Consider the alkenes (I) ethene, (II) propene, (III) 2-methylpropene and (IV) 3,3,3-trifluoropropene. The order of reactivity towards electrophilic addition of $\mathrm{HBr}$ (fastest to slowest) is

The slow step is protonation to a carbocation, so reactivity follows cation stability. 2-Methylpropene gives a tertiary cation, propene a secondary cation, ethene a primary cation. The strongly electron-withdrawing $\mathrm{CF}{}_3$ group destabilises the developing cation and also lowers the pi electron density, making IV the slowest.