Free radical hydrobromination of propargyl bromide and bromoallene

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Abstract
Rearrangement is observed in the free-radical hydrobromination of propargyl bromide. Between −20 and +30 °C in n-pentane or ether solution, 1,2-dibromopropene is the major product. A detailed stereochemical and kinetic study of the reaction indicates that the 1,2-dibromopropene is formed via bromoallene which results from the loss of a bromine atom from the first formed 1,3-dibromo-2-propenyl radical. Only 1/3 to 1/2 of the bromoallene diffuses away from its bromine atom partner. The remainder of the bromoallene recombines within the solvent cage with its bromine atom partner at a rate which is competitive with its rate of rotation with respect to the bromine atom. When the free radical hydrobromination of propargyl bromide is carried out at −78° in liquid hydrogen bromide, the 1,3-dibromo-2-propenyl radical can be trapped to a large extent by the hydrogen bromide to form cis-1,3-dibromopropene. The stereochemistry of this addition reaction >99% trans. A small amount of 1,2-dibromopropene which is 77% trans is also formed. Under the same conditions a 3:1 mixture of 1,2- and 1,3-dibromopropenes is produced from bromoallene. The 1,3-dibromopropene produced from bromoallene is >95% cis, while the 1,2-dibromopropene consists of a 49:51 cis:trans mixture of isomers.

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