Difficulty: Advanced | Prerequisites: Enolate chemistry, aldol reactions, electrophilic aromatic substitution
Conjugate (1,4) addition places a nucleophile at the beta carbon of an alpha,beta-unsaturated carbonyl, while direct (1,2) addition places it at the carbonyl carbon. The Michael addition is a specific type of conjugate addition using stabilised carbon nucleophiles (enolates, malonates). Robinson annulation combines a Michael addition with an intramolecular aldol to build a six-membered ring in one pot. Diazonium chemistry converts arylamines into versatile intermediates that can be transformed into aryl halides, nitriles, or azo dyes.
Conjugate addition (1,4-addition)
Addition of a nucleophile to the beta carbon of an alpha,beta-unsaturated carbonyl system. The nucleophile ends up at the beta position, and the carbonyl is preserved. Think of it as the nucleophile sneaking past the carbonyl to attack one carbon further along.
Direct addition (1,2-addition)
Addition of a nucleophile directly to the carbonyl carbon. The oxygen becomes an alkoxide, and the product is an alcohol (after protonation). This is the "normal" addition to a carbonyl.
Michael addition (Michael reaction)
A conjugate addition where the nucleophile is a stabilised carbanion, typically an enolate, malonate, or similar resonance-stabilised species. Named after Arthur Michael. The product is a 1,5-dicarbonyl compound.
Michael donor
The nucleophilic component in a Michael addition (the enolate or stabilised carbanion).
Michael acceptor
The electrophilic component: an alpha,beta-unsaturated carbonyl (an enone, enal, or alpha,beta-unsaturated ester).
Robinson annulation
A two-step, one-pot sequence that combines a Michael addition with an intramolecular aldol condensation. The result is a cyclohexenone ring fused or appended to the starting material. Named after Robert Robinson.
Diazonium salt (ArN2⁺)
Formed by treating a primary arylamine with NaNO2 and HCl at 0 C. The nitrogen-nitrogen triple bond makes this a good leaving group for substitution reactions.
Diazotisation
The process of converting a primary arylamine (ArNH2) into a diazonium salt (ArN2⁺) using nitrous acid (generated in situ from NaNO2 and HCl).
Sandmeyer reaction
Replacement of the diazonium group (N2⁺) with a halide (Cl, Br) or a cyano group (CN) using a copper(I) catalyst (CuCl, CuBr, or CuCN).
Diazo coupling
Reaction of a diazonium salt with an electron-rich aromatic ring (such as an aniline or phenol) to form an azo compound (Ar-N=N-Ar'). These are brightly coloured and used as dyes.
Schiemann reaction
Conversion of a diazonium salt to an aryl fluoride by treating with HBF4 to form the tetrafluoroborate salt, then heating to release N2 and BF3, leaving the aryl fluoride.
Addition polymer
A polymer formed by the chain addition of alkene monomers. Each monomer unit adds across the double bond without loss of atoms. To find the monomer, identify the repeating unit in the polymer backbone and restore the double bond.
Alpha,beta-unsaturated carbonyls have two electrophilic sites: the carbonyl carbon (C-1) and the beta carbon (C-4 of the conjugated system). Which site reacts depends on the nucleophile.
When does 1,4-addition win?
Stabilised ("soft") nucleophiles favour conjugate addition: enolates, cuprates (R2CuLi, Gilman reagents), amines, thiols, and cyanide.
Thermodynamic control (equilibrium conditions) tends to favour 1,4-addition because the product retains the strong C=O bond.
When does 1,2-addition win?
Strong, unstabilised ("hard") nucleophiles favour direct addition: Grignard reagents (RMgBr), organolithiums (RLi), hydride reagents (NaBH4, LiAlH4).
Kinetic control at low temperature can also favour 1,2-addition.
Why conjugate addition to enones works but not to simple alkenes (Q9 from the exam review):
Adding a nucleophile to a plain alkene like cyclohexene produces a carbanion with no resonance stabilisation. Alkyl carbanions are extremely unstable (conjugate acid pKa around 50). In contrast, adding the same nucleophile to an enone (alpha,beta-unsaturated ketone) gives a carbanion that is stabilised by resonance with the carbonyl, placing negative charge on oxygen. The resulting enolate is far more stable (pKa of conjugate acid around 13 to 20). This thermodynamic difference is why conjugate addition to enones proceeds readily while the same addition to simple alkenes does not.
The Michael addition is a conjugate addition where the nucleophile is a stabilised carbon nucleophile (an enolate, malonate enolate, or similar). The product is a 1,5-dicarbonyl relationship: two carbonyl groups separated by three carbons.
Retrosynthetic approach to Michael products:
To work backwards from a Michael addition product, look for a 1,5-dicarbonyl pattern. Then disconnect the bond between the alpha and beta carbons (the C-C bond formed in the reaction):
One fragment becomes the Michael donor (the enolate source): the carbon that was the alpha carbon of the enolate, with its attached carbonyl.
