Number of Steps in Electrophilic Addition

[Milind_Vasudev_2H]

How do we know how many steps there are in the mechanism? For example in the course reader, when looking at the reaction between 2-Butene and HBr, how do we know that both H and Br are not bonded simultaneously but rather in two separate steps?

[Chem_Mod]

This is the nature of the SN1 addition reaction mechanism. It is a mix of knowing properties of your reagents, solvents, to determine whether or not they are likely to undergo a specific mechanism with different stages.
For that reaction, you have a nucleophile (butene), a proton donor (HBr) and then a nucleophile in the form of Br- that will attack the positive charge.