account for the fact that small concentrations of Br₂ are conducive to allylic substitution, while high conentrations give addition.

Hi,

when the alkene is treated with Br2 there are two possibilities:

1.allylic substitution

2. addition

the intermediate formed during addition will be in equilibrium with the reactant alkene.

so if we provide the low concentrations of halogen the intermediate doesnt soon encounter a halogen molecule to complete the addition......the allylic radical on the othe rhand once formed has little option but to wait for a halogen molecule, whatever the temperature or however low the halogen concentration.

All the Best...