Delocalized electrons Resonance contributors

Resonance structures are diagrammatic tools used predominately in organic chemistry to symbolize resonant bonds between atoms in molecules. The electron density of these bonds is spread over the molecule, also known as the delocalization of electrons. Resonance contributors for the same molecule all have the same chemical formula and same sigma framework, but the pi electrons will be distributed differently among the atoms. Because Lewis dot diagrams often cannot represent the tme electronic stmcture of a molecule, resonance stmctures are often employed to approximate the tme electronic stmcture. Resonance stmctures of the same molecule are connected with a double-headed arrow. While organic chemists use resonance stmctures frequently, they are used in inorganic stmctures, with nitrate as an example. 

Examine the geometry of the most stable radical. Is the bonding in the aromatic ring fuUy delocalized (compare to model alpha-tocopherol), or is it localized Also, examine the spin density surface of the most stable radical. Is the unpaired electron localized on the carbon (oxygen) where bond cleavage occurred, or is it delocalized Draw all of the resonance contributors necessary for a full description of the radical s geometry and electronic structure. 

Onto which atoms (carbon, nitrogen or both) is the unpaired electron in tricyanomethyl radical delocalized Rationalize your result by drawing resonance contributors. 

First, try to draw resonance contributors for both ground state and triplet anthrone. Then display a spin density surface for the triplet state of anthrone. (Note that the spin density surface shows the location of both unpaired electrons, one of which may be in a 7t orbital and one of which may be in a o orbital.) Where are the two unpaired electrons Are they localized or delocalized Given that spin delocalization generally leads to stabilization, would you expect the triplet state of anthrone to be stable ... 

The cyclopentadiene anion is stabilized by five equivalent resonance structures. The anion is an aromatic anion by virtue of it being a six-jr-electron system. The indenyl anion is stabilized by a total of seven resonance contributors. However, they are nonequivalent and all but one require that the aromatic cloud of the benzene ring is disrupted. Thus, while the negative charge is well delocalized, the resonance stabilization is less than that of the cyclopentadiene system. Thus the proton is not as easily removed, making indene a weaker acid. 

Only electrons can be delocalized. Unlike electrons, nuclei cannot be delocalized. They must remain in the same places, with the same bond distances and angles, in all the resonance contributors. The following general rules will help us to draw realistic resonance structures ... 

Direct C-G bond formation at the 2-position of imidazole and benzimidazole was reported when imidazole was deprotonated with potassium /-butoxide in liquid ammonia via electrochemically induced SrnI. Coupling products 340 and 341 were isolated in a 4 1 ratio when the reaction started from 4-methylimidazole 338. Only one product 343 was obtained when 2-methylimidazole was used. It was proposed that delocalization of the negative charge of 345 resulted in some carbanion character (resonance contributors 346-348), and thus subsequent SrnI reaction with an electron deficient aryl chloride at carbon (Scheme 81) <1995JOC8015>. 

The reduction in Eq. (a) occurs with conjugated organic substrates. The radical anions formed often contain the added electron in a highly delocalized orbital, and these species cannot be represented by classical structures. Similarly, the dianions formed in Eq. (b) should be represented with delocalized structures. Nevertheless, classical structures that indicate the major resonance contributors are used e.g., the dilithioanthracene dianion is represented as I ... 

Delocalized electrons are electrons shared by more than two atoms. A compound with delocalized electrons has resonance. The resonance hybrid is a composite of the resonance contributors, which differ only in the location of their lone-pair and tt electrons. 

A disadvantage to using dashed lines to represent delocalized electrons is that they do not tell us how many tt electrons are present in the molecule. For example, the dashed lines inside the hexagon in the representation of benzene indicate that the tt electrons are shared equally by all six carbons and that all the carbon-carbon bonds have the same length, but they do not show how many tt electrons are in the ring. Consequently, chemists prefer to use stmctures with localized electrons to approximate the actual structure that has delocalized electrons. The approximate stmcture with localized electrons is called a resonance contributor, a resonance structure, or a contributing... 

