SolutionsAspirin
Exercise 1
Why is the Kolbe–Schmitt carboxylation carried out with sodium phenoxide and not with phenol? What changes with potassium phenoxide at higher temperature?
CO2 is a weak electrophile: only phenoxide, much more nucleophilic than phenol, attacks it. With sodium, the cation coordinates the phenoxide oxygen and the CO2 and directs it ortho, giving salicylate. With the more weakly coordinating potassium, and above 200 °C, mainly 4-hydroxybenzoic acid, the thermodynamic product, is obtained.
Exercise 2
Write the mechanism of the Hock reaction (rearrangement of cumyl hydroperoxide in acid) and say which group migrates.
The acid protonates the terminal OH of the hydroperoxide, which leaves as water; at the same time, the phenyl group migrates from carbon to the electron-deficient oxygen (phenyl, with its higher migratory aptitude, migrates in preference to methyl). An oxocarbenium ion, PhO–C+(CH3)2, is formed, which water converts into a hemiacetal that breaks down into phenol and acetone.