Nitrile Acid Hydrolysis

Nitrile Reactions: Acid Hydrolysis to Carboxylic Acid

Treatment of a nitrile (R-CN) with aqueous acid and heat converts the C≡N triple bond completely to a carboxylic acid (R-COOH) plus ammonium ion (NH4+). The mechanism proceeds through nitrile hydration and tautomerization to an amide, followed by amide hydrolysis: water adds to the protonated amide, which collapses after proton transfers to release ammonia and produce the carboxylic acid. This is a reliable, high-yielding transformation used to convert cyano groups installed via nucleophilic substitution into carboxylic acids.


Key Points

  • Full hydrolysis commonly uses heating. Heating aqueous acid often speeds conversion past the amide stage, although sufficiently reactive substrates can hydrolyze without heating.
  • Amide is an intermediate. The mechanism passes through a protonated amide (imidic acid → amide → tetrahedral intermediate → carboxylic acid). This two-stage hydrolysis is key to understanding the full mechanism.
  • Nitrogen leaves as NH4+. The nitrile nitrogen is released as ammonia, which is immediately protonated to ammonium ion under acidic conditions.
  • Compare to base hydrolysis. OH-/heat also hydrolyzes nitriles to carboxylic acids (as carboxylate salts), but the mechanism differs: base attacks directly rather than requiring protonation first.

Quick Summary

FeatureDetails
TransformsR-CN → R-COOH
ReagentsH3O+, heat (or conc. H2SO4/H2O, reflux)
ProductCarboxylic acid + NH4+
Key featureAmide intermediate; two hydrolysis stages
  • Reagents/conditions: Dilute or concentrated aqueous acid (HCl, H2SO4), reflux/heat.
  • Outcome: R-CN + 2 H2O + H+ → R-COOH + NH4+. The nitrile is fully hydrolyzed.
  • Intermediate: The reaction passes through an amide stage, which is not isolated when the reaction is taken to full hydrolysis.
  • Comparison: NaOH/heat also works (gives carboxylate salt); DIBAL-H stops at aldehyde; LiAlH4 gives primary amine.

Mechanism - Two-Stage Hydrolysis (7 Frames)

The first stage is hydration and tautomerization of the nitrile to an amide. The second is nucleophilic acyl substitution: water adds to the protonated amide, and the tetrahedral intermediate collapses after proton transfers to give the carboxylic acid.

Step 1: H3O+ protonates nitrile nitrogen
Step 1 - Protonate nitrile nitrogen. H3O+ protonates the nitrile nitrogen, placing a positive charge on N and making the nitrile carbon highly electrophilic.
Step 2: Water attacks electrophilic nitrile carbon
Step 2 - Water attacks nitrile carbon. A water molecule attacks the electrophilic nitrile carbon. One pi bond shifts to nitrogen, leaving a C=N double bond and forming an oxonium intermediate.
Frame 3: Proton transfers and tautomerization give protonated amide
Step 3 - Proton transfer (tautomerization). Solvent-mediated proton transfers and C=N/C=O bond rearrangement lead to the protonated amide shown in the next frame.
Step 4: Water attacks protonated amide carbonyl
Step 4 - Water attacks protonated amide. H2O attacks the electrophilic carbonyl carbon of the protonated amide. The pi bond shifts to oxygen, forming a tetrahedral intermediate.
Step 5: C-N bond breaks, NH3 leaves
Step 5 - C-N bond breaks; NH3 leaves. After proton transfers make nitrogen an NH3+ leaving group, the tetrahedral intermediate collapses: C-N breaks as NH3 leaves (protonated to NH4+ in solution), and C=O reforms to give the protonated carboxylic acid.
Step 6: Deprotonation of protonated carbonyl
Step 6 - Deprotonation. Water deprotonates the protonated carbonyl oxygen, neutralizing the positive charge and forming the carboxylic acid.
Frame 7: Final product - carboxylic acid
Frame 7 - Product. The final carboxylic acid product (R-COOH).

Mechanistic Checklist (Exam Focus)

  • Protonate first. Under acidic conditions, always show protonation of the nitrile nitrogen before water attacks.
  • Show the amide intermediate. The mechanism passes through an amide; recognize this as the halfway point.
  • Protonate the amide. The second hydrolysis requires protonation of the amide carbonyl oxygen.
  • Track nitrogen fate. NH2 becomes NH3+ (good leaving group) and departs as NH3, immediately protonated to NH4+ in acid.
  • Two hydrolysis stages. First: nitrile hydration/tautomerization → amide. Second: amide hydrolysis → carboxylic acid.

