Resonance form three-atom groupings

Look back at the resonance forms of the acetate ion and the acetone anion shown in the previous section. The pattern seen there is a common one that leads to a useful technique for drawing resonance forms. In general, any three-atom grouping with a p orbital on each atom has two resonance forms. 

By learning to recognize such three-atom groupings within larger structures, resonance forms can be systematically generated. Look, for instance, at the anion produced when H+ is removed from 2,4-pentanedione by reaction with a base. How many resonance structures does the resultant anion have ... 

In the second resonance form, the unpaired electron is next to another double bond, giving another three-atom grouping and leading to another resonance form. 

Any three-atom grouping with a multiple bond adjacent to a nonbonding p orbital has two resonance forms. 

Strategy Look for three-atom groupings that contain a multiple bond next to an atom with a p orbital. Exchange the positions of the bond and the electrons in the p orbital to draw the resonance form of each grouping. 

Solution The unpaired electron is on a carbon atom next to a C=C bond, giving a typical three-atom grouping that has two resonance forms. 

The atoms X, Y, and Z in the general structure might be C, N, O, P, or S, and the asterisk ( ) might mean that the p orbital on atom Z is vacant, that it contains a single electron, or that it contains a lone pair of electrons. The two resonance forms differ simply by an exchange in position of the multiple bond and the asterisk from one end of the three-atom grouping to the other. 

The primary electronic effect is due to the delocalization of electron pairs between the ether oxygen and the carbonyl group of the ester function as expressed by the resonance structure 2. 2, and 2- Resonance structures 1 and 2 show the delocalization of an electron pair between the carbon and oxygen of the carbonyl group H 2) and resonance structures 2 and 3 show that the ether oxygen has one electron pair delocalized towards the same central carbon (2 3), The primary electronic effect can therefore be viewed as the result of two n-v interactions. The three atoms involved can be considered to be sp3 hybridized and on that basis, the three-dimensional representations 4 and 5 correspond respectively to the Z and E forms of the ester function. 

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