General methods of preparation of aldehydes and ketones A. Preparation of aldehydes and ketones
1.) Oxidation and catalytic dehydrogenation of alcohols We have already learnt that the oxidation of primary alcohol gives aldehydes and secondary
alcohol gives a ketone. Oxidising agents such as acidified Na Cr O KMnO PCC2 2 7 4, , are used for oxidation. Oxidation using PCC yield aldehydes . Other oxidising agents further oxidise the aldhydes / ketones in to carboxylic acids (Refer Unit No. - 11 Oxidation of alcohols)
When vapours of alcohols are passed over heavy metal catalyst such as Cu, Ag, alcohols give aldehydes and ketons. (Refer Unit No. - 11 Catalytic dehydrogenation of alcohols 2.) Ozonolysis of alkenes
We have already learnt in XI th standard that the reductive ozonolysis of alkenes gives aldehydes and ketones.
Alkenes react with ozone to form ozonide which on subsequent cleavage with zinc and water gives aldehydes and ketones. Zinc dust removes H2O2 formed, which otherwise can oxidise aldehydes / ketones.
CH3 CH = CH CH3 + O3 2 CH3 CHO
O
O O but - 2- ene
ethanal
Zn / H2O CH3 CH CH CH3 + H2 2O
CH3 CH = C CH3 + O3 CH3 CHO + CH3 C CH3
O
Zn / H2O O
O O
CH3
2 - methyl but-2-ene ethanal
propanone
CH3 CH C CH3
CH3
+ H2 2O
Terminal olefines give formaldehyde as one of the product.
Evaluate yourself - 1
What happens when the following alkenes are subjected to reductive ozonolysis.
- propene 2) 1 – Butene 3) Isobutylene
3. Hydration of alkynes We have already learnt in XI standard that the hydration of alkynes in presence of 40%
dilute sulpuric acid and 1% HgSO4 to give the corresponding aldehydes / ketones.
a) Hydration of acetylene yields acetaldehyde
HC CH + H OH H C = C OHHgSO4
H2SO4
Isomerises
ethyne enol
H H
ethanal
CH3 CHO
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150
b) Hydration of alkynes, other than acetylene gives ketones
CH3 C CH + H OH CH3 C = CH2 HgSO4
H2SO4
Isomerises
enol
OH O
propanone
CH3 C CH3
prop - 1- yne
4. From calcium salts of carboxylic acids Aldehydes and ketones may be prepared by the dry distillation of calcium salts of carboxylic
acids.
a) Aldehydes are obtained when the mixture of calcium salt of carboxylic acid and calcium formate is subjected to dry distillation.
Ca H C H + CaCO3 Dry distillation
O
O
O
Calcium methanoate
methanal
H C O
H C O
Ca + Ca
O
O
O
O
dry distillation 2 CH3 C H + 2CaCO3
O
Calcium ethanoate Calcium methanoate
ethanal
CH3 C O
CH3 C O O C H
O C H
b) Symmetrical ketones can be obtained by dry distillation of the calcium salt of carboxylic acid (except formic acid)
O
O
dry distillation CH3 C CH3 + CaCO3
O
Calcium ethanoate
Ca propanone
CH3 C O
CH3 C O
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| O | COCa |
|---|---|
| COO |
151
B. Preparation of aldehydes
1) Rosenmund reduction a) Aldehydes can be prepared by the hydrogenation of acid chloride, in the presence of
palladium supported by barium sulphate. This reaction is called Rosenmund reduction.
Example
CH3 C Cl + H2 CH3 C H + HCl
O O Pd/ BaSO4
Acetyl chloride Acetaldehyde
In this reaction, barium sulphate act as a catalytic poison to palladium catalyst, so that aldehyde cannot be further reduced to alcohol.
Formaldehyde and ketones cannot be prepared by this method.
2. Stephen’s reaction When alkylcyanides are reduced using SnCl / HCl2 , imines are formed, which on
hydrolysis gives corresponding aldehyde. +
32 H OSnCl /HCl 3 3 3(H)CH C N CH CH=NH CH CHO + NH3
3. Selective reduction of cyanides Diisobutyl aluminium hydride (DIBAL –H) selectively reduces the alkyl cyanides to form
imines which on hydrolysis gives aldehydes.
Example
CH3 CH CH CH2 CNCH2 i) AlH (iso-butyl)2
ii) H2O CH3 CH CH CH2 CHOCH2
hex - 4- ennitrile hex - 4- enal
C) Preparation of benzaldehyde
1. Side chain oxidation of toluene and its derivatives by strong oxidising agents such as KMnO4 gives benzoic acid.
When chromylchloride is used as an oxidising agent, toluene gives benzaldehyde. This reaction is called Etard reaction. Acetic anhydride and CrO3 can also be used for this reaction.
CH3
+ CrO2Cl2 CS2
CH (OCrOHCl2 2)
H3O+
CHO
methylbenzene benzaldehyde
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152
Oxidation of toluene by chromic oxide gives benzylidine diacetate which on hydrolysis gives benzaldehyde.
2) Gattermann – Koch reaction This reaction is a variant of Friedel – Crafts acylation reaction. In this method, reaction of
carbon monoxide and HCl generate an intermediate which reacts like formyl chloride.
CO, HCl
AlCl3 / CuCl
CHO
3) Manufacture of benzaldehyde from toluene Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde.
CH3
Toluene
Cl2 / hυ
CHCl2
Benzalchloride
H2O
373 K
CHO
Benzaldehyde
This is the commercial method for the manufacture of benzaldehye.
D) Preparation of ketones
1) Ketones can be prepared by the action of acid chloride with dialkyl cadmium.
Example
CH3 C Cl + Cl C CH3 2CH3 C CH3 + CdCl2
O O O
Cd
H3C CH3
Dimethyl cadmium
AcetoneAcetyl chloride
2) Preparation of phenyl ketones Friedel – Crafts acylation
It is the best method for preparing alkyl aryl ketones or diaryl ketones. This reaction succeeds only with benzene and activated benzene derivatives.
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Example
CH3 C Cl + AlCl3
CO CH3
O
acetylchloride benzene acetophenone
C6H5 C Cl + AlCl3
CO C6H5
O
benzoylchloride benzene benzophenone