Alkene Protonation Step: In this context, "alkene protonation" means adding a proton (H⁺) to an alkene (a molecule with a carbon-carbon double bond). This addition creates a carbocation (a positively charged carbon atom) as an intermediate.

Endergonic Reaction: An endergonic reaction is one that requires energy input to proceed; it’s not energetically favorable on its own. When an alkene is protonated, forming a carbocation intermediate, energy is required to reach this unstable, high-energy state.

Carbocation Stability and Transition State: When forming a carbocation, the reaction passes through a transition state, which is a high-energy state that comes just before the carbocation actually forms. The energy level of this transition state largely depends on how stable the resulting carbocation will be.

More Substituted Carbocations: A carbocation is more stable when it's more substituted (i.e., when the positively charged carbon is bonded to more alkyl groups). This is because alkyl groups help stabilize the positive charge through electron-donating effects.

Linking Stability to Transition State: Because a more substituted carbocation is more stable, the transition state leading to its formation is also more stable. This means it requires slightly less energy to reach this transition state compared to forming a less substituted, less stable carbocation.

So, in summary: Protonating an alkene to form a carbocation is an energy-requiring (endergonic) step. However, if the carbocation formed is highly substituted and stable, the transition state (which precedes the carbocation) will also be relatively stable, making it easier for the reaction to proceed in that direction.