The other fragment becomes the Michael acceptor (the alpha,beta-unsaturated carbonyl): restore the C=C double bond between the alpha and beta carbons of this fragment.
From the exam review (Q11): An amine (H2N-) acting as the nucleophile in conjugate addition to an alpha,beta-unsaturated ketone. The nitrogen adds at the beta carbon. If the substrate also has the right geometry, an intramolecular reaction can close a ring, with nitrogen becoming part of the ring.
From the exam review (Q12): Four products are shown, and the task is to identify the starting materials. For each, find the 1,5-dicarbonyl pattern (or the bond formed by conjugate addition), disconnect it, and assign the two fragments as donor and acceptor. The retrosynthetic arrows point from product to starting materials.
Products from malonate or ester enolate donors use Claisen-type donors.
Products from ketone enolate donors use simple ketone enolates.
Products from cuprate (organocuprate, R2CuLi) donors appear when a simple alkyl group has been added at the beta position with no additional carbonyl on that fragment.
From the exam review (Q18): A 1,5-dicarbonyl compound with an ester group is identified as a Michael addition product (not self-aldol, Dieckmann, or Robinson annulation) because no ring has formed and the disconnection pattern matches a Michael donor and acceptor.
The Robinson annulation is a tandem reaction that builds a new six-membered ring (a cyclohexenone) in one pot. It combines two named reactions back to back:
Michael addition: A ketone enolate (the donor) adds in a 1,4-fashion to a methyl vinyl ketone or similar alpha,beta-unsaturated ketone (the acceptor). This forms a 1,5-diketone.
Intramolecular aldol condensation: Under the same basic conditions, the 1,5-diketone cyclises via an intramolecular aldol, then dehydrates to give a 2-cyclohexenone.
Retrosynthetic analysis of Robinson annulation products:
Given a cyclohexenone target, look for:
The enone within the six-membered ring (the C=C-C=O portion of the cyclohexenone).
Break the ring at the bond formed by the Michael addition (the bond between the alpha carbon of the donor and the beta carbon of the acceptor).
Also break the bond formed by the aldol (the bond that closed the ring).
One fragment is the ketone donor. The other is the alpha,beta-unsaturated ketone acceptor (with its C=C double bond restored).
From the exam review (Q13): Four cyclohexenone products are shown, and for each, the starting ketone and the enone (typically methyl vinyl ketone or a substituted variant) are identified by this disconnection approach. The ketone contributes the portion of the ring away from the enone, and the MVK-type fragment contributes the C=C-C=O side.
Forming the diazonium salt (diazotisation)
A primary arylamine (ArNH2) is treated with sodium nitrite (NaNO2) and hydrochloric acid (HCl) at 0 C. The nitrous acid generated in situ converts the amine to a diazonium salt (ArN2⁺ Cl⁻). The reaction must be kept cold because diazonium salts decompose above roughly 5 C.
Sandmeyer reaction
Once the diazonium salt is formed, treating it with a copper(I) salt replaces the N2⁺ group:
CuCl gives ArCl (aryl chloride)
CuBr gives ArBr (aryl bromide)
CuCN gives ArCN (aryl nitrile)
This is important because direct halogenation or cyanation of an aromatic ring is often difficult or gives the wrong regiochemistry. The Sandmeyer reaction lets you place the amine first (using electrophilic aromatic substitution), then swap it for the desired substituent.
Schiemann reaction (Balz-Schiemann)
Treating the diazonium salt with HBF4 gives the tetrafluoroborate salt (ArN2⁺ BF4⁻). Heating this salt decomposes it to the aryl fluoride (ArF), with loss of N2 and BF3. This is the standard way to put fluorine on an aromatic ring.
Removal of the amino group
Treating the diazonium salt with hypophosphorous acid (H3PO2) replaces the N2⁺ with H, effectively removing the amino group. This is useful when the amine was needed as a directing group for earlier substitution steps but is not wanted in the final product.
From the exam review (Q17): An arylamine with Cl and Br substituents is converted to the corresponding arene (amino group removed) using 1) NaNO2, HCl; 2) H3PO2.
Diazo coupling
A diazonium salt reacts with a very electron-rich aromatic ring (an aniline or a phenol, typically) in an electrophilic aromatic substitution. The product is an azo compound (Ar-N=N-Ar'), which is deeply coloured. Diazo coupling only works when the aromatic partner is sufficiently activated; ordinary benzene does not react.
From the exam review (Q15): A diazonium salt reacts with N,N-dimethylaniline to form an azo dye. The diazonium acts as the electrophile, and the electron-rich ring of the dimethylaniline is the nucleophilic partner.
From the exam review (Q14): A full sequence identifies products A through D. Starting from an arylamine: diazotisation gives the diazonium salt (A), Sandmeyer with CuCN gives the aryl nitrile (B), treatment with HBF4 gives the BF4 salt (C), and heating gives the aryl fluoride (D).
Addition polymerisation joins alkene monomers end to end by opening their C=C double bonds. No atoms are lost in the process.
