Second resonance structures

Write a second resonance structure for each of the following ... 

The first and last structures are OK, but the second resonance structure is bad because there are too many charges. This resonance structure is not significant, because it will not contribute much character to the overall resonance hybrid. It is like the kiwi in our analogy above. Therefore, we would say that this compound has only two significant resonance structures. 

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... 

In the example above, the second resonance structure has an oxygen with a positive charge. But this oxygen does not have its octet, and therefore, this resonance structure is not significant. 

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. 

The second resonance structure above is an important contributor to the overall resonance hybrid. 

Stork enamine synthesis takes advantage of the fact that an aldehyde or ketone reacts with a secondciry cimine to produce an enamine. Enamines cire resonance stabilized (see Figure 15-25) and have multiple applications. In the first resonance structure, the nitrogen is the nucleophile, while in the second resonance structure, the Ccirbanion is the nucleophile. Some commonly used secondary amines, pyrrolidine, piperidine, and morpholine, are shown in Figure 15-26. 

Larsen and Harsen describe the bonding as resulting from sp3 hybridization with one lobe (s + 3pz) parallel to the c axis, and the other three lobes pointing into the tetrahedral holes arranged trigonally below the atom. The latter combine with the orbitals of adjacent atoms, similarly pointing into the tetrahedral holes. A second resonance structure is obtained by reflecting the hybrids in the horizontal... 

Sometimes a given set of atoms can covalently bond with each other in multiple ways to form a compound. This situation leads to something called resonance. Each of the possible bonded structures is called a resonance structure. The actual structure of the compound is a resonance hybrid, a sort of weighted average of all the resonance structures. For example, if two atoms are connected by a single bond in one resonance structure and the same two atoms are connected by a double bond in a second resonance structure, then in the resonance hybrid, those atoms are connected by a bond that is worth 1.5 bonds. A common example of resonance is found in ozone, 0, shown in Figure 5-7. 

A second resonance structure can be written for a phosphorus ylide with a double bond between phosphorus and carbon. As a third-row element, phosphorus can have more than 8 electrons in its valence shell. 

Problem 1.11 Follow the curved arrows to draw a second resonance structure tor each species. 

Problem 15.25 Draw a second resonance structure for the allylic radical formed as a product of Step [1] in Figure 15.8. What hydroperoxide is formed using this Lewis structure ... 

For a double bond X=Y in which the electronegativity of Y > X, a second resonance structure can be drawn by moving the n eiectrons onto Y. 

Because A contains a positive charge and a ione pair on adjacent atoms, a second resonance structure B can be drawn. Because B has more bonds and aii second-row atoms have octets, B is more stabie than A, making it the major contributor to the hybrid C. Because the hybrid is more stable than either resonance contributor, the order of stability is ... 

Problem 16.7 Draw a second resonance structure and the hybrid for each species, and then rank the two resonance structures and the hybrid in order of increasing stability. 

Because this is an example of an allyl-type system p<=Y—Z ), a second resonance structure can be drawn that moves the lone pair and the jt bond. To delocalize the lone pair and make the system conjugated, the labeled carbon atom must besp hybridized with the lone pair occupying ap orbital. 

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. 

The carbonyl carbon of an amide is sp hybridized and has trigonal planar geometry. A second resonance structure can be drawn that delocalizes the nonbonded electron pair on the N atom. Amides are more resonance stabilized than other acyl compounds, so the resonance structure having the C=N makes a significant contribution to the hybrid. 

If you donate one or two electrons to an atom that already has an octet, regardless of whether it has a formal positive charge, another bond to that atom must break. For example, in nitrones (PhCH=NR-0) the N atom has its octet. A lone pair from O can be used to form a new N=0 it bond only if the electrons in the C=N it bond leave N to go to C, i.e., PhCH=NR-0 PhCH-NR=0. In the second resonance structure, N retains its octet and its formal positive charge. 

Carbonyl compounds C=0 have two major resonance structures, R.2C=0 <—> R2C-6. In the second resonance structure, C is electron-deficient, so carbonyl compounds are good electrophiles. Carbonyl groups with a-hydrogen atoms are relatively acidic compounds, because the carbanions produced upon removal of the H are stabilized by resonance with the carbonyl group 0=CR-CR2 <—> 0-CR=CR2. The enolate anions that are thereby obtained are nucleophilic at the a-carbon and on O. Under basic conditions, then, carbonyl compounds are electrophilic at the carbonyl carbon and nucleophilic at the a-carbons (if they have H atoms attached). All the chemistry of carbonyl compounds is dominated by this dichotomy. 

In the structural formulae of the dipeptides, the peptide bond is depicted in the common manner, i.e. with an sp -hybridised nitrogen and the double bond between carbon and oxygen. Actually, this is just one of two mesomeric forms, the second resonance structure (sp — N, double bond between N and C, positive charge on N and negative charge on O) being the more prominent one. 

To generate the second resonance structure from the first, we imagine one lone pair dropping down to form another bond, and pushing an adjacent bond off to form a lone pair. The arrows show this hypothetical shift of electrons, which leads to the resonance hybrid below. 

These rules are listed in order of importance. For instance, consider MeO—CH2 MeO=CH2. The second resonance structure is more important... 

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