Major resonance forms

None of the three major resonance forms of the intermediate formed by attack at the meta position has a positive charge on the carbon bearing the —CF3 group... 

Only major resonance forms are considered in providing these approximate answers. The electron-pair geometries on which the sketches are based are referred to by numbers in parentheses. 

The major resonance form will have the most covalent bonds and complete shells and the least amount of charge separation. It will seem to be the most stable of the possible resonance forms. Place any negative charges on electronegative atoms. Resonance structures with an incomplete shell are usually minor. Resonance forms having an electronegative atom with an incomplete octet are insignificant. 

Lewis structures—be able to rapidly draw any compound s major resonance form with the correct formal charges. 

Draw Lewis structures with resonance for the following charged species. Decide the major resonance form, if any. 

Write down the Lewis structure of the reactants, complete with formal charges, and draw any major resonance forms. Look for leaving groups, polarized single and multiple bonds, acids and bases. Classify into generic sources and sinks and then rank them. The reaction usually occurs between the best source and sink. Above all, note if the medium is acidic or basic. In basic media, find the best base, and then locate any acidic hydrogen within range (not more than 10 p Ta units above the pATabH of Ihe base). In acidic media, identify the best sites for protonation. Likewise, do not create a species that is more than 10 units more acidic than your acid. Understand what bonds have been made or broken, but do not lock into an arbitrary order as to which occurred first. 

Iodonium ylides derived from cyclic 1,3-dicarbonyl compounds, phenols, coumarin derivatives and hydrox-yquinones represent a particularly stable and synthetically important class of zwitterionic iodonium compounds [539,552], Several examples of these compounds are shown in Figure 2.15 in the major resonance form as enolate or phenolate zwitterionic structures 405-408. 

Ozone (trioxygen, O3) is an allotrope of oxygen that plays an jTflk important role in the ozone layer, protecting the Earth s surface from the harmful effects of ultraviolet radiation (Chapter 25 on the accompanying website). The ozone molecule is V-shaped and is a resonance hybrid described by two major resonance forms (Figure 14-48). 

Propanone, (CH3)2CO, the simplest ketone, can be described as a resonance hybrid of two resonance structures (Figure 14-51). The first resonance structure is known as the major resonance form and makes the largest contribution to the resonance hybrid the second resonance structure, with the separation of charge, makes a smaller contribution to the resonance hybrid. 

The major resonance form leaves the electrons in the cyclopentadiene ring to make both rings aromatic. Therefore, the central bond has more single bond character and rotation around that bond is easily achieved. 

The nitroso functional group is readily reduced in a thermodynamically favored and kinetically facile reaction to the hydroxylamine state (1). The relatively rapid kinetics of nitroso reduction can be reasonably explained on the basis of probable stabilities of transition state intermediates produced via electron addition. Unlike the nitro group, for which addition of an electron disrupts the major resonance of the ground state, addition of an electron to the nitroso group yields transitional intermediates with structures quite similar to the major resonance forms of the ground state nitroso group (Fig. 3). 

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