Chemistry Reference
In-Depth Information
involves no charges. Note that there is no net gain or loss of bond count between the
starting materials and the product.
b. The McLafferty rearrangement
Note: The radical cation present in the starting material is the result of the carbonyl
oxygen losing a single electron. This reaction is generally observed during electron
impact mass spectrometry.
The McLafferty rearrangement is a reaction generally seen as part of the fragmenta-
tion processes observed during mass spectrometry. It is, in fact, during electron
impact mass spectrometry that the illustrated starting radical cation is formed.
Since this is a radical mediated process, there are no charges involved in the
progression of the reaction mechanism other than the positive charge that remains
on the oxygen atom. As shown below, using arrow pushing, the first step of this
rearrangement involves transfer of a hydrogen atom to the carbonyl oxygen. This
occurs through homolytic bond cleavage and bond formation. The second step,
also progressing through a homolytic process, involves cleavage of a carbon-
carbon bond and liberation of ethylene.
c. 1,3-Dipolar cycloaddition
1,3-Dipolar cycloadditions are electrocyclic reactions where one of the starting
materials is charged. In fact, the charges on the starting material define the dipole.
Like all electrocyclic reactions, there is no net gain or loss of bond count.
However, in this case, while the starting material is charged, there are no charges
