Carbocation stability

Before we start talking about carbocation stability, we should have a starting discussion about some carbocation basics.

What is a carbocation?

A carbocation, also sometimes referred to as the carbonium ion, is an sp2 hybridized carbon atom with three groups bonded to it and a empty orbital.  Because it has an empty orbital, the sp2 carbon carries a positive charge on it, making it highly electrophilic (want to know more about electrophiles?  Check out this blog post).  The geometry of the carbocation is trigonal planar, meaning all bond angles are 120°.  The empty orbital allows for the cation to be attacked from either side.  Hence, in many cases, a loss of chirality will occur if the starting material was chiral and the mechanism goes through a carbocation intermediate (Like an SN1 reaction).

How do we get carbocations?

There are several ways to obtain a carbocation, but the first thing to understand is that carbocations are not inherently stable.  Translation: You will not find a bottle of carbocations just sitting on the shelf.

The most common way to get a carbocation is through heterolytic cleavage of a carbon-heteroatom bond, where the heteroatom gets both electrons in the bond.  The heteroatom is sometimes referred to as a “leaving group” and is an atom (or molecule even) that can easily carry a negative charge.  Some examples of this type of carbocation formation are below:

The other way to form a carbocation is through the first step of electrophilic addition to a double bond.  The most common electrophile to add is a proton.  In this method, the proton adds to one side of the double bond, creating a carbocation on what used to be the other side of the double bond.

How do we stabilize carbocations?

Now that we are all on the same footing, let’s talk about carbocation stability.  There are three major ways to do this:

• More alkyl groups: The first is though adding more alkyl groups to the carbocation. This is one of the more important things to understand in first semester organic chemistry.  If you take away one thing from this post, it should be that the more alkyl groups we add to a carbocation, the more stable that carbocation is:

The reason more alkyl groups (“R” groups) stabilize the carbocation is because of two factors, called inductive effects and hyperconjugation.  Inductive effects are relatively simple, it just means that alkyl groups are slightly electron rich and can donate some of this small electron density in to stabilize the carbocation.  In hyperconjugation, the electrons of a sigma orbital interact with the empty adjacent orbital to give an extended molecular orbital, which helps stabilize the cation.

• Adjacent double bonds: If we have a double bond one carbon away from the carbocation, it will also serve to stabilize it.  This is due to resonance and inductive effects of the double bond.  In essence, we are able to spread the unstable positive charge over multiple atoms, instead of just one.

• Lone pairs: The final way to stabilize a carbocation is with an adjacent atom with a lone pair of electrons.  As you know, lone pairs are non-bonding electron density hanging off an atom.  This atom can now serve as an electron donating group, adding negative electron density to stabilize the positive carbocation.  When we use resonance to observe this, it becomes much more obvious.

Now, we can add two more groups to our overall carbocation stability chart.  The allyl group and the benzyl group are a little more stable than a secondary carbocation.

Ok, I get carbocation stability, but what about destabilization?

• Double bonds: Having a carbocation on a double bond is very destabilizing.  A vinylic carbocation carries the positive charge on an sp carbon.  This is more electronegative than an sp2 carbon of an alkyl carbocation. Hence a secondary vinylic carbocation is less stable than a secondary alkyl carbocation.
• Electron withdrawing groups: Just like an electron donating group adds negative electron density and stabilizes a carbocation, an electron withdrawing group (such as CN) will destabilize it by trying to suck electron density away from something that is already electron deficient.

What happens to carbocations once they are formed?

• Reactions: Once a carbocation is formed, it can be a part of an organic chemistry reaction (and isn’t that really what we all want). There are a bunch of reactions that can have a carbocation as an intermediate, including E1/SN1 reactions, electrophilic aromatic substitution (EAS) and electrophilic addition to a double bond.
• Carbocation rearrangements: This is quite important. ANYTIME (did you see me put it in caps?  Must be important) you see a carbocation on an exam, the FIRST thing you need to do is look for a rearrangement. Carbocations are kinda crazy, but at the end of the day they want to be as stable as possible. This means that if I carbocation can go from a secondary cation to a tertiary cation, it will.  This can be done in several ways, but the most popular is the 1,2-hydride shift.  This involves moving an adjacent H^– to the carbocation.  The carbocation becomes neutral, and a cation forms on the carbon that lost the H^–.

There is one more way to rearrange a carbocation which involves ring formation and/or expansion, but that will be left for another time.

Reference: carbocations rock