Second-order resonance structures

Second-order resonance structures are like "assistant resonance structures." They are not as important as regular tfirst-orderf resonance stmctures. but can help us make predictions in certain instances. 

Explain why the three second-order resonance structures shown in Model 8 are NOT considered important resonance structures. That is, why are they "not as important" as a regular, first-order resonance structure such as the one at the far left of Model 8 (Hint Count formal charges.)... 

Add curved arrows to nitrobenzene at the top of the page showing how you would change it into one of the second-order resonance structures. Then use curved arrows to generate each of the subsequent second-order resonance structures. 

Draw second-order resonance structures of the aromatic starting material (benzaldehyde) that can be used to justify why the EAS reaction in the box yields mostly meta-bromobenzaldehyde. 

Composite Drawing of Aniline (showing a combination of all first- and second-order resonance structures)... 

A resonance effect is the donation or withdrawal of electron density through n bonds as demonstrated by first- or second-order resonance structures (as shown below, right). 

Electron donation by an alkyl group can be considered an inductive or a pseudo-resonance effect (See hyper conjugation). Hyperconjugation cannot be demonstrated with second-order resonance structures. 

Explain the following statement The second-order resonance structures help explain why substituents with a lone pair (such as -OH) activate the ortho and para positions toward electrophilic aromatic substitution (EAS). 

Draw three second-order resonance structures for benzoic acid. 

We have found that second-order resonance structures can be used to predict the regiochemistry of an EAS reaction. A more rigorous prediction comes from the heights of the activation barriers. According to the Hammond Postulate, the relative energies of the intermediates can be used to approximate the relative heights of these activation barriers. Simply put... The pathway with the most favorable carbocation intermediate will likely dominate the product mixture. 

Draw a second-order resonance structure demonstrating that the lone pair shown on the N of the acylated aniline is pulled (delocalized) toward the oxygen. 

For each compound above we can draw a second-order resonance structure to show donation of electron density into the bond marked with =>. This reduces the reactivity of the carbonyl toward a nucleophile. 

Use curved arrows to show a mechanism for Synthetic Transformation 26.3. Hint the first step is a tautomerization to generate an enol (it may help to draw a second-order resonance structure of this enol). 

Draw all reasonable second-order resonance structures for the following a,p-unsaturated carbonyl compound. Note Oxygen must always have an octet even in a second-order resonance structure ... 

Draw one or more second-order resonance structures that help explain why the lone pair on an aromatic amine (such as aniline) is imlikely to form a bond to iC compared to ordinary amines such as ammonia, in which the lone pair is localized on the nitrogen. 

Draw all three second-order resonance structures of aniline, and explain why aromatic amines are weak bases and poor nucleophiles (as compared to non-aromatic amines). 

As a result of this reaction, is the N made more nucleophihc or less nucleophiUc Draw second-order resonance structures to support your answer in part b. 

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