I’m telling you! You have to watch out for these carbocations in organic chemistry!
Anytime you make a carbocation (positive charge), you have to keep one eye open at ALL times. They have a tendency to move when you look away.
But…there is an exception. There is alway one in every crowd, right?
So, what’s the difference between these two reactions?
You are right! One has mercury (Hg) and the other does not. If we start with the same alkene in both reactions, will we get the same product or products?
I’ll give you a minute to think about that…
Let’s start by looking at what happens first in these organic chemistry reactions
It looks like we form a carbocation in both…but hold up! One has a strange 3-membered-dotted line-ring-thing with Hg…Whaaaaaat?
Ok, so both do form carbocations…but this is where the difference occurs.
When the bottom reaction happens, it forms a stable carbocation with mercury. You can think of the mercury (Hg) as acting like a guard that keeps the carbocation in check.
In the top reaction…no one is watching. Remember, I said keep one eye open?
The carbocation will shift if no one is watching!
Well, let me speak on behalf of the carbocation. It is only looking out for its best interest.
Molecules prefer to be neutral. However, if they find themselves charged, they are going to do what they can to make it more comfortable.
In the first example, a hydrogen next door (in orange) can shift places with the positive charge. This changes the carbocation from a secondary carbocation (20) to a tertiary carbocation (30).
Big improvement in stability.
The bottom structure has someone to help
Here, we can see the molecule is getting some help. The mercury atom is helping stabilize the carbocation. This way, it is not uncomfortable and doesn’t to need to shift.
Got it? Simple right?
The first example will give us two organic molecules are products
Here, we can get two products. One…where we would “expect” the OH group to go (bottom left). And the other is after the hydride shift (bottom right).
So…The answer is in the number of organic molecules formed
The reaction with with acid and water (H3O+) formes two products. And the one on the right (30 alcohol) is the major product.
The bottom reaction with mercury (Hg) forms only one product.