To determine which compound reacts most rapidly with HCl, we need to consider the nature of each compound in relation to carbocation stability and the mechanism of the reaction.
Among the given options, 2-methyl-2-hexene (b) will react the most rapidly with HCl. This is because it can form a highly stable tertiary carbocation, which promotes a faster reaction. When HCl adds to an alkene, it proceeds through a carbocation intermediate. The more stable the carbocation, the faster the reaction occurs.
In contrast, the other compounds, such as 5-methyl-1-hexene (a) and 4-methyl-1-hexene (e), would form less stable carbocations when reacting due to their primary and secondary nature, respectively. Therefore, they will react more slowly with HCl.
Overall, the reaction rate with HCl is greatly affected by the stability of the formed carbocation, making 2-methyl-2-hexene the compound that reacts most rapidly in this scenario.