Stability of Carbocations
Carbocations are highly reactive intermediates in organic chemistry. They are formed when a carbon atom loses a pair of electrons to form a cation, usually because a leaving group fell off of (or was removed from) the molecule. The stability of a carbocation is determined by the number of alkyl groups it has, as well as its hybridization state.
A primary carbocation (one with only one alkyl group attached to the carbon atom that lost the electrons) is the least stable and the most reactive. This is because the positive charge is tightly concentrated on the carbon atom, and has no where else to 'spread' the charge around to in order to feel more stable.
Secondary carbocations (two alkyl groups) are more stable and less reactive, and tertiary carbocations (three alkyl groups) are the most stable and least reactive. Note that alkyl groups are stabilizing.
The hybridization state also plays a role in stability. Carbocations that are sp3 hybridized are more stable than those that are sp2 hybridized. This is due to the fact that sp3 hybridized carbocations have a tetrahedral electron arrangement, which allows for the alkyl groups to be arranged in such a way as to minimize repulsion between the electrons in the alkyl groups and the electrons in the cationic carbon.
Some examples of tertiary carbocations include:
A primary carbocation (one with only one alkyl group attached to the carbon atom that lost the electrons) is the least stable and the most reactive. This is because the positive charge is tightly concentrated on the carbon atom, and has no where else to 'spread' the charge around to in order to feel more stable.
Secondary carbocations (two alkyl groups) are more stable and less reactive, and tertiary carbocations (three alkyl groups) are the most stable and least reactive. Note that alkyl groups are stabilizing.
The hybridization state also plays a role in stability. Carbocations that are sp3 hybridized are more stable than those that are sp2 hybridized. This is due to the fact that sp3 hybridized carbocations have a tetrahedral electron arrangement, which allows for the alkyl groups to be arranged in such a way as to minimize repulsion between the electrons in the alkyl groups and the electrons in the cationic carbon.
Some examples of tertiary carbocations include:
- 2,2-Dimethylpropyl cation (also known as isopropyl cation)
- 2,2,3-Trimethylbutyl cation
- 2,3,3-Trimethylbutyl cation
- 2,2,4-Trimethylpentyl cation
- 2,3,4-Trimethylpentyl cation
- 2,3,3-Trimethylpentyl cation
- 2,2,3,3-Tetramethylbutyl cation
- 2,2,3,4-Tetramethylpentyl cation
