Mechanism Lab Open the app

Radical conjugate addition

Meerwein arylation

How the Meerwein arylation works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.

Open the interactive version

In short. A 3-step mechanism. Typical reagents: ArN₂⁺ X⁻, activated alkene (e.g. methyl acrylate), cat. CuCl₂ / Cu(I). Conditions: Acetone/H₂O, 20–40 °C. Bonds formed: Ar–C (new C–C), C–Cl; bonds broken: C–N (as N₂), C=C π, Cu–Cl. Stereochemistry: Racemic α-chloro centre. Rate and selectivity: Radical chain; electron-poor alkenes work best.

At a glance

ReagentsArN₂⁺ X⁻, activated alkene (e.g. methyl acrylate), cat. CuCl₂ / Cu(I)
Solvent / conditionsAcetone/H₂O, 20–40 °C
Bonds formedAr–C (new C–C), C–Cl
Bonds brokenC–N (as N₂), C=C π, Cu–Cl
StereochemistryRacemic α-chloro centre
Rate / selectivityRadical chain; electron-poor alkenes work best

Step 1: SET reduction of the diazonium

+SET−N₂+Cu⁺NNCu21 · SET REDUCTION OF THE DIAZONIUMcopper(I)Cu⁺REDUCTANTbenzenediazoniumC₆H₄N₂⁺OXIDANTaryl radicalC₆H₅RADICALcopper(II)Cu²⁺BYPRODUCTSame SET / N₂-loss logic as Sandmeyer.

Cu(I) transfers a single electron to an aryldiazonium salt, triggering loss of N₂ and generating an aryl radical — the same electron-transfer/N₂-loss logic as the Sandmeyer reaction, but the radical is trapped differently here.

Step 2: Aryl radical adds to the alkene

+OOOO2 · ARYL RADICAL ADDS TO THE ALKENEaryl radicalC₆H₅RADICALmethyl acrylateC₄H₆O₂ELECTROPHILEα-ester radicalC₁₀H₁₀O₂RADICALAdds to the CH₂ terminus, leaving the stabilised α-carbonyl radical.

Rather than abstracting a halogen atom, this aryl radical adds directly across an activated alkene (typically a Michael acceptor), forming a new C–C bond and a stabilized alpha-radical on the other carbon.

Step 3: Cl atom transfer from CuCl₂

++OOClCu(II)ClClOOClCu3 · CL ATOM TRANSFER FROM CUCL₂α-ester radicalC₁₀H₁₁O₂RADICALCuCl₂REAGENTmethyl 2-chloro-3-phenylpropanoateC₁₀H₁₁ClO₂PRODUCTCuCl (Cu(I))CATALYSTβ-aryl alkyl halide formed; Cu(I) re-enters step 1.

That radical abstracts a halogen atom from CuCl₂ (regenerating Cu(I) to continue the cycle), giving a β-aryl alkyl halide — a radical-chain way to arylate an alkene, complementary to the ionic Friedel-Crafts and Heck-type arylation strategies covered elsewhere.

Abbreviations and terms

Ar
Aryl group. Any aromatic ring substituent such as phenyl.
X
Halogen / leaving group. Cl, Br or I (sometimes any leaving group).
SET
Single-electron transfer. Movement of one electron (not a pair), generating radicals/radical ions.

How it compares with related reactions

Meerwein arylationGiese reactionKharasch addition (peroxide-mediated HBr addition)
ReagentsArN₂⁺ X⁻, activated alkene (e.g. methyl acrylate), cat. CuCl₂ / Cu(I)R–Br (or R–I), Bu₃SnH (cat. or stoich.), AIBN, electron-poor alkeneHBr, ROOR (peroxide initiator) or light, alkene
ConditionsAcetone/H₂O, 20–40 °CBenzene or toluene, 80 °CHeat or hν, non-polar solvent
StereochemistryRacemic α-chloro centreNone at the radical stageAnti-Markovnikov; radical chain
Rate / selectivityRadical chain; electron-poor alkenes work bestChain propagation; radical addition is rate-limitingChain propagation; H-abstraction from HBr is fast

Test yourself

Try to answer each question before opening it.

Which reagents and conditions are typical for the Meerwein arylation?

ArN₂⁺ X⁻, activated alkene (e.g. methyl acrylate), cat. CuCl₂ / Cu(I). Conditions: Acetone/H₂O, 20–40 °C.

Which bonds are formed and which are broken?

Formed: Ar–C (new C–C), C–Cl. Broken: C–N (as N₂), C=C π, Cu–Cl.

What is the stereochemical outcome?

Racemic α-chloro centre

What controls the rate or the selectivity?

Radical chain; electron-poor alkenes work best

Study guides that help with this reaction

Related mechanisms

Step through it interactively, then test yourself in Quiz