Evidence for the Mechanism of Electrophilic Additions Carbocation Rearrangements

Evidence for the Mechanism of Electrophilic Additions Carbocation Rearrangements 

How do we know that the carbocation mechanism for electrophilic addition reactions of alkenes is correct The answer is that we don t know it s correct at least we don t know with complete certainty. Although an incorrect reaction mechanism can be disproved by demonstrating that it doesn t account for observed data, a correct reaction mechanism can never be entirely proved. The best we can do is to show that a proposed mechanism is consistent with all known facts. If enough facts are accounted for, the mechanism is probably correct. 

What evidence is there to support the carbocation mechanism proposed for the electrophilic addition reaction of alkenes One of the best pieces of evidence was discovered during the 1930s by F. C. Whitmore of the Pennsylvania State University, who found that structural rearrangements often occur during the reaction of HX with an alkene. For example, reaction of HC1 with 3-methyl-1-butene yields a substantial amount of 2-chloro-2-methylbutane in addition to the expected product, 2-chloro-3-methylbutane. 

2-Chloro-3-methy1butane 2-Chloro-2-methylbutane (approx. 50%) (approx. 50%) 

Carbocation rearrangements can also occur by the shift of an alkyl group with its ejection pair. For example, reaction of 3,3-dimethyJ-l-butene with HCI Leads to an equal mixture of unrearranged 2-chloro-3,3-dimethyTbutane and rearranged 2-chloro-2,3-dimethyibutane. In this instance, a secondary carbocation rearranges to a more stable tertiary carbocation by the shift of a methyl group. 

If the reaction takes place in a single step, it wonld be difficnlt to account for rearrangement, but if the reaction takes place in several steps through a carbocation intermediate, rearrangement is more easily explained. The secondary carhocation intermediate formed by protonation of 3-methylbut-l-ene evidently rearranges to a more stable tertiary carbocation by a hydride shift— the shift of a hydrogen atom and its electron pair (a hydride ion, H ) between neighboring carbons  

Note the similarities between the two carbocation rearrangements in both cases, a group ( H or CH3 ) moves to an adjacent positively charged 

A word of advice that we ve noted before and will repeat on occasion biological molecules are often larger and more complex in appearance than the molecules chemists work with in the laboratory, but don t be intimidated. When looking at any chemical transformation, whether biochemical or not, focus on the part of the molecule where the change is occurring and don t worry about the rest. The tertiary carbocation just pictured looks complicated, but all the chemistry is taking place in the small part of the molecule inside the red circle. 

---

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction. Rearrangements occur by shift of either a hydride ion, H (a hydride shift), or an alkyl anion, R-, from a carbon atom to the adjacent positively charged carbon. The result is isomerization of a less stable carbocation to a more stable one. 

---