[kevinklee2003]

An energy diagram for the dehydration of a Tertiary Alcohol is given.
The reaction is acid catalyzed (heated with H2SO4).

The question asks what Position B represents. Position B is the first stable intermediate that forms.

The answer is that Position B (show below in the attachments) is the protonated alcohol and NOT the carbocation intermediate.

I understand why it would make more sense for the next intermediate, which is at a much higher energy level (more unstable) to be the carbocation intermediate, but what completely threw me off here was that I thought that since the reaction was Acid Catalzyed, wouldn't include the catalyzed alcohol.

Most energy diagrams showing the enzyme catalyzed form do not include the catalyzed intermediate like here for example: http://guweb2.gonzaga.edu/faculty/cronk/CHEM240/images/reaction_coordinate_catalyst.gif

How do you know when the energy diagram is showing the more general energy diagram for the catalyzed reaction ,or the more specific one like the one in the problem?

[orgopete]

If you were to compare the two energy diagrams, which step(s) will represent the activation energy? Will the enzyme step be specific to the actual step? Here, I suggest you think about the principle as the actual mechanism of an enzyme catalyzed reaction may (must?) differ from a similar reaction that may be performed in the lab.

[kevinklee2003]

I see. The activation energy would be the difference between the highest peak and the initial starting energy of the reaction.

The enzyme would not be specific for that step.

So am I right in sumrising that the energy diagrams I am used to that show just one hump are really just a summary to show the activation energy and really there can be/are multiple steps involved? it's just that the most energetic step is the step with the highest energy transition state is what determines the activation energy?

So it's all the steps in between the initial state and the highest energy step.

And enzymes always bring the reaction through a different mechanism, i beleive?

[orgopete]

I agree with the principles described, the activation energy is the greatest hump that must be crossed.

I don't know whether an enzyme must operate by a different mechanism. Certainly some do. I think others my achieve the same net result by an entropic effect. If an enzyme were to align reactants, it could increase the rate and have the same result as reducing the activation energy as enthalpy and entropy are related. I don't know these facts, so perhaps someone more knowledgeable about enzyme thermodynamics could give a better answer.