Slowly, Slowly, Catchee Monkey (or Rate Determining Step)

So, you have written your rate equation and the next question asks you to suggest a rate determining step.

The key phrase here is suggest, when ever you see this the examiner is telling you that there are lots of possible answers, you just have to come up with one of them. There is an implication in the word "suggest" that there is some educated guess work involved.

So what do you suggest?

Let's look at an example...

Here is an overall reaction



Here is the rate equation



Question - Suggest a rate determining step.

In this relatively simple example, the rate equation  tells me that the OH- isn't involved in the slow step. It is only the (CH3)3CBr. The implication is that the OH- comes in to play in some unknown later fast step, I guess theoretically it could be involved in an earlier fast step but realistcally what can OH- do on its own in a first step?

So, all we know is that the rate determining step has (CH3)3CBr as a reactant and nothing else, giving us...

(CH3)3CBr -->

What about the products?

Well you would get away with "intermediates", i.e.

(CH3)3CBr --> intermediates

 but in this question you can do better than that

because if all you have is one molecule of (CH3)3Br on its own it must fall apart and logically what must fall off is the Br-. As it is a product in the full equation. This gives us

(CH3)3CBr --> (CH3)3C+  and  Br-                  as the slow (rate determining) step

and then theoretically

(CH3)3C+ and OH- -->   (CH3)3COH              as the the fast step.

Will that get you the marks in the exams? Yep
Is that what really happens? Doesn't matter