Meta-directing deactivators

Electron-withdrawing -I/-M groups (EWGs), which make the ring less nucleophilic than benzene, will deactivate the meta- position less than the ortho-lpara- positions. The carbocation produced from attack at the meta- position will therefore be the most stable because this does not reside adjacent to the EWG in any of the resonance forms. 

The EWG can destabilise both of these cations more effectively than the cation derived from meta- attack 

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Meta-directing deactivators, such as —CHO, act through a combination of electron-withdrawing inductive and resonance effects that reinforce each other and are felt most strongly at the ortho and para positions. As a result, the ortho and para intermediates are less stable so reaction with an electrophile occurs at the meta position (Figure 16.16). 

The iV,A/,ALtrimethylammonium group, -N(CH3)3, is one of the few groups that is a meta-directing deactivator yet has no electron-withdrawing resonance effect. Explain. 

Which of the following functional groups isn t a meta-directing deactivator ... 

Meta-directing deactivators Ortho- ajid para-directing deactivators ... 

Ortho/para directing activating Meta directing deactivating... 

Meta-directing deactivators Ortho- and para-directing dcactivators i... 

Substituents on the benzene ring affect both the reactivity of the rii toward further substitution and the orientation of that substitution. Grouj can be classified as ortho- and para-directing activators, ortho- an para-directing deactivators, or meta-directing deactivators. Sul stituents influence aromatic rings by a combination of resonance and indu five effects. Resonance effects are transmitted through -ir bonds indu tive effects are transmitted throu [Pg.654]

Reaction at pyridine is actually preferred to reaction at benzene - easily illustrated by the fate of 4-phenyl pyridine 167 under the two sets of conditions. Traditional nitration occurs only on the benzene ring 166 with the (presumably protonated) pyridine ring acting as a meta-directing deactivating substituent. With N205, nitration of the (presumably unprotonated) pyridine ring is preferred and occurs in the 3-position 168. 

Substituents are classified as (i) orf/zo-Zpara-directing activators (ii) ortho-/ para-directing deactivators or (iii) meta-directing deactivators. 

Substituents can be classified into three groups, as shown in Figure 9.16 meta-directing deactivators, ortho- and para-directing deactivators, and ortho- and para-directing activators. There are no meta-directing activators. 

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