Deshydratation du di-t-butylmenthylcarbinol dans l'acide acetique anhydre: Probleme de l'etape determinante

The products of the acid-catalysed dehydration of di-t-butylmethylcarbinol (1) in acetic acid are 2-t-butyl-3,3-dimethylbut-1-ene (2) and 2,3,3,4,4-pentamethylpent-1-ene (3). The rate constant for the rearrangement of 2 to 3 (k_rearr) is about 10 times smaller than that of dehydration (k_I). The kinetic isotope effect on the dehydration rate of d₃-methyl-(1) is that expected for a secondary isotope effect upon carbonium ion formation; there appears to be no primary kinetic isotope effect. The rate determining step is, then, probably heterolysis of the protonated alcohol, contrary to what is observed in aqueous media where carbonium ion deprotonation is rate determining. At low acidity (H₀ > - 1) the (log k_I)/ — H₀ correlation has a slope close to unity but at higher acidity log k_I increases faster than —H₀.