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Aldehydes and ketones are two chemical molecules with a carbonyl group in their molecular structure. A double bond exists between the carbon and oxygen atom in the carbonyl group. These molecules do not have reactive groups such as OH or Cl in the molecular structure, and hence these molecules are generally simple.
An aldehyde is a carbonyl compound with an alkyl group on one end and hydrogen on the other. The letters R and Ar stand for alkyl and aryl members, respectively. The aldehyde is expressed as –CHO in its condensed form.
Ketone is a carbonyl group member with an alkyl or aryl group on both ends of the carbonyl group. RC(=O)R’ is the compound formula. R and R’ are the distinct carbon-containing substituents in this scenario.
Preparation of Aldehydes and Ketones
From Alcohol
Primary alcohols are oxidised using various oxidising agents like sulphuric acid, potassium permanganate, acidified potassium dichromate, etc. to produce aldehydes.
RCH2OH (Primary Alcohol) + [O] → RCHO (Aldehyde) + H2O
Aldehydes are distilled to prevent further oxidation to carboxylic acids. All these reactions occur in a controlled environment.
Ketones can be made by oxidising secondary alcohols using similar oxidising agents.
Catalytic dehydrogenation of alcohols
Dehydrogenation of alcohols is a method generally used in industries for producing aldehydes and ketones. In this procedure, an alcohol vapour is passed over reduced metal catalysts such as copper at 573 K.
Primary alcohols are used to produce aldehydes, whereas secondary alcohol is used for ketones.
CH3CH2OH (Ethyl alcohol) → CH3CHO (Acetaldehyde) + H2
From Carboxylic Acids
Catalytic Decomposition – Catalytic decomposition of carboxylic acids is achieved by heating a mixture of methanoic acid or other acids to 573 K. It is generally done in the presence of a catalyst such as a manganous oxide to produce aldehydes and ketones.
CH3COOH (Ethanoic acid) + HOOCH (Methanoic acid) → HCHO (Methanal) + CO2 + H2O
Ketones can be made by passing fatty acid vapour over MnO at 573 degrees Celsius.
CH3COOH(Ethanoic acid) + C2H5COOH(Propanoic acid) → CH3COC2H5(Butan-2-one) + CO2+ H2O
Distillation of Calcium Salts – The calcium salts of acids can also be used to make aldehydes and ketones.
Ca(HCOO)2 (Calcium formate) → HCHO (Methanal) + CaCO3
However, due to the low yield, this process is not ideal for the synthesis of aldehydes. This is due to the formation of three products when a mixture of two calcium salts is heated.
From Hydrocarbons
The hydration of alkynes in the presence of oil. H2SO4 and HgSO4 as catalysts can produce aldehydes and ketones. When water is added to alkynes, unstable enol intermediates develop, which then recombine to produce aldehydes or ketones.
Ketones are formed when alkynes other than acetylene are hydrated.
Aldehydes and ketones are made from alkynes through hydroboration-oxidation reaction. Borane reacts with an alkyne to make vinyl borane, which produces aldehydes and ketones when oxidized with H2O2. A single ketone is produced by symmetrical non-terminal alkynes, whereas unsymmetrical non-terminal alkynes produce a mixture of both potential ketones, with methyl ketones predominating.
Aldehydes are formed from terminal alkynes. To circumvent problems caused by diborane’s double bond addition, bulky sterically hindered boranes such as bis (1,2-dimethyl propyl) borane, also known as disiamylborane are used.
Alkenes undergo ozonolysis when they react with ozone to make ozonide, which is then cleaved with zinc dust and water to produce aldehydes and ketones. Aldehydes are created if the carbon creating the double bond has an H-atom linked to it; otherwise, ketones are formed.
Walker’s Process
An acidified solution of palladium chloride in water is used for this process. The solution contains a small quantity of cupric chloride that acts as a catalyst in the presence of oxygen. The alkene can then be converted into aldehydes and ketones through this process.
CH3CH=CH2 (Propene) + PdCl2 + H2O → CH3COCH3 (Acetone) + Pd + 2HCl
Conclusion
Aldehydes and ketones are two simple chemical compounds with a carbonyl group. They are used in various industrial processes as a reagent, starting material, or for production. These chemicals can be produced from different agents such as alcohol, hydrocarbons, and carboxylic acids and are extremely important to study from the point of view of your exam.
