Final answer:
A low bromine concentration favors allylic substitution due to the reaction kinetics involving a stable allylic carbocation, while alkene addition typically requires higher concentrations of bromine to proceed effectively.
Step-by-step explanation:
When examining reactions involving bromine and organic compounds, the conditions of the reaction can favor different outcomes. A low bromine concentration tends to favor allylic substitution over alkene addition. This is because allylic substrates react by a SN1 mechanism, where the rate-determining step is the formation of a relatively stable allylic carbocation intermediate. This process does not require a high concentration of bromine because, once the carbocation is formed, it is relatively easy for a bromide ion to attack and complete the substitution.In contrast, the addition of bromine to an alkene to form a disubstituted alkyl halide involves a mechanism where the alkene reacts directly with a bromine molecule. This addition reaction, which is used as a test for the presence of alkenes due to the disappearance of the characteristic brownish color of bromine, requires a higher concentration of bromine to proceed efficiently.