preparation of aldehydes and ketones
Answers
Answer:
Preparation of Aldehyde and Ketone
Aldehyde and Ketone preparation is possible by oxidation of primary and secondary alcohol by agents such as PCC (pyridinium chlorochromate), Collins reagents (Chromium trioxide-pyridine complex), and Cu at 573 K.
Ketones and aldehydes from Alcohol
Collins Reagents (Chromium trioxide-pyridine complex)
Collin’s reagent or chromium trioxide-pyridine complex is a good oxidizing reagent for conversion of primary alcohol to aldehydes. Additionally, an advantage of Collins reagent is that it helps to cease further oxidation of aldehydes to carboxylic acids. However, the reaction with Collins reagent is possible in a non-aqueous medium such as CH2Cl2.
PCC (pyridinium chlorochromate)
The mixture of pyridine along with CrO3 and HCl in dichloromethane leads to the formation of Pyridine chlorochromate or PCC (C5H5NH+CrO3 Cl–).
Oxidizing agents for Preparation of aldehydes and Ketones
Ketones can be prepared by using similar oxidizing agents from secondary alcohols.
Secondary Alcohol to Ketones
Formation by Dehydrogenation of Alcohols
This preparation method applies in case of conversion of volatile alcohols to aldehydes. It is generally used in industrial application. Vapours of alcohol are passed through heavy metal catalysts such as Cu or Ag in this technique. Primary alcohol produces aldehyde whereas secondary alcohol produces ketones, respectively.
For example, alcohols undergo dehydrogenation when vapours of primary alcohol or secondary alcohol pass through copper gauze at a temperature of 573 K. Refer to the example below to see how n-propyl alcohol leads to the formation of propionaldehyde in the dehydrogenation process.
Preparation of Aldehydes and Ketones by Dehydrogenation
It is possible to use different metal catalysts such as copper or silver under heating conditions during dehydrogenation of alcohol. However, this technique is apt for conversion of valuable alcohols to aldehydes. Furthermore, it is useful in industrial applications.
Moreover, this is one of the better methods for preparation of aldehydes and ketones because further oxidation is not possible of aldehydes. Hence, there is no risk of conversion of aldehydes to carboxylic acids.
Ketone and Aldehyde Preparation from Hydrocarbons
This method is further divided into two separate methods. They are
By ozonolysis of alkenes
By hydration of alkynes
Ozonolysis of Alkenes
Formation of aldehyde and ketone is possible by ozonolysis of alkenes. Ozonolysis is a reaction method in which addition of ozone molecules or O3 to an alkene compound leads to the formation of ozonide. Reduction of the ozonide compound with the help of zinc dust and water produces the smaller molecules, which in this case will be the respective aldehydes and ketones.
The reaction produces aldehydes, ketones and in some cases both the compounds on the basis of the substitution arrangement of the alkene compounds.
Refer to the example below to observe how an alkene on ozonolysis leads to the formation of the ozonide compound. This compound further undergoes subsequent reductive cleavage with Zn dust and water or H2/Pd to produce carbonyl compounds.
ozonide to Aldehydes and Ketones
Zinc Dust helps in removing the H2O2 thereby ceasing the chances of further oxidation of carbonyl compounds to acids. Refer to the examples below to see how choosing a suitable alkene for this method helps to achieve the proper aldehyde or ketone.
ozonolysis
Formation of Aldehydes and Ketones by Ozonolysis
Reductive ozonolysis
Hydration of Alkynes
Alkynes follow Markovnikov’s rule in the presence of a proper catalyst to produce ketones. All alkynes react with water in the presence of HgSO4 and H2SO4 to form ketones. However, the reaction of ethyne with water in the presence of the catalyst (HgSO4 and H2SO4) leads to the formation of acetaldehyde. This is an only exception where alkyne on hydration produces acetaldehyde. Rest all the alkyne on hydration produces ketones.