← Labs|Organic Synthesis — Multi-step Pathways with Mechanism

Synthesis Routes

Retrosynthesis

• Target → disconnect bonds → synthons

• Synthon = theoretical fragment (acyl cation, enolate)

• Synthon → synthetic equivalent (real reagent)

• Work backwards from target to available starting materials

• Priority: disconnect at functional groups, use reliable reactions

Benzene → Aniline

Start: Benzene · Target: Aniline (C₆H₅NH₂) · 2 steps

Benzene

HNO₃ + H₂SO₄ (conc.)

→

50°C, controlled

Fe/HCl or Sn/HCl

→

Heat, then NaOH

Step 1: EAS — Nitration

C₆H₆→C₆H₅NO₂

Reagent: HNO₃ + H₂SO₄ (conc.)

Conditions: 50°C, controlled

H₂SO₄ activates HNO₃ → NO₂⁺ (nitronium ion). Strong electrophile attacks ring.

JEE Key Points for Benzene → Aniline

• Nitration: EAS via NO₂⁺. Rate-determining step = Wheland intermediate formation

• NH₂ group of aniline activates ring strongly → o/p director

• Cannot nitrate aniline directly in HNO₃/H₂SO₄ (oxidizes NH₂ to NO₂)

• Acetylation of aniline first (protection), then nitrate, then hydrolyse

General Synthesis Strategies for JEE

Functional group interconversion

OH → X → CN → COOH → ester → alcohol (chain)

C-C bond forming reactions

Aldol, Grignard, Reformatsky, Diels-Alder, Wittig

Aromatic substitution order

EAS before adding electron-withdrawing groups. Protect NH₂ as amide before nitration.

Retrosynthetic symbols

⇒ means "comes from". Disconnection at weakest/most accessible bond.