Here’s another great example of how one reagent used in a certain context performs one thing, and in a different context performs a completely different task. These are the types of inconsistencies which lead students to find organic chemistry difficult, and in many cases to mistakenly believe that it is arbitrary. It is not.
First example. In Org 1 we learn that alkenes can be converted to alcohols when treated with aqueous acid (i.e. H3O+) :
Although these technical details are never mentioned, the laboratory procedure for this reaction involves treating the alkene with concentrated acid,
often at high temperatures [edit: see link in comments for a specific example, done with cooling, not heat. Thanks to Mary Beth for the suggestion] Somewhat forcing conditions, in other words.
All’s well and good. However, skip ahead a semester, to the addition of Grignard reagents to ketones. After the Grignard reagent adds to the ketone, we add H3O+ in a “workup” step to protonate the negatively charged oxygen (alkoxide) to give the alcohol.
Although often not mentioned, in practice, this is done with dilute acid, at room temperature, and briefly (often in a separatory funnel). Mild conditions, in other words.
I recently met a student who was confused by the following problem:
See the issue? We are doing a Grignard in the presence of an alkene. How is the student supposed to know that the second step is merely a workup step and not a hydration of the alkene?
One answer is, “experience”. If you do enough Grignard problems, you see that H3O+ step at the end and you come to associate it with working up a Grignard.
A second answer is “context”. If one is doing problems from a chapter on Grignard reactions, it’s unlikely (although still possible) that alkene addition reactions will be thrown in there alongside.
Regardless, there is an aspect of ambiguity in the use of H3O+ in these two contexts. It’s not unreasonable to suggest that adding the word “workup” or “dilute” in the case of the second reaction would in some cases prevent the type of error shown here.