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

Edited By admin | Updated on Sep 25, 2023 25:24 PM | #NEET

Quick Facts

  • Preparation of Aldehydes is considered one the most difficult concept.

  • 34 Questions around this concept.

Solve by difficulty

In the following sequence of reactions, the alkene affords the compound B

CH_{3}CH= CHCH_{3}\overset{O_{3}}{\rightarrow}A\xrightarrow[Zn]{H_{2}O}B

The compound B is 

Reaction by which Benzaldehyde cannot be prepared:

Consider the following reaction:

Ozonolysis of an organic compound gives formaldehyde as one of the products. This confirms the presence of

One mole of a symmetrical alkene on ozonolysis gives two moles of an aldehyde having a molecular mass of 44 u . The alkene is

Cemmensen reduction of a ketone is carried out in the presence of which of the following?

The best reagent to convert pent­-3-­en­-2-­ol into pent­-3-­en­-2-­one is

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Concepts Covered - 3

Preparation of Aldehydes and Ketones

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 1o ROH first to an aldehyde and then to carboxylic acids. Further, 2o alcohols are oxidised to ketones and 3o 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. 1o 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
1o alcohols undergo dehydrogenation to give aldehydes only. 2o alcohols give ketones and 3o alcohols give alkenes. Some examples include.

Preparation of Aldehydes

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 \mathrm{\equiv } 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 \mathrm{\equiv } C). This reaction is known as Stephen reduction. The reaction occurs as follows:

Preparation of Ketones

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:

Study it with Videos

Preparation of Aldehydes and Ketones
Preparation of Aldehydes
Preparation of Ketones

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Books

Reference Books

Preparation of Aldehydes and Ketones

Chemistry Part II Textbook for Class XII

Page No. : 361

Line : 36

Preparation of Aldehydes

Chemistry Part II Textbook for Class XII

Page No. : 362

Line : 6

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