Three Factors That Affect Carbocation Stability.pdf - Three...

Three Factors That Affect Carbocation Stability
Three main factors increase the stability of carbocations:
•
Increasing the number of adjacent carbon atoms: methyl (least stable carbocation) <
primary < secondary < tertiary (most stable carbocation)
•
Adjacent pi bonds that allow the carbocation p-orbital to be part of a conjugated pi-
system system (“delocalization through resonance”)
•
Adjacent atoms with lone pairs
1. Three Factors That Stabilize Carbocations
If electrons were money, carbocations would be the beggars of organic chemistry. Packing a mere
six valence electrons, these electron-deficient intermediates figure prominently in many reactions
we meet in organic chemistry, such as
•
nucleophilic substitution (S
N
1) and elimination (E1) reactions
•
additions of electrophiles to double and triple bonds
•
electrophilic aromatic substitution
•
additions to carbonyl compounds and enolate chemistry (albeit in masked form)
That’s a huge chunk of sophomore O -chem, right there. Being electron-deficient (and therefore
unstable), formation of a carbocation is usually the rate-limiting step in these reactions. Knowing
that, then think about this: what happens to the rate of the reaction when the carbocation
intermediate is made more stable? Well, the energy of the transition state leading to the reaction will
be lower. Therefore, the activation energy will be lowered.
What’s that going to do to the rate of the reaction? Since the activation energy is lowered, the
reaction is going to speed up.
So what are some of the factors that stabilize carbocations?
If you look through all of your organic chemistry textbook, you’ll find 3 main structural factors that
help to stabilize carbocations.
1. Neighboring carbon atoms.
2. Neighboring carbon-carbon multiple bonds.
3. Neighboring atoms with lone pairs.
Why is this? It all goes back to the core governing force in chemistry: electrostatics. Since “opposite
charges attract, like charges repel”, you would b e right in thinking that carbocations are
stabilized by nearby electron-donating groups.

2. Carbocations Are Stabilized By Neighboring
Carbon Atoms.
The stability of carbocations increases as we go from primary to secondary to tertiary carbons.
There’s two answers as to why this is. The age -old answer that is still passed around in many
introductory textbooks points to carbons (alkyl groups in particular) as being “electron - releasing”
groups through inductive effects. That is, a carbon (electronegativity 2.5) connected to hydrogen
(electronegativity 2.2) will be electron rich, and can donate some of those electrons to the
neighboring carbocation. In other words, the neighboring carbon pays the carbocation with
electrons it steals from the hydrogens. The second, (and theoretically more satisfactory
explanation) is hyperconjugation , which invokes stabilization through donation of the electrons in