Deactivating groups, aromatic

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

The way in which various substituents affect the polarization of a carbonyl group is similar to the way they affect the reactivity of an aromatic ring toward electrophilic substitution (Section 16.5). A chlorine substituent, for example, inductively withdraws electrons from an acyl group in the same way that it withdraws elections from and thus deactivates an aromatic ring. Similarly, amino, methoxvl, and methylthio substituents donate electrons to acyl groups by resonance in the same way that they donate electrons to and thus activate aromatic rings. 

Deactivating group (Section 16.4) An electron-withdrawing substituent that decreases the reactivity of an aromatic ring toward electrophilic aromatic substitution. 

For the aromatic carboxylic acids, substituents on the aromatic ring may also influence the acidity of the acid. Benzoic acid, for example, has = 4.3 x 10 . The placements of various activating groups on the ring decrease the value of the equilibrium constant, and deactivating groups increase the value of the equilibrium constant. Table 12-2 illustrates the influence of a number of para-substituents upon the acidity of benzoic acid. 

The cyanide groups (L" = C6H5CH2NC, L = CN) in I deactivate the aromatic ring(s) by a negative inductive effect. 

The sulphonation of toluene (Expt 6.37) with concentrated sulphuric acid at 100-120°C results in the formation of toluene-p-sulphonic acid as the chief product, accompanied by small amounts of the ortho and meta isomers these are easily removed by crystallisation of the sodium salt of the para isomer in the presence of sodium chloride. Sulphonation of naphthalene at about 160°C yields largely the 2-sulphonic acid (the product of thermodynamic control) (Expt 6.38) at lower temperatures (0-60 °C) the 1-sulphonic acid (the product of kinetic control) is produced almost exclusively. In both cases the product is isolated as its sodium salt. In anthraquinone the carbonyl groups deactivate the aromatic nucleus towards electrophilic attack and vigorous conditions of sulphonation are required, i.e. oleum at about 160 °C. The product is largely sodium anthraquinone-2-sulphonate (Expt 6.39). 

The heteroaromatic substitution reflects the Friedel-Crafts reaction with the opposite reactivity and selectivity. The synthetic advantages and disadvantages are also opposite to those of concern for the selectivity of monosubstitution - whereas introduction of a carbonyl group deactivates the aromatic ring toward further substitution in the electrophilic process, in contrast it activates the heteroaromatic... 

A second limitation is that aromatic compounds substituted with moderately or strongly deactivating groups cannot be alkylated. The deactivated ring is just too poor a nucleophile to react with the unstable carbocation electrophile before other reactions occur that destroy it. 

Alkyl groups activate the aromatic rings by increasing the electron density of the ring, whereas halogens deactivate the aromatic rings... 

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