A symmetrical alkene is one like ethene where the groups at both ends of the carbon-carbon double bond are the same. The reaction happens at room temperature in the presence of organic peroxides or some oxygen from the air. Alkenes react very slowly with oxygen to produce traces of organic peroxides – so the two possible conditions are equivalent to each other.

The reaction is a simple addition of the hydrogen bromide. For example, with ethene:

With a symmetrical alkene you get exactly the same product in the absence of the organic peroxides or oxygen – but the mechanism is different.