Carboxylic Acids

Preparation of Carboxylic Acids

1. By oxidation of alkenes:

Alkenes can be oxidized to carboxylic acids with hot alkaline KMnO₄.



Alternatively, ozonides can be subjected to an oxidative workup that yields carboxylic acids.


RCH=CHR' RCO₂H + R'CO₂H



2. By oxidation of aldehydes and primary alcohols:

Aldehydes can be oxidized to carboxylic acids with mild oxidizing agents such as Ag(NH₃)⁺₂OH^- . Primary alcohols can be oxidized with KMnO₄.





3. By oxidation of alkylbenzenes:

Primary and secondary alkyl groups (but not 3^o groups) directly attached to a benzene ring are oxidized by KMnO₄ to a -CO₂H group.






4. By oxidation of methyl ketones:

Methyl ketones can be converted to carboxylic acids via the haloform reaction.




5. By hydrolysis of cyanohydrins and other nitriles:

Aldehydes and ketones can be converted to cyanohydrins, and these can be hydrolyzed to a-hydroxy acids. In the hydrolysis the -CN group is converted to a -CO₂H group.






6. By carbonation of Grignard reagents:

Grignard reagents react with carbon dioxide to yield magnesium carboxylates. Acidification produces carboxylic acids.






Reaction of Carboxylic Acids

1. As acids

RCO₂H + NaOH RCO₂^-Na⁺ + H₂O

RCO₂H + NaHCO₃ RCO₂^-Na⁺ + H₂O + CO₂



2. Reduction






3. Conversion to acyl chlorides

RCO₂H + SOCl₂ RCOCl + SO₂ + HCl

3RCO₂H + PCl₃ 3 RCOCl + H₃PO₃

RCO₂H + PCl₅ RCOCl + POCl₃ + HCl



4. Conversion to acid anhydrides





5. Conversion to esters






6. Conversion to lactones







7. Conversion to amides and imides








8. Conversion to lactams









9. a-Halogenation (The Hell-Volhard-Zelinsky Reaction)






10. Decarboxylation





11. Decarboxylation:

i) Kolbe electrolysis

an aqueous solution of the sodium or potassium salt of a carboxylic acid is subjected to electrolysis. At the anode the carboxylate ion loses an electron to become a carboxyl radical.




Then the carboxyl radical decarboxylates and the alkyl radicals that are produced combine to form an alkane.





ii) Hunsdiecker reaction

The silver salt of a carboxylic acid is heated with bromine in CCl₄. A carboxyl radical is produced in a two-step process as follows:



Then the carboxyl radical decarboxylates. The resulting alkyl radical abstracts a bromine atom from to produce an alkyl bromide and regenerate a carboxyl radical.


Reactions of Acyl Chlorides

1. Conversion to acids






2. Conversion to anhydrides





3.Conversion to esters






4. Conversion to amides






5. Conversion to ketones





6. Conversion to aldehydes





Reactions of Acid Anhydrides
1. Conversion to acids






2. Conversion to esters





3. Conversion to amides and imides









4. Conversion to ketones








Reactions of Esters


1. Hydrolysis





2. Conversion to other esters: transesterification








3. Conversion to amides






4. Reaction with Grignard Reagents




5. Reduction






Transesterification:
An alcohol is capable of displacing another alcohol from an ester. This alcoholysis (cleavage by an alcohol) of an ester is called trans esterification. Trans esterification is catalysed by acid (H₂SO₄ or dry HCl) or base (usually alkoxide ion). Trans esterification is an equilibrium reaction. To shift the equilibrium to the right, it is necessary to use a large excess of the alcohol.



The difference in the boiling points of the alcohols allows the equilibrium to be shifted toward the higher-molecular weight ester by distilling the methanol out of the reaction mixture.



Reactions of Amides

1. Hydrolysis







2. Conversion to nitriles: dehydration






3. Conversion to imides