Methods of Preparation of Aldehydes and Ketones - Practice Questions & MCQ

Oxidation
Oxidation of ROH involves the cleavage of (O-H) and (C-H) bonds to form (C=O) bond. Such reactions are also called dehydrogenation reactions because H₂ is lost from an alcohol molecule. Strong oxidising agents such as acidic KMnO₄ or acidic K₂Cr₂O₇ oxidise 1^o ROH first to an aldehyde and then to carboxylic acids. Further, 2^o alcohols are oxidised to ketones and 3^o alcohols do not undergo oxidation, but under drastic conditions such as strong oxidising agents(KMnO₄) and at high temperatures cleavage of (C-C) bonds takes place and a mixture of RCOOH containing lesser number of C atoms is formed. 1^o ROH can be converted upto aldehyde stage either by the use of CrO₃ in anhydrous medium or with PCC or with Jones reagent.
Some examples include:

Dehydrogenation
1^o alcohols undergo dehydrogenation to give aldehydes only. 2^o alcohols give ketones and 3^o alcohols give alkenes. Some examples include.

Rosenmund Reduction
Partial hydrogenation of benzoylchloride with finely divided Pd as catalyst in the prsence of BaSO₄ and S or quinoline in boiling xylene as solvent gives benzaldehyde. This reaction is called as Rosenmund reduction. The catalyst under the above condition is called Lindlar's catalyst or poisoned Pd. The Lindlar's catalyst also reduces (C  C) to (C=C) in syn-addition.
It is BaSO₄ that prevents the aldehyde from being further reduced to alcohols and acts as a poison to the Pd catalyst. Small amount of sulphur and quinoline is very effective in poisoning the catalyst in aldehyde reduction. Moreover, S and quinoline react with small amount of H₂ to give H₂S gas and hydroquinoline, thereby limiting H₂ for further reduction of aldehyde to alcohol.
The reaction occurs as follows:

Stephen Reduction
Nitriles are partially reduced to corresponding imine with SnCl₂ in the presence of HCl, which on hydrolysis gives corresponding aldehyde. It does not reduce (C=C) or (C  C). This reaction is known as Stephen reduction. The reaction occurs as follows.

From Acyl chlorides
Formyl chloride gives an aldehyde and all other halides five ketone. The reaction occurs as follows.

From Nitriles
Grignard reagents give aldehydes with hydrogen cyanide (HCN) and ketones with alkyl cyanides (RCN). The reaction occurs as follows: