Preparation of Aldehydes and Ketones
Ketones and aldehydes are both simple chemical molecules with a carbonyl group. The carbonyl group has a double bond between carbon and oxygen. Because the carbon atom in the carbonyl group lacks reactive groups like OH or Cl, these chemical molecules are simple.
Preparation of Aldehydes and Ketones
Some of the general methods of preparation of aldehydes and ketones are discussed below:
- By oxidation – Aldehydes can be prepared by the oxidation of primary alcohols with normal oxidizing agents such as acidified potassium dichromate, potassium permanganate, chromium oxide, and sulphuric acid.
RCH2OH (Primary Alcohol) + [O] → RCHO (Aldehyde) + H2O
CH3CH2OH (Ethanol) + [O] → CH3CHO (Acetaldehyde) + H2O
Aldehydes undergo readily oxidation to carboxylic acids. therefore, to prevent further oxidation of aldehydes, these are distilled off as soon as they are formed. Oxidation of aldehydes is carried out under controlled conditions.
Ketones can be prepared by the oxidation of secondary alcohols with similar oxidizing agents.
C(CH3)(CH3)HOH (Isopropyl alcohol) + [O] → C(CH3)(CH3)=O (Acetone) + H2O
C(CH3)(H3CH2C)HOH (sec-Butyl alcohol) + [O] → C(CH3)(CH3H2C)=O (Ethyl methyl ketone) + H2O
- By the catalytic dehydrogenation of alcohols – Aldehydes and Ketones can be prepared by the dehydrogenation of alcohols. It is carried out by passing the vapour of alcohol over reduced copper at 573 K.
Primary alcohols give aldehydes while secondary alcohols give ketones.
CH3CH2OH (Ethyl alcohol) → CH3CHO (Acetaldehyde) + H2
CH3—C(OH)H—CH3 (Propan-2-ol) → CH3—C(CH3)=O (Propanone)
From Carboxylic Acids
- By Catalytic Decomposition – Aldehydes and ketones can be prepared by catalytic decomposition of carboxylic acids. This can be done by heating a mixture of methanoic acid or other acids to 573 K in the presence of manganous oxide which acts as a catalyst.
CH3COOH (Ethanoic acid) + HOOCH (Methanoic acid) → HCHO (Methanal) + CO2 + H2O
Ketones can be prepared by passing the vapor of fatty acids over MnO at 573 K.
CH3COOH (Ethanoic acid) + HOOCC2H5 (Propanoic acid) → CH3COC2H5 (Butan-2-one) + CO2 + H2O
- By Distillation of Calcium Salts – Aldehydes and ketones can also be prepared by distilling the calcium salts of the acids.
Ca(HCOO)(HCOO) (Calcium formate) → HCHO (Methanal) + CaCO3
This method is, however not very suitable for the preparation of aldehydes because the yield is very low. This is because when a mixture of two calcium salts is heated three products are formed. For example, dry distillation of a mixture of calcium formate gives a mixture of formaldehyde. Similarly, this method cannot be used for the preparation of unsymmetrical ketone because it gives a mixture of three ketones.
For example, dry distillation of a mixture of calcium acetate will give a mixture of acetone.
Ca(CH3COO)(CH3COO) (Calcium acetate) → CH3COCH3 (Propanone)+ CaCO3
Cyclic ketones are formed when calcium salts of dicarboxylic acids are heated.
- By hydration of alkynes – Aldehydes and ketones can be prepared by the hydration of alkynes in the presence of dil. H2SO4 and HgSO4 as catalysts. Water adds to alkynes to form unstable enol intermediates which rearrange to form aldehydes or ketones.
HC≡CH (Acetylene) + H2O → CH2=CH(OH) (Vinyl alcohol) ⇌ CH3CHO (Acetaldehyde)
Hydration of alkynes other than acetylene gives ketones.
- By Hydroboration-oxidation reaction: The alkynes can be converted into aldehydes and ketones by hydroboration- oxidation reaction. Borane adds to an alkyne forming vinylic borane, which on oxidation with H2O2 gives aldehydes and ketones. The symmetrical non-terminal alkynes give a single ketone while unsymmetrical non-terminal alkynes give a mixture of both possible ketones in which the methyl ketones predominate.
CH3C≡C—CH3 (But-2-yne) → CH3—C(BH2)=CH—CH3 (Vinyl borane) → CH3—C(OH)=CH—CH3 (Enol) ⇌ CH3—C=O—CH2CH3 (Butan-2-one)
Terminal alkynes give aldehydes. However, to avoid complications due to double bond addition of diborane, bulky sterically hindered boranes such as bis (1,2-dimethyl propyl) borane commonly known as disiamylborane is used in place of diborane.
CH3CH2CH2C≡CH (Pent-1-yne) → CH3CH2CH2CH2CHO (Pentanal)
- By ozonolysis of alkenes: Alkenes react with ozone to form ozonide which on subsequent cleavage with zinc dust and water gives aldehydes and ketones. It is clear that if the carbon forming the double bond carries an H-atom attached to it, aldehydes are formed otherwise ketones are formed.
- By Wacker’s process: Alkenes can be converted to aldehydes and ketones by treating with an acidified aqueous solution of palladium chloride containing a catalytic amount of cupric chloride in the presence of air or oxygen. This method is known as Wacker’s process.
CH3CH=CH2 (Propene) + PdCl2 + H2O → CH3COCH3 (Acetone) + Pd + 2HCl
From Gem Dihalides
The gem dihalides containing two halogen atoms on the same carbon atom on hydrolysis give a carbonyl group.
CH3CHCl2 (1,1-Dichloroethane) → [CH3CH(OH)2] (Unstable) → CH3CHO (Ethanal)
Preparation of Aldehydes
From Acid Chlorides
Acid chlorides are converted into aldehydes by catalytic hydrogenation in the presence of palladium catalyst supported over barium sulfate. The catalytic mixture is poisoned by the addition of a small amount of sulphur or quinoline. This reaction is known as Rosenmund reduction.
From Reduction of alkyl cyanides and esters
Alkyl cyanides when reduced with stannous chloride and hydrochloric acid in absolute ether followed by hydrolysis give aldehydes. This reaction is known as Stephen reduction.
From aromatic hydrocarbons
Aromatic aldehydes are prepared from aromatic hydrocarbons by the following methods:
- By oxidation of methylbenzene: Strong oxidizing agents such as acidified or alkaline KMnO4, acidified K2Cr2O7, conc. HNO3, etc. oxidize toluene and its derivatives to benzoic acid through the oxidation of intermediate aldehydes.
However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents which convert the methyl group to an intermediate that is difficult to oxidize further. The following methods are used for this purpose:
- Use of chromic oxide in acetic anhydride: Aromatic aldehydes are prepared when the alkyl side chain aromatic ring is oxidized using chromium trioxide and acetic anhydride. The aldehyde formed immediately gets acetylated with acetic anhydride forming gem diacetate and this does not get further oxidized. Therefore, the function of acetic anhydride is to prevent further oxidation of the aldehyde to acid. The gem diacetate formed is isolated and hydrolyzed with aqueous acid to give corresponding aromatic aldehyde.
Similarly, p-nitrotoluene gives p-nitrobenzaldehyde.
It may be noted that the further oxidation of benzaldehyde to benzoic acid is prevented by trapping the aldehyde as gem-diacetate with acetic anhydride. The gem diacetate cannot be further oxidized.
- Use of chromyl chloride: Alkyl benzenes can also be oxidized to benzaldehyde with a solution of chromyl chloride in CCl4. A brown chromium complex is formed which is decomposed by water to give corresponding benzaldehyde. This reaction is called the Etard reaction.
In case, the side chain contains a group higher than the —CH3 group, the end carbon atom of the chain is oxidized by CrO2Cl2 to the —CHO group.
- By Gattermann Koch reaction: Benzene or its derivatives are converted to benzaldehyde or substituted benzaldehyde by treatment with carbon monoxide and hydrogen chloride in the presence of anhydrous AlCl3 or cuprous chloride.
