Chapter 8, Problem 8.54P

Interpretation Introduction

Interpretation:

Whether each step in the given mechanism is reasonable or not is to be determined. An explanation is to be given for any steps that are not reasonable.

Concept introduction:

Reasonableness of a multistep mechanism and the individual steps in it are evaluated on the basis of a few rules. Proton transfer reactions must be compatible with the reaction conditions. Strong acids cannot exist under basic conditions and strong bases cannot exist under acidic conditions. Proton transfer reactions are very fast compared to other reactions, so they can be added at any stage as long as they are compatible with the conditions. An intramolecular proton transfer is generally not reasonable, particularly if the solvent is acidic or basic. In this case, a solvent mediated proton transfer is more likely. Another important rule is related to the number of species that participate in an elementary step. Typically only one or two species react in a given step. Steps containing three or more molecules/ions are extremely rare, and therefore unreasonable. In an ${\text{S}}_{\text{N}}\text{1 or E1}$ reaction, if a $\text{1, 2-hydride or 1, 2-methyl shift}$ leads to a more stable carbocation, this will occur preferably over a competing elementary step.