of fatty acids: Aldehydes are obtained by  heating calcium salts of fatty acids with calcium formate to about 675 K.
    


Ketones are formed by distilling calcium salt of the fatty acid alone.        
                                                                                                             


Similarly, mixed ketones can also be obtained by similar reactions.



It must be noted here that the yield of aldehydes and ketones is poor for this reaction because a number of other products may also be obtained. For example, the reaction of calcium acetate and calcium formate also gives acetone and formaldehyde along with acetaldehyde.
    
(b) By passing vapours of fatty acids over manganous oxide: in this method, formic acid alone gives formaldehyde. Acetic acid gives acetone and the mixture of two acids i.e. formic and acetic acids produce acetaldehyde.

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