Carbocation formation and

The acid-catalyzed mechanism involves carbocation formation and substituent migration assisted by the hydroxy group. 

Boruah, R. C. Skibo, E. B. Determination of the pKa values for the mitomycin C redox couple by tritration, pH rate profile, and Nemst-Clark fits. Studies of methanol elimination, carbocation formation, and the carbocation/quinone methide equilibrium. J. Org. Chem. 1995, 60, 2232-2243. 

As shown in Table 4.38, three major reaction pathways are available to hypova-lent metals in the presence of an alkene (A) and (C) dative and synergistic coordination (B) carbocation formation and (D) and (E) metallacyclic and migratory insertions. The latter types are of particular importance in metal-catalyzed alkene polymerizations and will be given primary attention in the discussion that follows. 

SAMPLE SOLUTION (a) Isopropyl bromide, (CH3)2CHBr, is a secondary alkyl halide, whereas isobutyl bromide, (CH3)2CHCH2Br, is primary. Because the ratedetermining step in an SN1 reaction is carbocation formation and secondary car-bocations are more stable than primary ones, isopropyl bromide is more reactive Jthan isobutyl bromide in nucleophilic substitution by the SN1 mechanism. ... 

B is correct n1 mechanism depends on die rate-limiting step/ which is the carbocation formation and is nucleophile independent all subsequent steps occur at a much faster rate and do not affect the rate of the reaction. 

The reaction is highly dependent on the alkyne used. When structurally simple 3-hydroxy alkynes such as propargyl alcohol are used, acceptable yields of the addition products are obtained, whereas reactions with alkynes prone to carbocation formation and rearrangement, such as 1-ethynyl-cyclohexanol, give a complex mixture of products.68,73... 

Two idealized mechanistic scenarios for electron deficient [1,2] alkyl shift reactions can be envisaged a concerted event, in which alkyl migration is essentially synchronous with carbocation formation and a stepwise process, involving initial formation of a discrete cationic species followed by subsequent alkyl migration. Of the two, the concerted scheme is intuitively associated with a lower activation barrier. It should be noted that these sequences clearly represent extreme positions of a continuum of substrate and condition dependant mechanistic hybrids. With a view toward developing a predictive sense for the [1,2] alkyl shift, these mechanistic issues are relevant, in that the specific migrating bond for a given substrate will be influenced... 

The ( )-olefin GPP (3) can then be easily isomerized to the (Z)-configured neryl diphosphate (NPP 4). This isomerization most reasonably proceeds via carbocation formation and diphosphate addition to the tertiary carbon to give the conformationally flexible linalyl diphosphate (LPP, 5) (Scheme 6.2). In addition there are also enzymes known that... 

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