Nitrile addition — carbocation trapping
Ritter reaction
How the Ritter reaction works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: t-BuOH (or 2° ROH / alkene), R–C≡N, conc. H₂SO₄. Conditions: Strong acid, 0–25 °C; aqueous work-up. Bonds formed: C–N; C=O (amide); bonds broken: C–O; C≡N π. Stereochemistry: Planar cation: stereochemistry lost. Rate and selectivity: Needs a stable carbocation (3°, benzylic).
At a glance
Step 1: Ionisation to the carbocation
A secondary or tertiary alcohol ionizes under strongly acidic conditions, generating a stabilized carbocation — the same first step as SN1.
Step 2: Nitrile N traps the cation
A nitrile's nitrogen lone pair, despite being a fairly weak nucleophile, is available and traps the carbocation, since nitriles tolerate the strongly acidic conditions that would destroy most other nucleophiles.
Step 3: Water adds; amide forms
Water adds to the resulting nitrilium ion and tautomerizes, giving an amide — a useful way to convert a tertiary alcohol directly into an amide without ever isolating a discrete amine intermediate.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
- SN1
- Unimolecular nucleophilic substitution. Two steps: leaving group departs (rate-determining) to give a carbocation, then nucleophile attacks.
How it compares with related reactions
| Ritter reaction | Acid-catalyzed hydration of an alkene | Acid-catalyzed hydrolysis of a nitrile | |
|---|---|---|---|
| Reagents | t-BuOH (or 2° ROH / alkene), R–C≡N, conc. H₂SO₄ | Dilute H₂SO₄ or H₃PO₄ (H₃O⁺), H₂O | Nitrile R–C≡N, H₃O⁺ (conc. HCl or H₂SO₄), H₂O |
| Conditions | Strong acid, 0–25 °C; aqueous work-up | 25–100 °C; heat and excess water favour the alcohol | Reflux, several hours |
| Stereochemistry | Planar cation: stereochemistry lost | Planar carbocation: racemic; Markovnikov regiochemistry | No new stereocentre; α-stereocentres retained |
| Rate / selectivity | Needs a stable carbocation (3°, benzylic) | Protonation of the alkene is rate-determining | Water attack on the protonated nitrile is rate-limiting; amide hydrolysis is the slower, forcing stage |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Ritter reaction?
t-BuOH (or 2° ROH / alkene), R–C≡N, conc. H₂SO₄. Conditions: Strong acid, 0–25 °C; aqueous work-up.
Which bonds are formed and which are broken?
Formed: C–N; C=O (amide). Broken: C–O; C≡N π.
What is the stereochemical outcome?
Planar cation: stereochemistry lost
What controls the rate or the selectivity?
Needs a stable carbocation (3°, benzylic)
Study guides that help with this reaction
- SN1, SN2, E1 or E2? How to decide which mechanism wins
- Nucleophiles, electrophiles and leaving groups: what makes a reaction go
Related mechanisms
- Acid-catalyzed hydration of an alkene
- Acid-catalyzed hydrolysis of a nitrile
- Br₂ addition to an alkene (vicinal dihalide)
- HBr adds across ethylene (Markovnikov)
- Halohydrin formation from an alkene + Br₂/H₂O
- Henry (nitroaldol) reaction
- Hydride adds to acetaldehyde's carbonyl
- LiAlH₄ reduction of a nitrile to a primary amine