Aryl halides resonance structures

The Gabriel synthesis of amines uses potassium phthalimide (prepared from the reaction of phthalimide with potassium hydroxide). The structure and preparation of potassium phthalimide is shown in Figure 13-13. The extensive conjugation (resonance) makes the ion very stable. An example of the Gabriel synthesis is in Figure 13-14. (The N2H4 reactant is hydrazine.) The Gabriel synthesis employs an 8, 2 mechanism, so it works best on primary alkyl halides and less well on secondary alkyl halides. It doesn t work on tertiary alkyl halides or aryl halides. 

Oxidative addition can also occur at C(sp2)-X bonds (i.e., at vinyl and aryl halides). It always occurs with retention of configuration about the double bond. Oxidative addition at C(sp2)-X obviously cannot proceed by an Sn2 mechanism. The SRN1 mechanism is a possibility. Another possibility is that the Pd coordinates first to the 77 bond of, say, vinyl iodide, to form a 7r complex. The metal-lacyclopropane resonance structure of the 77 complex can be drawn. An electro-cyclic ring opening of the metallacyclopropane (Chapter 4) can then occur. One Pd-C bond breaks, and I- leaves to give a new cationic Pd-vinyl complex. When I associates with the Pd, the overall result is oxidative addition. 

This model may also be appropriate to other LL substrates, such as carbonyl groups (6), activated double bonds (5, 19), and aryl halides (7), that also obey the Ritchie equation. This seems appropriate because these LL substrates are highly polar and because particularly in solution the zwitterion I is a major contribution to their overall resonance structure ... 

The higher reactivity and possible hydrolysis of the aryl C-X (F or Cl) bond in 4-halophenylsulfone required that ice-cold water be employed during the precipitation and, especially, during the sodium hydroxide neutralization steps. Hydroxide ions and, under some conditions, water may attack the activated halide to possibly produce an undesired phenolic side-product that will lower the yields of sulfonated dihalodiphenyl sulfone. The H NMR spectrum of sulfonated diflorodiphenyl sulfone is shown in Fig. 6.5 as an example for analysis of the monomers. The resonance assignments for the aromatic protons H, H, and H, shown in the Fig. 6.5, are in accordance with the structure of the sulfonated monomers. 

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