Chapter 2, Problem 7EQ

One structure for the acetoxonium ion is

Clearly, the receptor is the positively charged $\_\_\_\_$ atom. However, there are available two different kinds of pushable electrons, namely, pi electrons in the carbon-oxygen double bond or unshared electrons on the oxygen atom. The proper choice in this case is dictated by the following considerations. In structure (3) the carbon atom possesses a formal positive charge and also has only six electrons in its outer shell (it lacks a stable octet of electrons). If the pi electrons are pushed, namely,

structure (4) is obtained in which the oxygen possesses a formal positive charge but, in addition, both the oxygen and the carbon atom lack a stable octet. This structure will be very unstable. Structure (4), despite the fact that it can be generated by properly pushing electrons, is not included in the resonance hybrid for the acetoxonium ion. If, on the other hand, the unshared electrons on the oxygen are pushed, a more acceptable structure is obtained, namely,

In structure (5) the oxygen atom possesses a formal positive charge, and both the carbon and the oxygen atom have a stable octet. Structure (5) is included in the resonance hybrid.