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Tertiary carbons form the most stable carbocations due to a combination of factors. The three alkyl groups on the tertiary carbon contribute to a strong inductive effect . This is because each alkyl group will share its electron density with the central carbocation to stabilize it.
The stabilities of the carbocations formed by this dissociation are known to follow the trend tertiary > secondary > primary > methyl. Therefore, since the tertiary carbocation is relatively stable and therefore close in energy to the R-X reactant, then the tertiary transition state will have a structure that is fairly similar to the R-X reactant.
The tert-butyl cation is a relatively stable carbenium ion. [1] A carbocation is an ion with a positively charged carbon atom.
The chemical basis for Markovnikov's Rule is the formation of the most stable carbocation during the addition process. Adding the hydrogen ion to one carbon atom in the alkene creates a positive charge on the other carbon, forming a carbocation intermediate.
A carbocation may be stabilized by resonance by a carbon–carbon double bond or by the lone pair of a heteroatom adjacent to the ionized carbon. The allyl cation CH 2 =CH−CH + 2 and benzyl cation C 6 H 5 −CH + 2 are more stable than most other carbenium ions due to donation of electron density from π systems to the cationic center. [20]
The stabilities of the carbocations formed by this dissociation are known to follow the trend tertiary > secondary > primary > methyl. Therefore, since the tertiary carbocation is relatively stable and therefore close in energy to the R-X reactant, then the tertiary transition state will have a structure that is fairly similar to the R-X reactant.
For instance a tertiary carbocation is more stable than a secondary carbocation and therefore the S N 1 reaction of neopentyl bromide with ethanol yields tert-pentyl ethyl ether. Carbocation rearrangements are more common than the carbanion or radical counterparts. This observation can be explained on the basis of Hückel's rule.
Although the initial carbocation is already tertiary, the oxygen can stabilize the positive charge much more favorably due to the complete octet configuration at all centers. It can also be seen as the -OH's lone pairs pushing an alkyl group off as seen in the asymmetrical pinacol example.