In Chapter 2, we saw several patterns for drawing

Chapter 11, Problem 11.56

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In Chapter 2, we saw several patterns for drawing resonance structures of ions or uncharged compounds. There are also several patterns for drawing resonance structures of radicals, although we have only encountered one such pattern (allylic or benzylic radicals). One other pattern is characterized by a lone pair on an atom that is adjacent to an atom with an unpaired electron, as seen in the following example. The unpaired electron is delocalized by resonance, with radical character being spread over the nitrogen and phosphorus atoms. Note that phosphorus is a third-row element, so it can have more than eight electrons. We will make use of a related pair of resonance structures in this problem. N P N P The rearrangement shown below (Ar = aromatic group) represents the conversion of a sulfenate to a sulfoxide, which is initiated by heat. The transformation is believed to occur via a two-step process in which the C!O bond undergoes homolytic cleavage followed by a recombination of the resulting radicals to form the product (J. Am. Chem. Soc. 2000, 122, 33673374): O S Ar S Ar Heat O (a) Draw a mechanism of this rearrangement, consistent with the description provided. (b) Provide a reasonable rationalization for the fact that the C!O bond is cleaved preferentially over the O!S bond in the first step of the mechanism.