Core Concepts
In this article, we will explore the stability of carbocations in alkenes via substitution and rearrangement, including stabilizing factors such as resonance.
Topics Covered in Other Articles
- Alkenes
- Carbocation Stability
- Understanding E1 vs E2 Reactions
- Functional Groups in Organic Chemistry
Definitions
- Carbocation: A carbon with a positive charge, resulting from an incomplete octet.
- Primary carbocation: a positively charged carbon with only one carbon substituent directly attached.
- Secondary carbocation: a positively charged carbon with two carbon substituents directly attached.
- Tertiary carbocation: a positively charged carbon with three carbon substituents directly attached.
Substitution and Stability
The number of carbon substituents attached to a carbocation is directly proportional to the stability of the carbocation. The stability of alkene carbocations (or vinylic carbocations) follows the same pattern as alkane carbocations. Tertiary carbocations are more stable than secondary carbocations, which are in turn more stable than primary carbocations. Alkene carbocations are very rarely found in nature because of their severe instability. However, they can be prepared in the lab through a variety of methods, including deprotonation with a strong base.
To better understand the relative stability of alkene carbocations, we can draw on knowledge of alkane carbocation stability. Of alkenes and alkanes, carbocations are most stable in tertiary alkenes. Tertiary alkane carbocations are approximately equal in stability to secondary alkene carbocations. Secondary alkane carbocations are approximately equal in stability to primary alkene carbocations. Primary alkane carbocations follow in relative stability, with methyl groups being the least stable for carbocations.
See below for a visual depiction of this pattern.
Why is substitution important?
There are two main reasons why substitution lends stability to carbocations: Hyperconjugation and Inductive Effects. We will look at each of these phenomena below!
- Hyperconjugation
Hyperconjugation occurs when a carbon atom has attached electron donating groups. Methyl groups are strong examples of electron donating groups as there is overlap of the methyl’s C-H sigma bond and the empty p orbital of the carbocation. Increasing the number of carbon groups (electron donating groups) increases the delocalization of the positive charge, thus increasing the stability.
- Inductive effect
The inductive effect is similar to hyperconjugation in that it results in delocalization of the positive charge. However, unlike with hyperconjugation, the inductive effect results in unequal delocalization. The unequal delocalization results from the electronegativity differences of involved atoms; carbon, which is a highly electropositive atom, will push the shared bonding electrons toward the carbocation. In turn, as shown in the above diagram, a partial positive charge will form on the ethyl group.
Alkene Carbocation Unique Stabilization – Resonance
When a carbocation is adjacent to a pi-bonded carbon (alkene), the pi bond may shift positions in the structure. This shifting, which we call resonance, will reduce the impact of the positive charge by spreading it across multiple carbon atoms.
Rearrangements
There are two different types of rearrangements that may occur which ultimately increase a structure’s stability – an alkyl shift, and a halide shift. Both shifts occur in a 1,2 fashion, meaning that the alkyl or halide will shift to the most adjacent carbon.
- In an alkyl shift, a methyl group on a more highly substituted carbon will shift to the carbocation. Because of the shift of the methyl group, the positive charge will then be on the previously higher substituted carbon.
- In a halide shift, a hydrogen on an adjacent carbon will shift to the carbocation. As a result of the shift, the positive charge will then be on the carbon from which the hydrogen was donated.
