Toluene activation

In this equilibrium the Nd-species to which a diene is coordinated is active in polymerization, whereas the Nd-species to which an arene is coordinated is inactive. According to the authors the experimentally determined ranking of activities toluene > mesitylene > toluene (+ 7% hexamethylbenzene) correlates with the electron richness (i.e. Lewis basicity) of the aromatic compounds. The polymerization activity decreases with increasing Lewis basicity of the aromatic compound as the equilibrium is shifted and the concentration of the active species is reduced. These considerations were supported by the following experimental results (Table 18). 

The most suitable solvents for the polymerization by metal chloride-based catalysts are aromatic hydrocarbons, especially toluene. This is due to the following reasons i) they are good solvents for both catalyst and polymer, and ii) they interact only weakly with the active species, and therefore do not reduce its activity. Toluene is preferable to benzene from the viewpoint of toxicity. Though aliphatic hydrocarbons (cyclohexane, hexane, etc.) are also useful, their ability to dissolve catalysts and polymers is lower than that of aromatic hydrocarbons. 

The benzoylation yield of deactivated fluorobenzene (181, R=F) increases ten-fold on going from atmospheric pressure to 0.5 GPa. In contrast, for activated toluene (181, R=Me) the yield decreases as the pressure increases (Scheme 7.46). These results can be explained in terms of competition between the favourable effect of pressure which allows the transition states between ArH and the 1 1 acyl chloride complex (TSl) or the acyllium (TS2) to be reached. 

Different studies were then conducted to extend the reaction to more deactivated aromatic compounds, particularly anisole and toluene, as model substrates [13]. With such aromatic compounds hydroxyalkylation always leads to the bisaryl-methane type product unless the aldehyde is deactivated. Interesting results have been obtained by use of chloral, fluoral [14], or hexafluoromethylacetone. The reactivity of toluene, anisole, and phenol is compared in Table 4. At 5 % conversion the reaction is very selective with the less active toluene and very dirty with phenol. 

Methylation of activated toluenes. Arylacetonitriles and methyl arylacetates are monomethylated at the benzylic position on heating with (MeO)2CO and K2COJ in an autoclave at I80°C. 

The same reactivity pattern for the activated toluene and the deactivated chlorobenzene is generally observed in electrophilic substitution reactions. Indeed, the results lor nitration obey a linear free energy relationship based on o as shown In Figure 1. ... 

If acetoxylation were a conventional electrophilic substitution it is hard to understand why it is not more generally observed in nitration in acetic anhydride. The acetoxylating species is supposed to be very much more selective than the nitrating species, and therefore compared with the situation in (say) toluene in which the ratio of acetoxylation to nitration is small, the introduction of activating substituents into the aromatic nucleus should lead to an increase in the importance of acetoxylation relative to nitration. This is, in fact, observed in the limited range of the alkylbenzenes, although the apparently severe steric requirement of the acetoxylation species is a complicating feature. The failure to observe acetoxylation in the reactions of compounds more reactive than 2-xylene has been attributed to the incursion of another mechan-104... 

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

A methyl group is an electron releasing substituent and activates all of the ring carbons of toluene toward electrophilic attack The ortho and para positions are activated more than the meta positions The relative rates of attack at the various positions m toluene compared with a single position m benzene are as follows (for nitration at 25°C)... 

Alkyl groups are as we saw when we discussed the nitration of toluene in Sec tion 12 10 activating and ortho para directing substituents Aryl and alkenyl substituents resemble alkyl groups in this respect they too are activating and ortho para directing... 

Stereoselectivity was also observed in the Friedel-Crafts reaction of optically active plienyloxirane with toluene and anisole. The product diarylethanol had an enantiomeric ratio of 60 40 (37). 

Whereas the above reactions are appHcable to activated aromatics, deactivated aromatics can be formylated by reaction with hexamethylenetetramine in strong acids such as 75% polyphosphoric acid, methanesulfonic acid, or trifluoroacetic acid to give saUcylaldehyde derivatives (117). Formyl fluoride (HCOF) has also been used as formyl a ting agent in the Friedel-Crafts reaction of aromatics (118). Formyl fluoride [1493-02-3] in the presence of BF was found to be an efficient electrophilic formyl a ting agent, giving 53% para-, 43% ortho- and 3.5% meta-tolualdehydes upon formylation of toluene (110). 

In addition, boron, aluminum, and gallium tris(triduoromethanesulfonates) (tridates), M(OTf)2 and related perduoroalkanesulfonates were found effective for Friedel-Crafts alkylations under mild conditions (200). These Lewis acids behave as pseudo haUdes. Boron tris(tridate) shows the highest catalytic activity among these catalysts. A systematic study of these catalysts in the alkylation of aromatics such as benzene and toluene has been reported (201). 

Resorcinol Derivatives. Aminophenols (qv) are important intermediates for the syntheses of dyes or active molecules for agrochemistry and pharmacy. Syntheses have been described involving resorcinol reacting with amines (91). For these reactions, a number of catalysts have been used / -toluene sulfonic acid (92), zinc chloride (93), zeoHtes and clays (94), and oxides supported on siUca (95). In particular, catalysts performing the condensation of ammonia with resorcinol have been described gadolinium oxide on siUca (96), nickel, or zinc phosphates (97), and iron phosphate (98). 

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of organic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution ate embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible appHcation of PEI. 

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