The only time resonance contributors obtained by moving electrons away from the more electronegative atom should be shown is when that is the only way the electrons can be moved. In other words, movement of the electrons away from the more electronegative atom is better than no movement at all, because electron delocalization makes a molecule more stable (Section 7.6). For example, the only resonance contributor that can be drawn for the following molecule requires movement of the electrons away from oxygen ... 

Since the ability to delocalize electrons increases the stability of a molecule, we can conclude that a resonance hybrid is more stable than the predicted stability of any of its resonance contributors. The resonance energy associated with a compound that has delocalized electrons depends on the number and predicted stability of the resonance contributors The greater the number of relatively stable resonance... 

Chemists use resonance contributors—stractures with localized electrons—to approximate the actual structure of a compound that has delocalized electrons the resonance hybrid. To draw resonance contributors, move only tt electrons, lone pairs, or unpaired electrons toward an sp hybridized atom. The total number of electrons and the numbers of paired and unpaired electrons do not change. 

The extra stability a compound gains from having delocalized electrons is called resonance energy. It tells us how much more stable a compound with delocalized electrons is than it would be if its electrons were localized. The greater the number of relatively stable resonance contributors and the more nearly equivalent they are, the greater is the reso-... 

When a proton is lost from the / ara-nitroanilinium ion, the electrons that are left behind are shared by five atoms. (Draw resonance contributors if you want to see which atoms share the electrons.) When a proton is lost from /jara-nitrobenzoic acid, the electrons that are left behind are shared by two atoms. In other words, loss of a proton leads to greater electron delocalization in one base than in the other base. Because electron delocalization stabilizes a compound, we now know why the addition of a nitro substituent has a greater effect on the acidity of an anilinium ion than on benzoic acid. Now continue on to Problem 14. 

Second, the weaker the basicity of Y, the smaller is the contribution from the resonance contributor with a positive charge on Y (Section 17.2) the less the carboxylic acid derivative is stabilized by electron delocalization, the more reactive it will be. 

Major resonance contributor I he Lewis structures most neariy representative of the electron distribution in a delocalized molecule. 

Electron delocalization in a,(3-unsaturated carbonyl compounds is represented by three principal resonance contributors ... 

As the extent of electron delocalization into the ring increases, the geometry at nitrogen flattens. p-Nitroaniline, for example, is planar. Write a resonance contributor for p-nitroaniline that shows how the nitro group increases electron delocalization. 

A/-Nitroso amines are stabilized by electron delocalization. Write the two most stable resonance contributors of A/-nitrosodimethylamine, (CH3)2NN0. 

Chemists use an alternative method to show the equivalent resonance contributors, as with the formate anion 79 discussed in Section 5.9.3. If benzene is drawn with the n-bonds localized as in 87A, there is no indication of electron delocalization and this single structure does not adequately represent the structure of benzene. If the double bonds are moved from their position in 87A to give 87D, 87D is also inadequate because all the double bonds in 87A are single bonds in 87D and vice versa. The actual structure of benzene is not 87A or 87D, but it can be represented by both structures—called resonance contributors. Imagine the electrons shifting back and forth between 87A and 87D (the electrons are moving within the framework of 7t-orbitals) to represent the electron delocalization in benzene. 

Benzene was introduced in Chapter 5 (Section 5.10). Chapter 21 will discuss many benzene derivatives, along with the chemical reactions that are characteristic of these compounds. In the context of dissolving metal reductions of aldehydes, ketones, and alkynes, however, one reaction of benzene must be introduced. When benzene (65) is treated with sodium metal in a mixture of liquid ammonia and ethanol, the product is 1,4-cyclohexadiene 66. Note that the nonconjugated diene is formed. The reaction follows a similar mechanism to that presented for alkynes. Initial electron transfer from sodium metal to benzene leads to radical anion 67. Resonance delocalization as shown shordd favor the resonance contributor 67B due to charge separation. 

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