Aniline Diazotization Hno2 H2so4

Aromatic Reactions: Diazotization of Aniline (HNO₂/H₂SO₄)

Aromatic Reactions: Diazotization of Aniline (HNO₂/H₂SO₄, 0–5 °C)

Diazotization converts anilines (Ar–NH₂) into aryl diazonium salts that become Sandmeyer, Schiemann, or azo intermediates. Nitrous acid is generated in situ from NaNO₂ and cold sulfuric acid, protonated to H₂NO₂⁺, and dehydrated to nitrosyl cation (NO⁺)—that electrophile, not HNO₂ itself, is what the amine attacks. Nitrosation gives an N-nitrosamine that tautomerizes to the diazohydroxide, and proton-assisted dehydration removes water to furnish Ar–N₂⁺ HSO₄⁻. Everything stays between 0 and 5 °C to prevent the diazonium from hydrolyzing to phenol or ejecting N₂ prematurely.

Key Emphasis (Teaching Pivots)

Electrophile clarity: Nitrosyl cation (NO⁺), generated from HNO₂ + H₂SO₄, is the species attacked by aniline—never draw direct attack on neutral HNO₂.
Sequence discipline: NO⁺ formation → N-nitrosation (diazohydroxide) → protonation/dehydration → cold storage of Ar–N₂⁺. Skipping a frame hides a mechanistic requirement.
Temperature lock: Keep everything at 0–5 °C. Warming accelerates hydrolysis to phenol or uncontrolled N₂ loss before the diazonium is captured.
Amine-class logic: Primary aryl amines diazotize. Secondary amines stall as nitrosamines, tertiary amines merely protonate—call those outcomes explicitly on exams.
Downstream mindset: Arenediazonium salts are transient reagents. Plan the follow-up (Sandmeyer, Schiemann, azo coupling, hydrolysis) before diazotizing.

Quick Summary

Reagents/conditions: NaNO₂ + H₂SO₄ (aq, cold) → HNO₂ → NO⁺; strict 0–5 °C; counter-ion is typically HSO₄⁻.
Outcome: Aniline (or another primary aryl amine) converts to the corresponding arenediazonium salt (Ar–N₂⁺ HSO₄⁻) ready for Sandmeyer, Schiemann, or azo chemistry.
Class rules: Primary aryl amines succeed; secondary amines form nitrosamines; tertiary amines do not generate diazonium; aliphatic diazonium ions collapse even cold.
Notes: Nitrosyl formation must precede nitrosation; proton shuttles deliver the diazohydroxide; protonation plus dehydration release water and lock in Ar–N₂⁺.

Mechanism — Diazotization of Aniline (8 Frames)

Step 1: Mixed acid generates NO⁺ — **Step 1 — Generate nitrosyl cation (NO⁺).** Cold H₂SO₄ protonates nitrous acid to H₂NO₂⁺, which ejects water to reveal the electrophile that aniline will attack.

Step 2: Nitrosation of aniline — **Step 2 — Nitrosation to the N-nitrosamine.** The aniline lone pair attacks H₂NO₂⁺/NO⁺; water departs and the N–NO linkage forms the classic N-nitrosammonium motif.

Step 3: Water pulls the first N–H — **Step 3 — Water removes the anilinium proton.** H₂O abstracts one N–H to begin forging the Ar–N=N linkage that will become the diazohydroxide.

Step 4: Bisulfate finishes the diazohydroxide — **Step 4 — Bisulfate (HSO₄⁻) pulls the second N–H.** The nitrosamine collapses to the diazohydroxide (Ar–N=N–OH) once both benzylic protons are removed.

Step 5: Hydronium reprotonates the diazohydroxide oxygen — **Step 5 — Hydronium restores –N=NOH.** Any diazohydroxide present as its conjugate base (N=N–O⁻) is reprotonated by H₃O⁺ under the acidic conditions.

Step 6: Sulfuric acid delivers a second proton to –N=NOH — **Step 6 — Sulfuric acid gives the second proton.** –N=NOH becomes –N=NOH₂⁺, turning the N–O bond into a better leaving group for dehydration.

Step 7 — Collapse to Ar–N≡N⁺. The aniline nitrogen donates into the N=N bond as the N–O bond breaks to oxygen, setting up loss of water and nitrogen.

