Carbocations show up VERY often in Organic Chemistry
This should come as no surprise to all of you OChem students. They seem to be everywhere. However, there is a way to see if these sneaky charges will shift on us.
Ok – Let’s start by looking at an example with…well…a carbocation.
At first, we would probably think that the second structure would be the major product. Not so my friends…not so.
Here, we get the isopropyl instead of the propyl. Why?
Well, let’s take a closer look at this reaction. And I mean before the alkyl halide adds to the benzene.
Why does the carbocation cation “move” in this example?
You see…molecules do not like to be charged. They prefer to be neutral. However, if they find themselves “charged up”, they have a way out.
Does anyone remember carbocation stability? I see you in the back raising you hand…well?
Yes! That is correct! Tertiary > Seconday > Primary
Carbocation stability is the key!
In the above example, you can see that we get a primary carbocation at first. However, if a hydrogen moves from the secondary carbon to the primary, we can get a secondary carbocation.
This is known as a hydride-shift. A hydrogen can move from one atom to the atom next door if it will increase the stability of the carbocation (30 > 20 > 10).
So you see…the hydrogen is moving…not the carbocation.
OK – now let’s look at a couple of organic molecules and try to anticipate this moving carbocation.
If you are wondering, there is a way to find out if a shift will occur BEFORE you start the question.
Carbocations are key to SN1 reactions. And here, we know we will generate a carbocation.
Start by looking to the carbons next door to your alkyl halide and check for a 30 or 40 center. If you have one of them…then you will get a shift!
You can also look AFTER you generate your carbocation. Either way works.
Hydride-shift with a 30 carbons…but what about 40 carbons?
We get an alkly-shift with 40 carbons.
But wait! Did you see the cool thing that happens in both examples?
Both SN1 examples give you a 30 carbocation in the end. So cool, right?