This reaction is known as the Gattermann Koch reaction.
From hydrocyanic acid
Aldehydes can be prepared from hydrocyanic acid on treatment with Grignard reagent and subsequently followed by hydrolysis gives an aldehyde.
HC≡ N + CH3MgBr → CH3—CH=NMgBr → CH3CHO + NH3 + Mg(OH)Br
Hydrocyanic Acid Addition Product Acetaldehyde
Phenolic aldehydes are obtained by treating the phenol with chloroform in an aqueous sodium hydroxide solution at about 343 K. This reaction is called the Reimer-Tiemann reaction.
Preparation of Ketones
From acyl chlorides
Ketones can be prepared from acyl chlorides by treating with dialkyl cadmium. The dialkyl cadmium needed for this purpose is prepared by the reaction between the Grignard reagent and anhydrous cadmium chloride.
2RMgX (Grignard reagent) + CdCl2 → R2Cd (Dialkyl cadmium) + 2Mg(X)Cl
2CH3CH2MgBr (Grignard reagent) + CdCl2 (Cadmium chloride) → (CH3CH2)2Cd (Diethyl Cadmium) + 2MgBrCl
From nitriles by Grignard reagent
Both aliphatic and aromatic ketones can be prepared by treating an alkyl or aryl nitrile with a suitable Grignard reagent followed by acid hydrolysis.
1-Phenylpropanone can also be prepared by the action of ethyl magnesium bromide on benzonitrile followed by acid hydrolysis.
From benzene or substituted benzene by Friedel Crafts reaction
Aromatic ketones can be prepared by Friedel Crafts acylation or benzoylation by treating aromatic hydrocarbons with acid chlorides in the presence of a Lewis acid like anhydrous Aluminium chloride.
Friedel Crafts reaction is a typical example of an electrophilic substitution reaction. Benzophenone can also be prepared by Friedel Crafts’ reaction of carbonyl chloride with an excess benzene.
From phenyl esters by Fries rearrangement
Phenolic esters or phenyl esters on heating with anhydrous AlCl3 in the presence of CS2 as solvent undergo a rearrangement in which the acyl group migrates from the phenolic oxygen atom to the o- and p-positions of the benzene ring to give a mixture of o- and p-phenolic ketones.
This reaction is called Fries rearrangement.
Question 1: In the preparation of acetaldehyde from ethyl alcohol, it is distilled out as soon as it is formed. Explain.
Aldehyde is easily oxidizable to acetic acid. Therefore, to prevent its oxidation, it is distilled out as soon as it is formed.
Question 2: Aliphatic aldehydes do not show position isomers. Why?
In the case of aliphatic aldehydes, the —CHO group is always present at the end. Therefore, they do not show position isomerism.
Question 3: What is formalin?
Formalin is a 40% aqueous solution of formaldehyde.
Question 4: What reacts with aldehydes and ketones?
Aldehydes and Ketones react with primary amines to form a class of compounds called imines. An unshared pair of electrons on the nitrogen of the amine is attracted to the partial-positive carbon of the carbonyl group.
Question 5: What products are formed when aldehydes and ketones?
Most aldehydes and ketones react with 2°- amines to give products known as enamines. These are acid-catalyzed reversible reactions in which water is lost. enamines are easily converted back to their carbonyl precursors by acid-catalyzed hydrolysis.
Question 6: Are Aldehydes and Ketones acidic or basic?
Aldehydes and Ketones are hydrogen bond acceptors; this makes them have considerable solubilities in water. Ketones such as acetone are good solvents because they dissolve both aqueous and organic compounds. Acetone is a polar, aprotic solvent. Reactions with acids:- The carbonyl oxygen is weakly basic.
Question 7: Are aldehydes alcohols?
Alcohol with its —OH group bonded to a carbon atom that is bonded to no or one other carbon atom will form an aldehyde. Alcohol with its —OH group attached to two other carbon atoms will form a ketone.