Retrosynthesis: how to plan a multi-step synthesis backwards
Work backwards from the target. Find a bond you know how to make, break it on paper (a disconnection), and write the two idealised fragments (synthons) — one nucleophilic, one electrophilic. Replace each synthon with a real reagent that behaves that way, then repeat on the simpler pieces until you reach available starting materials. Reverse the whole thing and you have the forward synthesis.
Why backwards is easier than forwards
Working forwards, every intermediate offers a dozen plausible reactions and you have no way to tell which leads anywhere. Working backwards, the target is fixed, and each bond in it either corresponds to a reaction you know or it does not. The search space collapses.
The notation is a double-lined arrow (⇒) meaning "can be made from". It is not a reaction arrow — it points from product to precursor, the opposite of the eventual synthesis.
The three words
- Disconnection — breaking a bond on paper, in the retrosynthetic direction.
- Synthon — an idealised charged fragment left by the disconnection. Usually one is a carbanion (nucleophilic, "d") and one a carbocation (electrophilic, "a"). Synthons are conceptual; you would not put them in a flask.
- Reagent equivalent — the real, bottle-able compound that behaves like that synthon. A Grignard reagent is the equivalent of a carbanion; an alkyl halide is the equivalent of a carbocation; a carbonyl carbon is an electrophile equivalent too.
The polarity rule. A useful disconnection produces one nucleophilic and one electrophilic synthon. If both fragments come out nucleophilic, that bond cannot be made that way — disconnect somewhere else.
Where to cut: the good disconnections
Disconnect at a bond you already know a reliable reaction for. In a two-semester course, that is a short list.
| Bond in the target | Forward reaction that makes it | What you need |
|---|---|---|
| C–C next to an alcohol | Grignard + aldehyde or ketone | RMgBr and a carbonyl |
| C–C next to a nitrile | SN2 with cyanide | Primary alkyl halide and NaCN |
| C–C between α and β carbons of an enone | Aldol condensation | Two carbonyls, one enolisable |
| C–C on an aromatic ring | Friedel–Crafts acylation, then reduce | Acid chloride and AlCl₃ |
| C–O of an ether | Williamson ether synthesis | Alkoxide and a primary halide |
| C–O of an ester | Fischer esterification or acid chloride + alcohol | Acid or acid chloride, plus alcohol |
| C–N of an amine | Reductive amination | Carbonyl, amine and a hydride source |
Prefer a disconnection that cuts the molecule near the middle. Two medium fragments are usually easier to source than one large piece and a methyl group.
Functional group interconversion
Not every step makes a bond. Often you need to change one group into another so that a disconnection becomes available — an FGI. The common ones are worth knowing cold:
- Alcohol → aldehyde or ketone (PCC), or → carboxylic acid (Jones, KMnO₄)
- Alkene → alcohol (hydration, Markovnikov or anti-Markovnikov, your choice of reagent)
- Alkyl halide → alkene (E2) or → alcohol (SN2 with hydroxide)
- Ketone → alkane (Clemmensen or Wolff–Kishner) — the standard fix after a Friedel–Crafts acylation
- Nitrile → carboxylic acid (hydrolysis) or → amine (reduction) — a one-carbon extension either way
A worked plan: 1-phenylpropan-1-ol from benzene
Target: PhCH(OH)CH₂CH₃.
- Spot the alcohol. A secondary alcohol with two different groups on the carbinol carbon is the signature of a Grignard addition to an aldehyde. Disconnect one of the two C–C bonds at that carbon.
- Choose which side. Cutting the bond to the ethyl gives ethyl carbanion (⇒ EtMgBr) plus benzaldehyde. Cutting the bond to the ring gives phenyl carbanion (⇒ PhMgBr) plus propanal. Both are legitimate; take the first, since benzaldehyde is a standard bottle reagent.
- Now get benzaldehyde from benzene. An aldehyde directly on a ring is awkward, so run an FGI: benzaldehyde ⇐ benzyl alcohol ⇐ ... or, more simply, acylate and adjust. For a clean two-step course answer, disconnect instead as PhMgBr + propanal, and make PhMgBr from bromobenzene, which comes from benzene by bromination.
Forward synthesis: benzene + Br₂/FeBr₃ → bromobenzene; + Mg in dry ether → phenylmagnesium bromide; + propanal, then H₃O⁺ → 1-phenylpropan-1-ol.
Notice how the second disconnection was chosen because it made the starting material reachable, not because it was more elegant. That is the whole skill.
Mistakes that sink synthesis questions
- Disconnecting a bond you cannot actually make. Every disconnection must correspond to a real forward reaction. If you cannot name it, do not draw it.
- Two nucleophilic synthons. Check the polarity of both fragments before you commit.
- Ignoring the order of aromatic substitutions. The first group directs the second. Get the order wrong and the product is the wrong isomer, or the reaction fails entirely. See the EAS guide.
- Forgetting to protect. A Grignard destroys any free OH, NH or COOH in the molecule. Protect the carbonyl as an acetal, or the alcohol as a silyl ether, and deprotect afterwards.
- Counting carbons only at the end. Count them at every step. A synthesis that quietly gains or loses a carbon is wrong no matter how good the reasoning looked.
- Stopping at the first plan. Generate two disconnections and compare. The better one usually needs fewer FGIs.
How to practise
Take any molecule from a later chapter of your textbook and ask one question: which bond here do I know how to make? Then disconnect it and see what you are left with. Do that ten times and the pattern recognition starts arriving faster than the rules.
How Organic Chemistry AI helps here
Retrosynthesis is where a single missed constraint — a directing effect, an unprotected alcohol, a carbocation rearrangement — invalidates an entire route. The tutor will take a question about a proposed route and talk through whether a step survives, and Check My Homework will take your own synthesis and give a verdict with feedback rather than just handing you a different answer. The bundled study library includes retrosynthesis and protecting groups among its 39 chapters.
Check My Homework takes the route you drafted and gives a verdict with feedback — more useful on a synthesis than simply being shown a different answer.
Frequently asked
What is the difference between a synthon and a reagent?
A synthon is the idealised charged fragment a disconnection leaves behind — a carbanion or a carbocation. A reagent is the real compound that behaves like it. The carbanion synthon 'CH₃CH₂⁻' has the reagent equivalent ethylmagnesium bromide; you would never put a bare carbanion in a flask.
How do I decide where to disconnect first?
Look for bonds next to functional groups, because those are the bonds your known reactions make. Prefer a cut near the middle of the molecule, prefer one that produces one nucleophilic and one electrophilic fragment, and prefer one whose fragments are commercially plausible.
When do I need a protecting group?
Whenever a reagent would attack a group you want to keep. The standard case is a Grignard or hydride in a molecule that also contains a free OH, NH or a second carbonyl you want untouched — protect the carbonyl as an acetal, run the reaction, then hydrolyse the acetal back in aqueous acid.
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