Nitrobenzene Reduction Sn Hcl

Aromatic Reductions: Nitrobenzene → Aniline (Sn/HCl)

Aromatic Reactions: Reduction of Nitrobenzene to Aniline with Tin and HCl (Sn/HCl)

Tin shavings or powder in concentrated hydrochloric acid deliver the classic Sn/HCl (Béchamp-style) reduction. The nitro group adsorbs/protonates under strongly acidic conditions, then undergoes proton-coupled electron transfer (PCET) to nitrosobenzene, N‑phenylhydroxylamine, and finally the anilinium chloride salt. Only after consumption of Sn/HCl is the mixture basified (NaOH) to free the neutral aniline. Acidic media suppress azo/azoxy coupling, keep the amine protonated (so it cannot poison the reductant), and funnel oxygen away as water while tin oxidizes to SnCl₂/SnCl₄/SnO₂.

Key Emphasis (Teaching Pivots)

Intermediate map: Always show nitro → nitroso → N‑phenylhydroxylamine → anilinium (→ aniline after base). Skipping stages costs mechanism credit.
Acidic medium purpose: HCl protonates O/N, keeping reactive intermediates on-path and suppressing azoxy/azo/hydrazo coupling.
Reagent role: Sn⁰/SnCl₂ provides electrons; the nitro oxygen departs as water while tin oxidizes to SnCl₄/SnO₂.
Workup: The reaction mixture contains anilinium chloride. Only after filtration is NaOH added to liberate neutral aniline.

Quick Summary

Reagents/conditions: Excess tin (granules or powder) + conc. HCl, 20–70 °C; optional NaOH(aq) workup for the free base.
Outcome: Nitrobenzene → anilinium chloride (reaction medium) → aniline (after NaOH).
Mechanism class: Polar PCET at the metal/acid interface (closed-shell). No catalytic Pd/H₂ required.
Selectivity: Aryl halides typically survive; strongly acid-labile groups or base-sensitive protecting groups may require alternative routes.
Oxygen sink: The nitro oxygen atoms leave as water; tin is oxidized to SnCl₄/SnO₂ sludge.

Mechanism — Acidic Tin Reduction + Basic Workup (7 Frames)

Step 1: Sn/HCl activation of the nitro group — **Step 1 — Acidic tin activation.** Sn⁰ meets HCl to produce SnCl₂ while H₃O⁺ protonates the nitro oxygen, priming Ar–NO₂ for proton-coupled electron transfer.

Step 2 — Nitro → nitroso. Tin delivers electrons while acid supplies protons, ejecting water and giving nitrosobenzene bound to the surface.

Step 3: Hydride and proton delivery into the nitroso unit — **Step 3 — Hydride + proton delivery.** A formal hydride from tin plus protonation at oxygen hydrate Ar–N=O to phenylhydroxylamine (Ar–NHOH).

Step 4: Protonation of the hydroxylamine oxygen — **Step 4 — Prime the N–O bond.** HCl protonates the hydroxylamine oxygen, weakening the adjacent N–O bond for reductive cleavage.

Step 5: Tin-driven N–O bond cleavage — **Step 5 — Tin-driven cleavage.** Sn⁰/SnCl₂ injects electrons into the protonated N–O σ* orbital, expelling water and revealing an anionic nitrogen.

Step 6: Reprotonation of the anionic nitrogen — **Step 6 — Immediate reprotonation.** The N⁻ intermediate is quenched by HCl, furnishing anilinium (Ar–NH₃⁺) still in the acidic medium.

Step 7: NaOH workup to release aniline — **Step 7 — NaOH workup.** Aqueous NaOH deprotonates anilinium to the free-base aniline alongside tin hydroxide residues.

Mechanistic Checklist

Track every organic intermediate (Ar–NO₂ → Ar–NO → Ar–NHOH → Ar–NH₃⁺ → Ar–NH₂).
Depict PCET arrows: electrons originate from Sn/SnCl₂; protons arise from H₃O⁺/HCl; oxygen leaves as H₂O.
Show anilinium (acidic stage) before the NaOH workup. The free base only appears after the workup.
Mention that poorly controlled acid levels can divert Ar–NO and Ar–NHOH into azoxy/azo/hydrazo by-products.

Worked Examples

1. Nitrobenzene → Aniline (textbook Sn/HCl)

Sn / HCl reagent button — Sn, HCl (then NaOH)

Nitrobenzene is reduced to anilinium chloride in acid. Filtration and NaOH(aq) workup release the free aniline for downstream Sandmeyer or azo coupling steps.

2. p-Chloronitrobenzene → p-Chloroaniline

Aryl chlorides typically survive Sn/HCl, so this route is preferred when Pd/C hydrogenation might hydrogenolyze the C–Cl bond.

3. m-Dinitrobenzene → m-Phenylenediamine

Sn/HCl reagent — Sn/HCl, controlled heat

Both nitro groups reduce under Sn/HCl; staged tin additions and careful stirring prevent tarry by-products and ensure full conversion to the diamine.

Scope & Limitations

Works well: Most aromatic mono- and polynitro compounds; electron-poor rings simply require longer reflux or additional tin portions.
Functional-group tolerance: Aryl halides, ketones, and esters usually survive; acid-labile protections (acetals) or base-sensitive motifs (to be basified later) need planning.
Side reactions: Poor acid control or insufficient tin lets Ar–NO and Ar–NHOH condense to azoxy/azo/hydrazo compounds. Maintaining strongly acidic, reducing conditions suppresses these detours.

Edge Cases & Exam Traps

Reporting “aniline” before the base step—remember that the acidic stage furnishes anilinium chloride.
Omitting the nitroso or hydroxylamine intermediates in curved-arrow mechanisms.
Forgetting that azoxy/azo species appear only when the reaction mixture becomes neutral/basic or tin is exhausted.

Practical Tips

Charging: Suspend the nitroarene in conc. HCl, warm gently, then add tin in small portions to manage H₂ evolution and exotherm.
Stirring: Finely divided tin accelerates reduction; maintain vigorous agitation so fresh metal contacts the acidic solution.
Workup: Filter hot to remove tin oxides, rinse with hot acid, then cautiously add NaOH(aq) (pH > 10) to liberate the amine before extraction.
Safety: HCl fumes and nascent hydrogen demand a hood; dried tin residues can be pyrophoric—keep them wet until disposal.

Exam-Style Summary

Sn/HCl reductions march nitro groups through nitroso and N‑phenylhydroxylamine intermediates to anilinium chloride; a subsequent NaOH workup frees aniline. Acidic media prevent coupling sidetracks, and tin functions as the stoichiometric reductant while oxygen leaves as water.

Interactive Toolbox

Mechanism Solver — Use Mechanism Solver to see each Sn/HCl reduction frame with overlays and narrated curved arrows.
Reaction Solver — Use Reaction Solver to test substituted nitroarenes under Sn/HCl and catch halogen or over-reduction risks before lab day.
IUPAC Namer — Use IUPAC Namer to practice naming nitro starting materials, anilinium intermediates, and the liberated aniline.

Study Tools