Resonance electron-withdrawing

Substitution of a methyl group for hydrogen on pyrrole N or C atoms leads to upfield shifts (ca. 0.1-0.3 p.p.m.) in the remaining ring proton resonances. Electron-withdrawing substituents (e.g. formyl, acyl, alkoxycarbonyl) on N cause substantial downfield shifts of the a-proton resonances and much smaller downfield shifts for those of the -protons (Table 4). The downfield shifts experienced by the ring proton resonances in pyrroles... 

Substituents such as NOz, C=N, S02CH3, CONH2, and COOR can donate an electron pair to reaction centers such as OH, NH2, OR, and SH. In the presence of a strong-resonance electron-withdrawing group and a strong-... 

Resonance effects Resonance electron-withdrawing groups make the carbonyl carbon more electrophilic and increase the rate resonance electron donors make the carbonyl carbon less electrophilic and decrease the reaction rate. 

JV-acetyl derivatives of these azidoimidazoles give a 3 2 mixture of azido and tetrazole forms. It has been suggested that the azide form is much more prevalant in azoles than in azines and thiazoles. Substituents affect the equilibrium and a shift to the tetrazole form is mainly governed by resonance electron withdrawal. Not only electron-withdrawing power, but the size of a substituent group can also affect the equilibrium, for in... 

Meta-directing deactivators act through a combination of inductive and resonance effects that reinforce each other. Inductively, both ortho and para intermediates are destabilized because a resonance form places the positive charge of the carbocation intermediate directly on the ring carbon atom that bears the deactivating group (Figure 16.17). At the same time, resonance electron withdrawal is also felt at the ortho and para positions. Reaction with an electrophile therefore occurs at the meta position. 

The Hammett Op constants are inadequate for aromatic reactions in which crossconjugation between substituents is possible. For example, when the substiment X is electron-supplying by resonance and Y is electron-accepting, cross-conjugation is possible as shown in equation 9. This direct cross-conjugation is not possible for meta substituents. To allow for these interactions the cr-constants resonance electron-supplying substituents respectively, and some values for these constants are shown in Table 1. 

Solution The nitro-substituted compound is the stronger acid because the nitro substituent withdraws electrons both inductively (through the cr bonds) and by resonance (through the TT bonds). Withdrawal of electrons through tt bonds is called resonance electron withdrawal. We have seen that electron-withdrawing substituents increase the acidity of a compound by stabilizing its conjugate base. 

Donation of electrons through tt bonds is called resonance electron donation withdrawal of electrons through IT bonds is called resonance electron withdrawal. 

Solution to 21a The left-hand ring is attached to a substituent that activates that ring by resonance electron donation. In contrast, the right-hand ring is attached to a substituent that deactivates that ring by resonance electron withdrawal. 

Alkenes that undergo polymerization by an anionic mechanism are those that can stabilize the negatively charged propagating site by resonance electron withdrawal (Table 27.5). 

Resonance electron-withdrawing (or donating) eroup = group that withdraws or donates electrons via 7T bonds as demonstrated by 2" order resonance structures (e.g. NO2) — usually a STRONG effect. 

Memorization Task 19.6 Identifying Resonance Electron-withdrawing Groups... 

A group with a % bond next to an aromatic ring will be a resonance electron-withdrawing group... 

As stated earlier, resonance electron-withdrawing and donating effects are strong in comparison to other effects (e g. steric or inductive effects). Circle the answer that completes the sentence... 

No ordinary EAS reactions work with very deactivated rings. As a mle of thumb, a ring is considered very deactivated if it has two resonance electron-withdrawing groups (and no resonance electron-donating groups to counterbalance this). 

---