4

What is the rate determining step in the following energy profile? To clarify, the reaction is:

A -> B -> C

The energy of A is greater than B which in turn is also greater than C. The intermediate A-B is higher energy that the B-C intermediate but the energy gap between B and the intermediate B-C is greater than from A to the A-B intermediate. I hope this is clear.

Energy Profile

ruskind
  • 177
  • 1
  • 5
  • you do realise that you've answered your question. – DarrenRhodes Dec 21 '15 at 16:55
  • yes i was hoping to get some other opinion because the article on wikipedia has no reference and i have read some understandably more confused answers elsewhere. – ruskind Dec 21 '15 at 17:01
  • 1
    Try this ref if you have access to a library, J. J. Murdoch: What is the Rate-Limiting Step of a Multi-Step Reaction? J. Chem. Ed., 1981, vol. 58, p. 32. – DarrenRhodes Dec 21 '15 at 17:07
  • 1
    Here's a good starting point https://thewinnower.com/papers/mechanism-of-the-reduction-of-a-carboxylic-acid-by-borane-revisited-and-revised to get into the nuts and bolts of this thinking. – DarrenRhodes Dec 21 '15 at 17:15

2 Answers2

2

The second step is rate-determining. According to Wikipedia:

Given a reaction coordinate (energy diagram), the rate determining step can be determined by taking the largest energy difference between any starting material or intermediate on the diagram and any transition state that comes after it. That transition state will then be the rate-determining step of a given reaction. The transition state with highest absolute energy may not necessarily correspond to the rate determining step.

ruskind
  • 177
  • 1
  • 5
1

The second step is the rate determining step in the case that this is not an equilibrium reaction; however, if B can go back to A(therefore it is an equilibrium rxn) then it is easier for B to proceed to C instead of going back to A, right? The graph can go in both ways and then the first step will be the determining. It has the highest point of energy needed to be overcame.

Steven Razzouk
  • 19
  • 2
  • This doesn't seem right. As drawn, species A and B should not be in equilibrium given the lower barrier to C. A and B would be in a pre-equilibrium if the transition state for B->C were higher in energy than that from B->A, but in that case, we've firmly established that B->C is rate determining, which can be confirmed via the definition in the accepted answer. – Zhe Feb 26 '19 at 20:23
  • Well I never said that they are on equilibrium I was just saying that if this reaction was reversible then what I said above in the first part would apply – Steven Razzouk Feb 27 '19 at 21:37
  • I still do not think that is correct. If it were reversible, it could not have the reaction coordinate diagram shown in the OP's question based on the arguments in my first comment. – Zhe Feb 28 '19 at 00:07
  • I respect your opinion on what you think but I did confirm my answer with two different professors that I currently have just because of your skepticism... Thanks anyway for the feedback. – Steven Razzouk Mar 01 '19 at 01:12
  • Sigh... I'm trying not be offended here. This is not my first foray into physical organic chemistry, and it's not like I've never taught this material before... – Zhe Mar 02 '19 at 01:21