13.10
Preparation of Carboxylic Acids: Hydrolysis of Nitriles
Nitriles (R–CN) can be converted into carboxylic acids (R–COOH) upon treatment with aqueous acids, i.e., upon hydrolysis of nitriles. Under base-catal…
Carboxylic acids can be prepared from nitriles via hydrolysis under aqueous acidic conditions. In base-catalyzed conditions, carboxylate anions are generated.
In acid-catalyzed hydrolysis, first, the nitrile nitrogen atom is protonated, rendering the cyano carbon atom more electrophilic.
This leads to an attack by the weak nucleophile—water. Next, deprotonation of the oxygen atom generates an imidic acid.
Then, the imidic acid tautomerizes to a more stable form, an amide, via two steps. The imidic nitrogen is first protonated to form a resonance stabilized protonated amide, which upon subsequent deprotonation, forms the amide.
Finally, the amide undergoes multi-step hydrolysis to generate the carboxylic acid.
Hydrolysis of nitriles is part of the two-step conversion of haloalkanes to carboxylic acids with introduction of an additional carbon to the starting haloalkane.
For example, ibuprofen—an anti-inflammatory drug—is synthesized via nitrile formation and its subsequent base hydrolysis.
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Q1: How does acid-catalyzed hydrolysis convert nitriles to carboxylic acids?
In acid-catalyzed hydrolysis, the nitrile nitrogen is first protonated, making the cyano carbon more electrophilic. Water then attacks this activated carbon. The resulting imidic acid tautomerizes to an amide through protonation and deprotonation steps. Finally, the amide undergoes multi-step hydrolysis to generate the carboxylic acid product.
Q2: What is the difference between acid-catalyzed and base-catalyzed nitrile hydrolysis?
Acid-catalyzed hydrolysis produces carboxylic acids through an imidic acid intermediate and amide formation. Base-catalyzed hydrolysis directly generates carboxylate anions instead. Both pathways convert nitriles to carboxylic acid products, but base conditions yield the deprotonated carboxylate form rather than the protonated acid.
Q3: How are haloalkanes converted to carboxylic acids using nitriles?
Haloalkanes are first converted to nitriles via SN2 reaction with sodium cyanide, introducing one additional carbon. The resulting nitrile is then hydrolyzed under aqueous acidic or basic conditions to yield carboxylic acids. This two-step process effectively extends the carbon chain while converting the halide to a carboxylic acid functional group.
Q4: What solvents are used in nitrile hydrolysis reactions?
Nitrile hydrolysis reactions use dimethyl sulfoxide, alcohols, or water as solvents to facilitate the aqueous hydrolysis conditions necessary for converting nitriles to carboxylic acids. The choice of solvent depends on reaction conditions and substrate compatibility. These solvents enable efficient nucleophilic attack and product formation.
Q5: Why are tertiary alkyl halides rarely used in nitrile synthesis for carboxylic acid preparation?
Tertiary alkyl halides are seldom used because the nitrile synthesis step relies on the SN2 mechanism, which requires nucleophilic attack at a primary or secondary carbon. Tertiary carbons are sterically hindered and favor SN1 pathways instead, making them unsuitable for efficient nitrile formation and subsequent carboxylic acid synthesis.
Q6: What is the role of the imidic acid intermediate in acid-catalyzed nitrile hydrolysis?
The imidic acid is a key intermediate formed after water attacks the protonated nitrile. It tautomerizes to a more stable amide form through protonation of the imidic nitrogen, creating a resonance-stabilized protonated amide. Subsequent deprotonation yields the amide, which then undergoes hydrolysis to form the final carboxylic acid.
Q7: How is ibuprofen synthesized using nitrile hydrolysis?
Ibuprofen is synthesized via nitrile formation followed by base-catalyzed hydrolysis. The nitrile intermediate is generated from a haloalkane precursor, then hydrolyzed under basic aqueous conditions to produce the carboxylic acid component of ibuprofen. This demonstrates the practical application of nitrile hydrolysis in pharmaceutical synthesis.