14.23
Amides to Carboxylic Acids: Hydrolysis
Amides can undergo either acid-catalyzed hydrolysis or base-promoted hydrolysis through a typical nucleophilic acyl substitution. Each hydrolysis requ…
Amides undergo hydrolysis in aqueous acid or aqueous base to give carboxylic acids. These reactions are slow and require heating.
Acid-catalyzed amide hydrolysis begins with the carbonyl oxygen protonation, followed by the nucleophilic addition of water to the carbonyl carbon, forming a tetrahedral intermediate.
Further, deprotonation of the tetrahedral intermediate and protonation of the nitrogen converts the amino group into a better leaving group.
Next, the carbonyl group is re-formed by eliminating ammonia. Final deprotonation yields a carboxylic acid.
Notably, ammonium ion, a weak acid, is formed as a by-product, driving the equilibrium towards products.
Basic hydrolysis of amides is another nucleophilic acyl substitution involving hydroxide as a nucleophile.
The mechanism begins with the nucleophilic attack of the hydroxide ion at the carbonyl carbon, forming a tetrahedral intermediate.
Subsequently, the carbonyl group is reconstructed with the departure of an amide ion.
Final deprotonation yields a carboxylate ion and ammonia. This step drives the reaction to completion, pushing the equilibrium towards the product.
Acidification of the carboxylate ion gives free acid.
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Q1: What happens to amides during acid-catalyzed hydrolysis?
Acid-catalyzed amide hydrolysis converts amides to carboxylic acids through nucleophilic acyl substitution. The carbonyl oxygen is first protonated, making the carbonyl carbon more electrophilic. Water then attacks this carbon, forming a tetrahedral intermediate. After deprotonation and nitrogen protonation, ammonia leaves as the leaving group, and final deprotonation yields a carboxylic acid product.
Q2: Why does ammonium ion formation drive acid-catalyzed amide hydrolysis forward?
Ammonium ion is a weak acid that forms as a by-product during acid-catalyzed amide hydrolysis. Its formation shifts the equilibrium toward carboxylic acid products, driving the reaction to completion. This thermodynamic advantage makes the overall hydrolysis process favorable despite the reaction being inherently slow and requiring heating.
Q3: How does base-promoted amide hydrolysis differ from acid-catalyzed hydrolysis?
Base-promoted amide hydrolysis uses hydroxide ion as the nucleophile instead of water. Hydroxide attacks the carbonyl carbon to form a tetrahedral intermediate. The amide ion then departs, and deprotonation yields a carboxylate ion and ammonia. This step drives the reaction to completion, and acidification converts the carboxylate ion to free carboxylic acid.
Q4: What role does the tetrahedral intermediate play in amide hydrolysis?
The tetrahedral intermediate is a key reactive species formed when a nucleophile attacks the amide carbonyl carbon. In acid-catalyzed hydrolysis, water creates this intermediate; in base-promoted hydrolysis, hydroxide does. The intermediate then undergoes deprotonation and nitrogen protonation before the carbonyl group reforms and the leaving group departs.
Q5: Why do amide hydrolysis reactions require heating?
Amide hydrolysis reactions are inherently slow and require heating to proceed at practical rates. The reaction demands severe conditions because breaking the strong carbon-nitrogen bond and forming the carboxylic acid product requires significant activation energy. Heat provides the necessary energy to overcome this kinetic barrier.
Q6: What is the leaving group in base-promoted amide hydrolysis?
In base-promoted amide hydrolysis, the amide ion is the leaving group that departs after the tetrahedral intermediate forms. This departure allows the carbonyl group to reform. The amide ion is a strong base and poor leaving group, but the basic conditions and the formation of ammonia as a neutral product make this step favorable.
Q7: How does acidification complete base-promoted amide hydrolysis?
Base-promoted amide hydrolysis produces a carboxylate ion and ammonia. Acidification of the carboxylate ion converts it to free carboxylic acid, completing the hydrolysis process. This final step is necessary to obtain the neutral carboxylic acid product from the ionic intermediate formed under basic conditions.