Deactivating groups, in aromatic electrophilic

The pKa of p-(tiifluoromethyl)benzoic acid is 3.6. Is the trifluoromethyl substituent an activating or deactivating group in electrophilic aromatic substitution ... 

We recall that chlorine and bromine are deactivating groups in electrophilic aromatic substitution because they withdraw electron density by an inductive effect but are ineffective in the donation of electron density by resonance. The same features are important in controlling the rate of the second step of ketone halogenation. The bromine atom withdraws electron density from the carbonyl carbon atom, making the carbonyl oxygen atom less basic. Therefore, the enol forms more slowly in the second step. 

Exercise 22-24 Draw the structures of the intermediate cations for nitration of nitrobenzene in the 2, 3, and 4 positions. Use the structures to explain why the nitro group is meta-orienting with deactivation. Use the same kind of arguments to explain the orientation observed with —CF3, —CHO, —CH2Ci, and —NH2 groups in electrophilic aromatic substitution (Table 22-6),... 

Section 10.6), the strength of F-substituted carboxylic acids, the deactivating effect of the CF3 group in electrophilic aromatic substitutions, and the non-basic character of NF3 and (CF3)3N (see end-of-chapter problem 17.4). 

Such a representation is referred to as a local ionization potential map. Local ionization potential maps provide an alternative to electrostatic potential maps for revealing sites which may be particularly susceptible to electrophilic attack. For example, local ionization potential maps show both the positional selectivity in electrophilic aromatic substitution (NH2 directs ortho para, and NO2 directs meta), and the fact that TC-donor groups (NH2) activate benzene while electron-withdrawing groups (NO2) deactivate benzene. 

Reactivity in electrophilic aromatic substitution depends, then, upon the tendency of a substituent group to release or withdraw electrons. A group that releases electrons activates the ring a group that withdraws electrons deactivates the ring. 

In nucleophilic as in electrophilic aromatic substitution, then, a substituent group affects reactivity by its ability to attract or release electrons in nucleophilic as in electrophilic aromatic substitution, a substituent group exerts its effect chiefly at the position ortho and para to it. The kind of effect that each group exerts, however, is exactly opposite to the kind of effect it exerts in electrophilic aromatic substitution. In nucleophilic aromatic substitution electron withdrawal causes activation, and electron release causes deactivation. 

Electronegative groups in the aromatic are necessary for reaction the polyhalo groups deactivate the aromatic for electrophilic substitution by SO 3, increase the thermal stability and also block potential sites of substitution. 

Like the NO2 group, CN is deactivating in electrophilic aromatic substitution. There is much less literature to deal with than there was for the article on ether and hydroxyl groups. As in the case of the previous article, however, Roger Taylor s excellent monograph is a good guide to what is available and references to particular parts of this book will be appropriate. 

Substituent effects In electrophilic aromatic substitution, electron-withdrawing groups deactivate the ring toward attack, while in nucleophilic aromatic substitution, an electron-withdrawing group is required in order for the reaction to proceed. 

PROBLEM 14.45 Eixplain why the trifluoromethyl group is meta-directing in electrophilic aromatic substimtion. Would you expect CF3 to be activating or deactivating Why ... 

The pAT values of/substituted benzoic acids for the —PCI2 and —Si(CH3)3 groups are 3.6 and 4.3, respectively. Based on these data, determine whether these groups are activating or deactivating in electrophilic aromatic substitution. 

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