Substituted benzenes groups that withdraw electrons

Figure 16.12 Electrostatic potential maps of benzene and several substituted benzenes show that an electron-withdrawing group (-CHO or —Cl) makes the ring more electron-poor (yellow-green), while an electron-donating group (-OH) makes the ring more electron-rich (red).

Competition reactions between benzene and various substituted benzenes reveal that electron-withdrawing substituents on the arene increase reaction rates. All ortho, meta, and para forms of the arylsilanes are obtained. The major component of the arylsilane product formed in reactions of arylsilanes with electron-withdrawing groups, such as chlorobenzene, and anisole, is the ortho isomer. In contrast, toluene, which bears a less electronegative substituent, leads to meta isomer as major product. 

Iron complexes containing bidentate alkyl and aryl phosphorus ligands cleave a variety of C-H bonds under mild conditions, Hydrido acetylide complexes were prepared by oxidative addition of primary acetylenes in the Fe(DPPE)2 and the Fe(DMPE)2 systems [DPPE = bis(diphenylphos-phino)ethane, DMPE = bis(dimethylphosphino)ethane]. The Fe(DMPE)2 system also cleaves C-H bonds of activated methyl groups, aromatic compounds, and certain other sp hybridized molecules. The C-H cleavage reactions are reversible, resulting in equilibrium mixtures of isomeric products in many cases. Studies of substituted benzenes show that while product stability is favored by electron withdrawing substituents, steric effects play a predominant role in the determination of product distribution. 

Pyrrole is soluble in alcohol, benzene, and diethyl ether, but is only sparingly soluble in water and in aqueous alkaUes. It dissolves with decomposition in dilute acids. Pyrroles with substituents in the -position are usually less soluble in polar solvents than the corresponding a-substituted pyrroles. Pyrroles that have no substituent on nitrogen readily lose a proton to form the resonance-stabilized pyrrolyl anion, and alkaU metals react with it in hquid ammonia to form salts. However, pyrrole pK = ca 17.5) is a weaker acid than methanol (11). The acidity of the pyrrole hydrogen is gready increased by electron-withdrawing groups, eg, the pK of 2,5-dinitropyrrole [32602-96-3] is 3.6 (12,13). 

It has been documented in an earlier volume that appropriately substituted molecules with two strongly electron withdrawing substituents meta to one another in a benzene ring often possess diuretic properties and, even though the prototypes usually have two substituted sulfonamide moieties so disposed, other groups can replace at least one of them. An example of this is piretanide (24), where one such group is a carboxyl... 

A Ni(dppe)Br2-Zn system effectively catalyzes co-cydotrimerization of an allene with a propiolate. The reaction is highly regio- and chemoselective to afford a poly-substituted benzene derivative in good yield. (Scheme 16.82) [92], From the observation that no desired [2 + 2 + 2] product is obtained for the reaction of 1-hexyne and phenylacetylene with w-butylallene under similar conditions, the presence of an electron-withdrawing C02Me group in the alkyne moiety is essential for the success of the present [2 + 2 + 2]-co-cyclotrimerization. 

Like other electrophilic substitutions, nitration of a substituted benzene, where the substituent is electron withdrawing (N02, C02H, CN, and so on Table 22-6), generally produces the 1,3-isomer. To prepare the 1,4-isomer, less direct routes are necessary— the usual strategem being to use benzene derivatives with substituent groups that produce the desired orientation on... 

---