Multi-Step Synthesis & Retrosynthetic Analysis
Disconnection approach, synthons, functional group interconversions, protecting groups, and route planning
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🧠 Multi-Step Synthesis & Retrosynthetic Analysis
E. J. Corey's retrosynthetic logic: work backward from the target, applying disconnections to reveal simpler precursors and recognized synthons.
Core moves
- Disconnection — break a strategic bond; ⇒ arrow indicates "is made from".
- Synthons — idealized cation/anion fragments (acyl cation = RCO⁺, enolate = ⁻CH₂COR).
- Synthetic equivalents — real reagents that deliver a synthon (acyl chloride for RCO⁺, lithium enolate for the α-anion).
- Functional Group Interconversion (FGI) — rewrite a hard-to-disconnect FG into one that disconnects cleanly.
Strategic bonds to target first
- Bonds α,β to a carbonyl (aldol/Michael disconnections)
- Bonds to a ring (Diels-Alder, EAS, intramolecular aldol)
- C–heteroatom bonds (SN2, reductive amination, Williamson)
Protecting groups — when to use and remove acetals (carbonyls), TMS/TBS ethers (alcohols), Boc/Cbz (amines), and the principle of orthogonal protection.
Convergent vs linear synthesis — convergent routes (separate fragments joined late) carry far higher overall yield than long linear sequences.
📚 Practice Problems
1Problem 1hard
❓ Question:
Provide a retrosynthetic analysis and a forward synthesis of 2-phenylbutan-2-ol starting from benzene and any 2-carbon and 3-carbon building blocks.
💡 Show Solution
Retrosynthesis:
- Target: PhC(OH)(CH₃)(CH₂CH₃).
- Tertiary alcohol with two C–C disconnections at the central carbon ⇒ ketone + organometallic. Best disconnection: C(α)–C(Ph) bond, treating PhMgBr as the synthetic equivalent of Ph⁻ that attacks butan-2-one.
- Butan-2-one ⇐ from 2-butanol oxidation; 2-butanol ⇐ hydration of 1-butene or from acetaldehyde + ethyl Grignard.
- PhMgBr ⇐ PhBr + Mg/Et₂O; PhBr ⇐ benzene + Br₂/FeBr₃.
Forward synthesis:
- Benzene + Br₂ / FeBr₃ → bromobenzene.
- Bromobenzene + Mg, Et₂O → PhMgBr.
- PhMgBr + butan-2-one (commercially available) in Et₂O, then H₃O⁺ workup → 2-phenylbutan-2-ol.
Why this disconnection? It is convergent: it joins two pre-built fragments at the most strategic bond (the one whose disconnection produces simple, commercially available pieces), and it uses a single reliable C–C bond-forming reaction.
2Problem 2hard
❓ Question:
Synthesize 4-bromo-3-nitrobenzoic acid from toluene. State the reagents and the order of steps clearly, and explain why the order matters.
💡 Show Solution
Synthesis (order is critical):
- Bromination first: toluene + Br₂ / FeBr₃ → 4-bromotoluene (the methyl is an ortho/para director; para predominates by sterics).
- Nitration: 4-bromotoluene + HNO₃ / H₂SO₄ → 4-bromo-3-nitrotoluene. Both the methyl (o/p) and bromine (o/p, weakly deactivating) direct ortho to the methyl and meta to no useful effect — the position ortho to CH₃ and meta to Br is favored.
- Oxidation last: KMnO₄, hot, then H₃O⁺ → 4-bromo-3-nitrobenzoic acid. The methyl is oxidized to –COOH.
Why this order?
- If you oxidize first, –COOH becomes a strong meta-director and a deactivator → bromination/nitration would be slower and the regiochemistry would change.
- If you nitrate before brominating, –NO₂ on the ring deactivates EAS strongly and forces bromine meta to itself (wrong regiochemistry).
- Doing EAS while CH₃ is still on the ring keeps the ring activated and ortho/para directed; the final oxidation cleanly converts CH₃ → COOH without disturbing the substitution pattern.
⚠️ Common Mistakes: Multi-Step Synthesis & Retrosynthetic Analysis
Avoid these 3 frequent errors
🌍 Real-World Applications: Multi-Step Synthesis & Retrosynthetic Analysis
See how this math is used in the real world
📝 Worked Example: Stoichiometry — Limiting Reagent
mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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