Free-radical bromination of cyclopentane


In this note I describe bromination of cyclopentane according to patent US2342072.

Reaction plan

In order to increase yields, yet still perform a reasonably low number of distillations, I’m using a cyclopentane:bromine ratio of 3:1. This should result in yields of cyclopentyl bromide of approx. 60%.

The indicator of progress is evolution of hydrogen bromide. The indicator of completion is the loss of color.

First attempt (2017-08-06)

In a 250 ml round-bottom flask stopper I’ve mixed:

  • approx. 1 mole (70.1 g, 93.3 ml) of cyclopentane,
  • approx. 0.33 mole (52.7 g, 17 ml) of bromine.

The reaction proceeding to completion would evolve 0.33 mole of hydrogen bromide (approx. 7 L), so a gas washing flask was set up with a saturated solution of sodium thiosulphite, which would trap any acid as well as bromine vapor, and a silicone hose was used to direct the effluent into it. The hose, as expected, is not resistant to bromine, which penetrated the silicone, causing it to crack through the penetration depth, as well as permeating the entire thickness and causing a noticeable odor, however for the few hours that the reaction would proceed at most, it is satisfactory.

Trying to perform the reaction resulted in a series of failures.

First, it appears that the meager output of a 6 W UV tube I’ve used was completely absorbed by the soda-lime glass of the flask.

Second, while trying to relocate the apparatus so that sunlight could be used to initiate the reaction, I accidentally siphoned the gas washing solution in, which resulted in a quench, precipitation of a large quantity of sulfur, and the flask heating somewhat, still being OK to touch.


  • Two gas washing bottles must be used to form a trap, so that even if suction is generated on the reactor side, no washing solution enters it.
  • Sunlight (or, less preferably, a strong artificial UV source) should be used instead of weak artificial UV sources, such as banknote detectors.

Want to discuss this note? Drop me a letter.