Open sextet

This process is sometimes called the Whitmore 1,2 shift Since the migrating group carries the electron pair with it, the migration terminus B must be an atom with only six electrons in its outer shell (an open sextet). The first step therefore is creation of a system with an open sextet. Such a system can arise in various ways, but two of these are the most important ... 

After the migration has taken place, the atom at the migration origin (A) must necessarily have an open sextet. In the third step, this atom acquires an octet. In the case of carbocations, the most common third steps are combinations with a nucleophile (rearrangement with substitution) and loss of H (rearrangement with elimination). 

The mechanism is generally regarded as involving formation of a carbene. It is the divalent carbon that has the open sextet and to which the migrating group brings its electron pair ... 

The resultant carbonium has only six electrons, i.e. it is an open sextet. This provides the electron deficient sink that will accommodate the excess electrons that are at present contained by a nucleophilic migratory group. Suggest what will be the next step. 

Theoretically, any species that contains an unshared electron pair could act as a base. In fact, most ions and molecules that contain unshared electron pairs undergo some reactions by sharing their electron pairs. Conversely, many Lewis acids contain only six electrons in the highest occupied energy level of the central element. They react by accepting a share in an additional pair of electrons. These species are said to have an open sextet. Many compounds of the Group IIIA elements are Lewis acids, as illustrated by the reaction of boron trichloride with ammonia, presented earlier. 

Open sextet Refers to species that have only six electrons in the highest energy level of the central element (many Lewis acids). 

Carbonium ions with their open sextets of electrons abstract hydride from HBr to produce Br2 and abstract hydride from HI to produce I2 (Deno et al, 1962b). Since sulfuric acid is well-known to oxidize HBr to Br2 and HI to I2, it is attractive to propose that a derivative of H2SO4 with an open sextet of electrons is responsible for these oxidations. The simplest such derivative would be SO3H+, which would also be an equally attractive intermediate in the familiar sulfonation of aromatics such as benzene. 

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