In this tutorial, you will learn about the formation of carbocations and what factors cause carbocation rearrangements. To further understand rearrangements, we will go over examples of each type of rearrangement.
Topics covered in other articles
Carbocation Stability – Vocabulary
- Primary – A carbon connected to one other carbon atom
- Secondary – A carbon connected to two other carbon atoms
- Tertiary – A carbon connected to three other carbon atoms
- Hydride shift – When a hydrogen atom moves to a positively charged carbon
- Methyl shift – When a methyl group moves to a positively charged carbon
- Ring expansion – When a strained ring breaks a bond to form a new bond with a carbocation
When do carbocations form?
Carbocations form in intermediate steps of E1 and SN1 reactions. The formation of a carbocation is often the rate-determining step and the intermediate carbocation will undergo rearrangement to stabilize the positive charge.
What is the most stable form?
Carbocations are the most stable when the charge is on a tertiary carbon and least stable on a primary carbon. Carbocations will shift the positive charge to reach the most stable configuration. This is called a carbocation rearrangement.
In addition, the more resonance structures there are, the more stable the charge will be. Finally, if any atoms besides carbon have lone pairs, a double bond will form and the more electronegative atom will take the positive charge.
Example: Primary to tertiary carbocations
Example: Formation of double bond
Rearrangement of carbocations
There are several forms of carbocation rearrangements that can occur. First is a hydride shift, where a hydrogen atom switches places with the positive charge. A methyl shift is similar to a hydride shift, but instead of hydrogen shifting a whole methyl group is moved. Finally, a ring expansion, which occurs to relieve steric strain next to a carbocation. Cyclohexane has the least steric strain and so cyclohexane will not shift to cyclopentane or cycloheptane. Take a cyclobutane, for example, which has a large steric strain. If a carbocation is formed on a branched propyl group, the cyclobutane can expand to a cyclopentane.