Organic reaction mechanism carbocations

Saul Winstein (1912-1969) was bom in Montreal, Canada, and received his Ph.D. in 1938 at the California Institute of Technology. From 1942 to 1969, he was professor of chemistry at the University of California, Los Angeles, where he devoted his scientific career to the study of organic reaction mechanisms, particularly those involving carbocations. 

Markovnikov s rule was empirical that is, it was based on observation only. At the time it was proposed, the concept of organic reaction mechanisms had not yet been imagined so it was impossible to provide a theoretical basis for why the H added to one carbon and the X to the other. Now that we know the mechanism for the reaction, it is easy to understand why these reactions are regiospecific. In fact, if we write the mechanism for the additions to give both possible products, we can predict the preferred product simply on the basis of what we already know about carbocations. It is much better to make predictions based on the mechanism of the reaction because cases that are exceptions to the empirical rule will be readily apparent. 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

Organozirconates. Zwitterionic zirconocene-ate see Ate Complexes) intermediates have been evoked to explain some reaction mechanisms (see Section 2.3.3). In fact, electron transfer from an organic ligand to 16-electron metal center permits the generation of a carbocationic center which can, then, undergoes isomerization and rearrangement characteristic of carbocations. It is now possible to isolate and characterize stable zwitterionic phosphonium zirconate complexes (eqnation 18). ... 

The proposed mechanisms for acetal hydrolysis can be tested with the tools used to study other organic reactions. An A1 mechanism proceeding through transition structure II would lead to a carbocation centered on the departing R group, so racemization of a chiral center would be expected. On... 

The type of carbocation formed in a reaction mechanism can affect the products formed in some organic reactions, such as addition reactions to alkenes (see page 208). 

---