Mechanism Lab Open the app

Radical conjugate addition

Giese reaction

How the Giese reaction 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: R–Br (or R–I), Bu₃SnH (cat. or stoich.), AIBN, electron-poor alkene. Conditions: Benzene or toluene, 80 °C. Bonds formed: C–C (R–Cβ); C–H (Cα); bonds broken: C–Br; Sn–H; C=C π. Stereochemistry: None at the radical stage. Rate and selectivity: Chain propagation; radical addition is rate-limiting.

At a glance

ReagentsR–Br (or R–I), Bu₃SnH (cat. or stoich.), AIBN, electron-poor alkene
Solvent / conditionsBenzene or toluene, 80 °C
Bonds formedC–C (R–Cβ); C–H (Cα)
Bonds brokenC–Br; Sn–H; C=C π
StereochemistryNone at the radical stage
Rate / selectivityChain propagation; radical addition is rate-limiting

Step 1: Tin radical abstracts bromine

++Bu₃Sn•BrBu₃SnBr1 · TIN RADICAL ABSTRACTS BROMINEtributyltin radicalRADICALtert-butyl bromideSUBSTRATEtributyltin bromideBYPRODUCTtert-butyl radicalC₄H₉RADICAL

A radical chain carrier (Bu₃Sn•, generated from AIBN and Bu₃SnH) abstracts a halogen atom from an alkyl halide, generating a nucleophilic carbon radical — the same halogen-abstraction logic used throughout radical chain chemistry.

Step 2: Radical adds to the β-carbon

+OOOO2 · RADICAL ADDS TO THE Β-CARBONtert-butyl radicalC₄H₉RADICALmethyl acrylateC₄H₆O₂ELECTROPHILEα-carbonyl radicalC₈H₁₅O₂RADICAL

This carbon radical adds to the β-carbon of a Michael acceptor — radical conjugate addition, mechanistically parallel to the ionic Michael addition covered much earlier, but using a radical nucleophile instead of an enolate.

Step 3: α-Radical abstracts H from Bu₃SnH

++OOBu₃SnHOOBu₃Sn•3 · Α-RADICAL ABSTRACTS H FROM BU₃SNHα-carbonyl radicalC₈H₁₅O₂RADICALtributyltin hydrideREDUCTANTmethyl 4,4-dimethylpentanoateC₈H₁₆O₂PRODUCTBu₃Sn•RADICALBu₃Sn• continues the chain; tolerates free OH groups.

The resulting alpha-radical (stabilized by the adjacent carbonyl) abstracts a hydrogen atom from Bu₃SnH, giving the 1,4-addition product and regenerating Bu₃Sn• to continue the chain — this radical variant tolerates functional groups (like free alcohols) that would derail an ionic enolate under basic conditions.

Abbreviations and terms

R
R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
Bu3SnH
Tributyltin hydride. Radical H-atom donor (weak Sn–H bond).
AIBN
Azobisisobutyronitrile. Radical initiator: on heating releases N₂ and two 2-cyano-2-propyl radicals.

How it compares with related reactions

Giese reactionMeerwein arylationKharasch addition (peroxide-mediated HBr addition)
ReagentsR–Br (or R–I), Bu₃SnH (cat. or stoich.), AIBN, electron-poor alkeneArN₂⁺ X⁻, activated alkene (e.g. methyl acrylate), cat. CuCl₂ / Cu(I)HBr, ROOR (peroxide initiator) or light, alkene
ConditionsBenzene or toluene, 80 °CAcetone/H₂O, 20–40 °CHeat or hν, non-polar solvent
StereochemistryNone at the radical stageRacemic α-chloro centreAnti-Markovnikov; radical chain
Rate / selectivityChain propagation; radical addition is rate-limitingRadical chain; electron-poor alkenes work bestChain 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 Giese reaction?

R–Br (or R–I), Bu₃SnH (cat. or stoich.), AIBN, electron-poor alkene. Conditions: Benzene or toluene, 80 °C.

Which bonds are formed and which are broken?

Formed: C–C (R–Cβ); C–H (Cα). Broken: C–Br; Sn–H; C=C π.

What is the stereochemical outcome?

None at the radical stage

What controls the rate or the selectivity?

Chain propagation; radical addition is rate-limiting

Study guides that help with this reaction

Related mechanisms

Step through it interactively, then test yourself in Quiz