Step 8: Cold stabilization of the diazonium salt — **Step 8 — Cold stabilization.** The final arenediazonium is paired with HSO₄⁻ and must stay at 0–5 °C until Sandmeyer, Schiemann, hydrolysis, or azo coupling consumes it.

Mechanistic Checklist

Make NO⁺ explicitly (HNO₂ + H₂SO₄ → H₂NO₂⁺ → NO⁺ + H₂O); do not shortcut to “HNO₂ attacks.”
Show the nitrosation event on nitrogen, followed by the diazohydroxide tautomer.
Protonate the –N=NOH fragment and remove water to reveal Ar–N₂⁺.
Keep the counter-ion (HSO₄⁻ here) and the temperature callout visible.
Mention that secondary amines stop as nitrosamines and tertiary amines do not diazotize.

Worked Examples

1. Aniline → Benzenediazonium Hydrogen Sulfate

NaNO₂/H₂SO₄ reagent button — NaNO₂ + H₂SO₄ (0–5 °C)

Benzenediazonium product — Benzenediazonium HSO₄⁻

Classic diazotization: cold NaNO₂/H₂SO₄ converts aniline to benzenediazonium hydrogen sulfate, ready for Sandmeyer halogenation or azo coupling.

2. p-Toluidine → p-Methylbenzenediazonium

Electron donation from the para methyl group accelerates nitrosation; the diazonium is still only stable cold and should be captured immediately.

3. m-Methoxyaniline → m-Methoxybenzenediazonium

m-Methoxyaniline reactant — m-Methoxyaniline

Even meta-directing substrates diazotize. Keep them cold, then hand the diazonium to Sandmeyer copper halides, Schiemann (HBF₄), or azo coupling partners.

Scope & Limitations

Works: Primary aryl amines (anilines, heteroaryl analogs) with at least one aromatic carbon bearing the amine.
Doesn’t: Aliphatic primary amines (diazonium collapses), secondary amines (stop at nitrosamines), tertiary amines (no diazonium).
Functional sensitivity: Strong acid is present; protect acid-labile functionality or switch to HCl/BF₄⁻ variants if needed.
Operational: Prepare HNO₂ in situ, maintain 0–5 °C throughout nitrite addition and hold until the follow-up reaction begins.

Edge Cases & Exam Traps

Phenol formation: Hydrolysis to phenol happens when aqueous mixtures warm—do not attribute phenol to the diazotization step itself.
Counter-ion swapping: Cl⁻ or BF₄⁻ exchanges are common to improve handling; they are not different mechanisms.
Over-nitrosation/azo coupling: Strongly activated rings can self-couple if you linger warm; keep cold or capture immediately.
Router logic: Secondary amines lead to nitrosamines, tertiary amines only protonate—state that explicitly when rejecting substrates.

Practical Tips

Add cold NaNO₂ solution slowly into the pre-chilled anilinium sulfuric acid mixture with vigorous stirring.
Use an ice–salt bath or refrigerated jacket to keep 0–5 °C; verify a slight excess of nitrosating agent with starch–iodide paper at the end.
Charge the nucleophile/catalyst (CuX, HBF₄, activated ring partner) before warming so the diazonium is captured as soon as temperature rises.
Keep everything in solution—dry arenediazonium salts are shock-sensitive, even as BF₄⁻.

Exam-Style Summary

HNO₂/H₂SO₄ (0–5 °C) protonates nitrous acid to H₂NO₂⁺, loses water to NO⁺, and nitrosates aniline at nitrogen. Proton transfers yield the diazohydroxide, protonation of –N=NOH activates water as a leaving group, and dehydration produces Ar–N₂⁺ paired with HSO₄⁻. Only primary aryl amines diazotize; warming leads to phenols or N₂ loss.

Interactive Toolbox

Use Mechanism Solver to animate each of the eight diazotization frames (NO⁺ formation, nitrosation, proton shuttles, dehydration prep, and cold storage) with temperature warnings baked in.
Use Reaction Solver to send the arenediazonium into Sandmeyer, Schiemann, hydrolysis, or azo-coupling branches and see router warnings for secondary/tertiary amines.
Use IUPAC Namer to confirm names for benzenediazonium salts and their downstream Sandmeyer or phenol products (still no SMILES in the learner-facing text).

Study Tools