Resonance structures rules

Figure 3.22 shows resonance structures for a compound that has a CO double bond in conjugation with a CC double bond. The resonance structures are a combination of the types in Figures 3.18 and 3.19. Structure (a) has the octet rule satisfied at all atoms and has no formal charges, so it is more stable than the others and contributes the most to the resonance hybrid. Therefore, the actual structure most resembles this resonance structure (rule 4). In addition, the actual energy of the compound is also closer to the energy of the most important resonance structure. In other words, this compound has only a small resonance stabilization. However, even though structures (b) and (c) make... 

The rules to be followed when writing resonance structures are summarized m Table 1 5... 

These are the most important rules to be concerned with at present Additional aspects of electron delocalization as well as additional rules for Its depiction by way of resonance structures will be developed as needed in subsequent chapters... 

Although five equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion should be aromatic. The four-77-electron cyciopentadienyl carbocation and the five-7r-electron cyciopentadienyl radical are predicted to be unstable and antiaromatic. 

On reaction with acid, 4-pvrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hiickel 4n 4- 2 rule, explain why the protonated product is so stable. 

Anthracene has the formula Cl4Hln. It is similar to benzene but has 3 six-membered rings that share common C—C bonds, as shown below. Complete the structure by drawing in multiple bonds to satisfy the octet rule at each carbon atom. Resonance structures are possible. Draw as many as you can find. 

In this chapter, you will learn the tools that you need to draw resonance structures with proficiency. I cannot adequately stress the importance of this skill. Resonance is the one topic that permeates the entire subject matter from start to finish. It finds its way into every chapter, into every reaction, and into your nightmares if you do not master the rules of resonance. You cannot get an A in this class without mastering resonance. So what is resonance And why do we need it ... 

Now we know what curved arrows are, but how do we know when to push them and where to push them First, we need to learn where we cannot push arrows. There are two important rules that you should never violate when pushing arrows. They are the two commandments of drawing resonance structures ... 

From now on, we will refer to the second commandment as the octet rule. But be careful—for purposes of drawing resonance structures, it is only a violation if we exceed an octet for a second-row element. However, there is no problem at all with a second-row element having fewer than an octet of electrons. For example ... 

The arrow on the top structure violates the octet rule (giving carbon five bonds), and the arrow on the bottom structure does not violate the octet rule. The arrow on the bottom structure will therefore provide a valid resonance structure ... 

Once you learn to recognize this pattern (a lone pair next to a pi bond), you will be able to save time in calculating formal charges and determining if the octet rule is being violated. You will be able to push the arrows and draw the new resonance structure without thinking about it. 

There are three simple rules to follow when comparing resonance structures. At this point, you are probably thinking that it is hard enough to keep track of... 

Let s see the three rules for determining which resonance structures are significant ... 

There is one notable exception to this rule compounds containing the nitro group (-NO2) will often have resonance structures with more than two charges. Why The nitro group looks like this ... 

If we apply our rule (about limiting charge separation to no more than two charges), then we might say that the second resonance structure above has too many charges... 

Rule 3 Avoid drawing a resonance structure in which two carbon atoms bear opposite charges. Such resonance structures are generally insignificant, for example ... 

Inspection of the second resonance structure reveals that this nitrogen atom is actually sp hybridized, not sp. It might look like it is sp hybridized in the first resonance structure, but it isn t. Here is the general rule a lone pair that participates in resonance must occupy ap orbital. In other words, the nitrogen atom in the compound above is sp hybridized. And as a result, this nitrogen atom is trigonal planar rather than trigonal pyramidal. 

It is difficult to give a localized orbital description of the bonding in a period 3 hypervalent molecule that is based only on the central atom 3s and 3p orbitals and the ligand orbitals, that is, a description that is consistent with the octet rule. One attempt to do this postulated a new type of bond called a three-center, four-electron (3c,4e) bond. We discuss this type of bond in Box 9.2, where we show that it is not a particularly useful concept. Pauling introduced another way to describe the bonding in these molecules, namely, in terms of resonance structures such as 3 and 4 in which there are only four covalent bonds. The implication of this description is that since there are only four cova-... 

The use of resonance structures such as 7 and 8 to describe bond polarity led to a subtle change in the meaning of the octet rule, namely, that an atom obeys the octet rule if it does not have more than eight electrons in its valence shell. As a result, resonance structures such as 7 and 8 are considered to be consistent with the octet rule. However, this is not the sense in which Lewis used the octet rule. According to Lewis, a structure such as 7 would not obey the octet rule because there are only three pairs of electrons in the valence shell of carbon, just as BF3 does not obey the octet rule for the same reason. Clearly the octet rule as defined by Lewis is not valid for hypervalent molecules, which do, indeed, have more than four pairs of shared electrons in the valence shell of the central atom. 

A. General rules for drawing realistic resonance structures ... 

The following are rules that apply to drawing resonance structures. Remember that resonance relates to different ways of placing electrons in the structures, not ways of arranging the atoms themselves. 

Now that we have determined that structure I is correct for the cyanate ion, we still need to consider resonance structures. In keeping with the rules given earlier, the acceptable resonance structures that can be devised are... 

Although such textbook diagrams are called Lewis structures, they are not the electron-dot diagrams that G. N. Lewis originally wrote for such species. Lewis s depiction of S042-, for example, is reproduced in Fig. 3.90. This shows a normal-valent S2+ ion with shared-pair bonds to four O- ions, which is fully consistent with the octet rule, with no intrinsic need for multiple resonance structures to account for the observed Td symmetry. According to Lewis s original concept, each ion is... 

As previously recognized (e.g., Table 4.52), this corresponds to the ideal pattern of three cu bonds, three lone pairs, and sd2 (90°) hybridization, locking in a rigid octahedral structure. On the basis of the formally duodectet-rule-consistent parent species FeH3, the overall co-bonding is compactly described by the two resonance structures... 

Like phenanthrene, some PAHs have a unique Clar structure, whereas several alternative Clar structures are possible for other PAHs [21]. Thus, Clar s rule does not designate the resonance structure mainly responsible for the aromaticity of anthracene. Clar s model cannot differentiate between its outer and inner ring (Scheme 28.2). 

Figure 11.11 shows there are some molecules which can legitimately be drawn in several different ways using Lewis structures, each conforming to the octet rule. These are resonance structures, and are equally valid, but the true structure is a hybrid of the two or more possible structures. This is indicated by the double-headed arrow, where the electrons are moved, but the atoms stay in position. However, in this example, the carbon oxygen bonds are of equal length - they do not rapidly interconvert from one version to another. The true... 

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