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The transition states for SN1 reactions that showcases tertiary carbons have the lowest transition state energy level in SN1 reactions. A tertiary carbocation will maximize the rate of reaction for an SN1 reaction by producing a stable carbocation. This happens because the rate determining step of a SN1 reaction is the formation of the carbocation.
Carbocations were also found to be involved in the S N 1 reaction, the E1 reaction, and in rearrangement reactions such as the Whitmore 1,2 shift. The chemical establishment was reluctant to accept the notion of a carbocation and for a long time the Journal of the American Chemical Society refused articles that mentioned them.
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.
The migration of alkyl groups in this reaction occurs in accordance with their usual migratory aptitude, i.e.phenyl carbocation > hydride > tertiary carbocation (if formed by migration) > secondary carbocation (if formed by migration) > methyl carbocation. {Why carbocation? Because every migratory group leaves by taking electron pair with it.}
The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols. With primary and secondary alkyl halides, the alternative S N 2 reaction occurs.
In this case, tertiary carbocation will react faster than a secondary which will react much faster than a primary. It is also due to this carbocation intermediate that the product does not have to have inversion. The nucleophile can attack from the top or the bottom and therefore create a racemic product.
In the case of primary alkyl halides, the carbocation-like complex (R (+)---X---Al (-) Cl 3) will undergo a carbocation rearrangement reaction to give almost exclusively the rearranged product derived from a secondary or tertiary carbocation. [8] Protonation of alkenes generates carbocations, the electrophiles.
This product distribution can be rationalized by assuming that loss of the hydroxy group in 1 gives the tertiary carbocation A, which rearranges to the seemingly less stable secondary carbocation B. Chlorine can approach this center from two faces leading to the observed mixture of isomers.