13.9
Preparation of Carboxylic Acids: Overview
There are various methods for the preparation of carboxylic acids. For example, oxidation of primary alcohols or aldehydes using strong oxidizing agen…
Carboxylic acids can be prepared by oxidizing alkenes, alkynes, aldehydes, primary alcohols, or alkylbenzenes.
When alkenes react with hot basic KMnO4 or, undergo ozonolysis, they yield carboxylic acids.
Similarly, when alkynes are treated with concentrated KMnO4 or, O3 followed by H2O treatment, they undergo oxidative cleavage to form carboxylic acids.
In the case of terminal unsaturation, CO2 is released along with the acid that is formed.
1° alcohols and aldehydes are oxidized by strong oxidants such as Cr(VI) or Mn(VII) reagents, to generate carboxylic acids. Aldehydes can also be oxidized using mild reagents like Ag2O .
Aromatic carboxylic acids can be prepared by oxidizing alkylbenzenes, provided at least one benzylic hydrogen is present.
Here, the alkyl group is completely oxidized to give benzoic acid.
Further, the presence of non-oxidizable substituents results in substituted aromatic acids.
Alternatively, alkylbenzenes upon ozonolysis, followed by H2O2 treatment, leads to the conversion of the benzene ring to the carboxyl group.
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Q1: How do you prepare carboxylic acids from primary alcohols and aldehydes?
Primary alcohols and aldehydes are oxidized using strong oxidizing agents such as Cr(VI) or Mn(VII) reagents to generate carboxylic acids. Aldehydes can also be oxidized using milder reagents like Ag2O. These oxidation methods convert the alcohol or aldehyde functional group directly into a carboxylic acid.
Q2: What happens when alkenes are oxidized to form carboxylic acids?
Alkenes react with hot basic KMnO4 or undergo ozonolysis to yield carboxylic acids through oxidative cleavage of the double bond. The carbon-carbon double bond is broken and each carbon is oxidized to a carboxylic acid group, making this an effective method for preparing carboxylic acids from unsaturated starting materials.
Q3: How do internal and terminal alkynes differ in carboxylic acid preparation?
Internal alkynes treated with concentrated KMnO4 or ozonolysis undergo oxidative cleavage to yield carboxylic acids. Terminal alkynes follow the same process but release CO2 along with the acid formed. This difference occurs because terminal alkynes have a hydrogen on the triple bond carbon, which is oxidized away as carbon dioxide.
Q4: What is required for alkylbenzenes to be oxidized to aromatic carboxylic acids?
Alkylbenzenes must have at least one benzylic hydrogen present to be oxidized to aromatic carboxylic acids using KMnO4 or Na2Cr2O7 under vigorous conditions like heat and high oxidant concentration. The alkyl group is completely oxidized to yield benzoic acid or substituted aromatic acids depending on other substituents present.
Q5: What role do non-oxidizable substituents play in aromatic carboxylic acid synthesis?
Non-oxidizable substituents on alkylbenzenes remain intact during oxidation while the alkyl group is oxidized to a carboxylic acid. This allows preparation of substituted aromatic carboxylic acids where the substituent pattern is preserved. The oxidation selectively targets only the alkyl group, not other functional groups present on the benzene ring.
Q6: Can ozonolysis of alkylbenzenes produce carboxylic acids?
Yes, alkylbenzenes can undergo ozonolysis followed by H2O2 treatment, which leads to conversion of the benzene ring to the carboxyl group. This alternative method provides another route to aromatic carboxylic acids and demonstrates the versatility of ozonolysis in carboxylic acid preparation.
Q7: What oxidizing agents are most effective for converting aldehydes to carboxylic acids?
Strong oxidizing agents such as Cr(VI) or Mn(VII) reagents effectively oxidize aldehydes to carboxylic acids. Milder oxidizing agents like Ag2O can also be used for aldehyde oxidation. The choice of oxidant depends on the reaction conditions and compatibility with other functional groups in the molecule.