Identifying the monomer from a polymer:
Look at the repeating unit in the polymer backbone. Each repeating unit corresponds to one monomer molecule. To find the monomer:
Identify the two-carbon repeat (the backbone carbons that were once doubly bonded).
Restore the C=C double bond between those two carbons.
Attach any substituents that appear on those carbons in the polymer.
From the exam review (Q16): A polymer with a phenyl (styrene-like) repeating unit is shown. The monomer is styrene (vinylbenzene, CH2=CH-C6H5), identified by finding the two-carbon repeat and restoring the double bond.
"Conjugate addition and direct addition give the same product." They do not. 1,2-addition gives an allylic alcohol; 1,4-addition preserves the carbonyl and places the nucleophile at the beta carbon. The products are structurally different.
"Any nucleophile can do a Michael addition." The Michael addition specifically requires a stabilised carbanion (enolate, malonate, etc.) as the donor. A Grignard reagent doing conjugate addition is not called a Michael addition in the strict sense.
"Robinson annulation is just a Michael addition." Robinson annulation is a Michael addition followed by an intramolecular aldol condensation. Both steps are required; the Michael alone gives an open-chain 1,5-diketone without the ring.
"Diazonium salts are stable at room temperature." Aryl diazonium salts decompose readily above 5 C and must be used immediately after preparation, typically at 0 C.
"Diazo coupling works with any aromatic ring." The aromatic partner must be strongly activated (an amine or phenol on the ring). Unactivated benzene does not react.
⚠️ Retrosynthetic disconnection of Michael addition products is a core exam skill. Given a 1,5-dicarbonyl, you must be able to identify the donor and acceptor fragments.
⚠️ Robinson annulation retrosynthesis: given a cyclohexenone, identify the two starting materials by disconnecting the ring at the Michael bond and the aldol bond.
⚠️ The comparison of conjugate addition to enones vs simple alkenes (Q9) tests whether you understand resonance stabilisation of the resulting carbanion. Expect a question asking which reaction is favoured and why.
⚠️ Diazonium chemistry is tested as a multi-step synthesis problem: amine to diazonium salt to various products. Know the reagents for each transformation (NaNO2/HCl for diazotisation, CuX for Sandmeyer, HBF4/heat for fluoride, H3PO2 for removal).
⚠️ Polymer-to-monomer identification is typically a quick one-mark question. Practise finding the repeating unit and restoring the double bond.
True or False: A Grignard reagent (RMgBr) typically gives 1,4-addition to an enone. (False, Grignard reagents favour 1,2-addition. Cuprates give 1,4-addition.)
Fill in the blank: The Michael addition produces a ______ dicarbonyl relationship. (1,5)
True or False: Robinson annulation builds a five-membered ring. (False, it builds a six-membered ring, a cyclohexenone.)
Fill in the blank: Treating a diazonium salt with CuCN gives an aryl ______. (nitrile / cyanide)
True or False: Diazo coupling requires an electron-rich aromatic partner. (True.)
Q: What reagent would you use to add a butyl group to the beta position of 2-cyclohexenone?
A: A lithium dibutylcuprate (Gilman reagent), (Bu)2CuLi. Cuprates are the standard reagent for conjugate addition of alkyl groups to enones.
Q: Given a 1,5-diketone product, how do you determine the Michael donor and Michael acceptor?
A: Find the 1,5-dicarbonyl pattern. The bond between the alpha carbon of one carbonyl and the beta carbon of the other is the bond that formed. Break it: the fragment that becomes the enolate is the donor, and the fragment where you restore a C=C conjugated with the carbonyl is the acceptor.
Q: Draw the two starting materials for a Robinson annulation that produces 2-methyl-2-cyclohexenone.
A: The donor is acetone (providing the methyl and the alpha carbon). The acceptor is methyl vinyl ketone (MVK). The Michael addition joins them, and intramolecular aldol condensation closes the ring.
Q: What products can be made from a diazonium salt using the Sandmeyer reaction?
A: Aryl chlorides (CuCl), aryl bromides (CuBr), and aryl nitriles (CuCN). Additionally, aryl fluorides via the Schiemann reaction (HBF4, then heat) and simple removal of nitrogen using H3PO2.
Q: Why does conjugate addition of a malonate enolate to cyclohexenone proceed readily, but the same addition to plain cyclohexene does not?
A: Addition to cyclohexene would produce an unstabilised carbanion (conjugate acid pKa around 50), which is essentially impossible to form. Addition to cyclohexenone produces an enolate stabilised by resonance with the carbonyl (conjugate acid pKa around 13 to 20), making the reaction thermodynamically feasible.
Conjugate addition and the Michael reaction extend enolate chemistry from earlier units. The Robinson annulation connects Michael addition to the aldol condensation, combining two major reaction types into one synthetic strategy. Diazonium chemistry ties back to electrophilic aromatic substitution (EAS) and amine chemistry: the amine is placed on the ring via EAS, then converted to other functional groups via diazonium intermediates. Polymer chemistry connects to alkene reactivity from earlier in the course.
Related Terms / Search Tags
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