Worked Examples

Example A - Propanenitrile → Propanoic acid. Refluxing propanenitrile (CH3CH2CN) with dilute H2SO4 gives propanoic acid.
Example A reactant: propanenitrile
Reactant
Reagent: H3O+ heat
Reagent
Example A product: propanoic acid
Product
Example B - Benzonitrile → Benzoic acid. Benzonitrile hydrolysis gives benzoic acid. The aromatic ring is unaffected.
Example B reactant: benzonitrile
Reactant
Reagent: H3O+ heat
Reagent
Example B product: benzoic acid
Product
Example C - Mandelonitrile → Mandelic acid. The alpha-hydroxy nitrile (cyanohydrin) hydrolyzes to an alpha-hydroxy acid. The OH group survives acid conditions.
Example C reactant: mandelonitrile
Reactant
Reagent: H3O+ heat
Reagent
Example C product: mandelic acid
Product

Scope & Limitations

  • Works well: Simple alkyl nitriles, aryl nitriles, cyanohydrins (give alpha-hydroxy acids).
  • Functional groups tolerated: Aromatic rings, ethers, halogens (unless they undergo competing hydrolysis), alcohols.
  • Sensitive groups: Acetals, epoxides, and other acid-labile groups will be cleaved under these conditions.
  • Stereochemistry: Chiral centers alpha to the nitrile are generally retained (no epimerization at the alpha carbon).
  • Alternative: Base hydrolysis (NaOH/heat) also works and gives the carboxylate salt directly.

Edge Cases & Exam Traps

  • Stopping at amide: Milder conditions (lower temperature, shorter time) can stop at the amide. Full conversion to carboxylic acid often uses prolonged heating.
  • Cyanohydrin trap: Cyanohydrins (alpha-hydroxy nitriles) can revert to aldehyde + HCN under certain conditions. Under acidic hydrolysis with heat, they typically give alpha-hydroxy acids.
  • Base vs acid: NaOH/heat gives the carboxylate (requires acid workup for neutral carboxylic acid). H3O+/heat gives the carboxylic acid directly.
  • DIBAL-H confusion: DIBAL-H at -78 C stops at aldehyde. LiAlH4 gives primary amine. Only H3O+/heat or OH-/heat gives carboxylic acid.
  • Two-step synthesis: Alkyl halide + NaCN → nitrile; then H3O+/heat → carboxylic acid. This is a common one-carbon homologation sequence.

Basic nitrile hydrolysis and the workup endpoint

Aqueous hydroxide can hydrolyze a nitrile through an amide. Heating commonly makes the conversion practical. Unlike the acidic pathway above, the first nucleophile is OH− and the nitrile need not be protonated first.

  1. Hydroxide attacks the nitrile carbon; one C≡N pi pair moves to nitrogen.
  2. Proton transfer from water gives a hydroxy imine. Proton transfers and tautomerization produce the amide, without removing nitrogen.
  3. Hydroxide adds to the amide carbonyl. Tetrahedral collapse and proton transfer give the carboxylate and ammonia.

Benzonitrile therefore gives benzoate in base. A separate acid workup gives benzoic acid and protonates ammonia to ammonium. The nitrile carbon becomes the carboxyl carbon: neither stage shortens the carbon skeleton. See the base amide-hydrolysis mechanism for the second part.

benzonitrile gives benzoate; OH-, water, heat.
Net organic transformation; reagents and product accounting are described above.

Practical Tips

  • Full conversion: Reflux for several hours in dilute H2SO4 or HCl to ensure complete hydrolysis past the amide stage.
  • Product isolation: Keep the mixture sufficiently acidic to extract the neutral carboxylic acid into an organic solvent; if it has been made basic, reacidify before extraction.
  • Monitor by TLC: The amide intermediate can be detected; continue heating until it disappears.
  • Scale considerations: Large-scale reactions may require longer reflux times due to heat transfer limitations.

Exam-Style Summary

R-CN + H3O+/heat → R-COOH + NH4+. The mechanism first hydrates and tautomerizes the nitrile to an amide, then hydrolyzes the amide to the carboxylic acid. Key intermediates are the protonated nitrile, imidic acid/amide, and tetrahedral intermediate. Nitrogen is expelled as ammonia (protonated to NH4+).


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