The relative rates of reaction of ethane, toluene, and ethylbenzene with bromine atoms have been measured.

the most reactive hydrocarbon undergoes hydrogen atom abstraction a million times faster than does the least reactive one.

arrange these hydrocarbons in order of decreasing reactivity.

Reactivity: Ethyl benzene > Toluene > Ethane

Explanation:

The order of reactivity depends on how easily the most reactive hydrogen can be abstracted.

The order of reactivity of hydrogen is : 1° < 2° < 3° < Benzyllic based on the stabilizing effects like inductive effect, hyperconjugation , resonance effect.

Hence the order of decreasing reactivity : Ethyl Benzene, Toluene , Ethane.