Resonance electron donation

Problem 16.8 Write resonance structures for chlorobenzene to show the electron-donating resonance effect of the chloro group. 

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

It should be noted that the electron-donating resonance effects just considered are the result of lone pair electrons feeding in to the jr electron system. Potentially, any substituent with a lone pair might do the same, yet we did not invoke such a mechanism with chlorine substituents above. As the size of the atom increases, lone pair elechons will be located in orbitals of higher level, e.g. 3p rather than 2p as in carbon. Consequently, the ability to overlap the lone pair orbital with the it electron system of the aromatic ring will diminish, a simple consequence of how far from the atom the electrons are mostly located. Chlorine thus produces a low resonance effect but a high inductive effect, and the latter predominates. 

The inductive effect in 4-hydroxybenzoic acid is electron withdrawing, but this is a modest effect compared with the electron-donating resonance effect. This significantly destabilizes the conjugate base, and 4-hydroxybenzoic acid (p a 4.6) is less acidic than benzoic acid. The last example, 4-acetylbenzoic acid, is the most acidic of the group, and this is primarily the result of an electron-withdrawing resonance effect, though there is also a favourable inductive effect. 

The electron-donating resonance effect applies with decreasing strength in the following order ... 

Finally, two or even three amino or dimethylamino groups in the para position stabilize trityl cations so efficiently because of their pronounced electron-donating resonance effect that the associated non-ionized neutral compounds are no longer able to exist at all but heterolyze quantitatively to salts. These salts are the well-known tri-phenylmethane dyes. 

Problem 16.46 shows the mechanism of the addition of HBr to 1-phenylpropene and shows how the aromatic ring stabilizes the carbocation intermediate. For the methoxyl-substituted styrene, an additional resonance form can be drawn in which the cation is stabilized by the electron-donating resonance effect of the oxygen atom. For the nitro-substituted styrene, the cation is destabilized by the electron-withdrawing effect of the nitro... 

One further point inductive effects and resonance effects don t necessarily act in the same direction. Halogen, hydro.xyl, alko.xyl, and amino substituents, for instance, have electron-H /l7/i/nnv//i inductive effects because of the electro-negativity of the —X, -O, or -N atom bonded to the aromatic ring but have electron-donating resonance effects because t)f the lone-pair electrons on those same —X, —O, or —N atoms. When the hvo effects act in opposite directions, the stronger of the two dominates. 

Like other compounds with carbon-carbon double bonds, enols are electron rich, so they react as nucleophiles. Enols are even more electron rich than alkenes, though, because the OH group has a powerful electron-donating resonance effect. A second resonance structure can be drawn for the enol that places a negative charge on one of the carbon atoms. As a result, this carbon atom is especially nucleophilic, and it can react with an electrophile to form a new bond to carbon. Loss of a proton then forms a neutral product. 

Substituents with an electron-donating resonance effect have the general structure -V, where the Y atom has a lone pair of electrons available for donation to the ring ... 

Rings substituted by a group with an electron-donating resonance effect have this general structure. 

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