Toluene substitution reactions

The aromatic ring has high electron density. As a result of this electron density, toluene behaves as a base, not only in aromatic ring substitution reactions but also in the formation of charge-transfer (tt) complexes and in the formation of complexes with super acids. In this regard, toluene is intermediate in reactivity between benzene and the xylenes, as illustrated in Table 2. 

These effects can be attributed mainly to the inductive nature of the chlorine atoms, which reduces the electron density at position 4 and increases polarization of the 3,4-double bond. The dual reactivity of the chloropteridines has been further confirmed by the preparation of new adducts and substitution products. The addition reaction competes successfully, in a preparative sense, with the substitution reaction, if the latter is slowed down by a low temperature and a non-polar solvent. Compounds (12) and (13) react with dry ammonia in benzene at 5 °C to yield the 3,4-adducts (IS), which were shown by IR spectroscopy to contain little or none of the corresponding substitution product. The adducts decompose slowly in air and almost instantaneously in water or ethanol to give the original chloropteridine and ammonia. Certain other amines behave similarly, forming adducts which can be stored for a few days at -20 °C. Treatment of (12) and (13) in acetone with hydrogen sulfide or toluene-a-thiol gives adducts of the same type. 

Equation (7-85) is a selectivity-reactivity relationship, with lower values of Sf denoting lower selectivity. Lower values ofpt correspond to greater reactivity, with the limit being a partial rate factor of unity for an infinitely reactive electrophile. This selectivity-reactivity relationship is followed for the electrophilic substitution reactions of many substituted benzenes, although toluene is the best studied of these. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

The substitution reaction of toluene with Br2 can, in principle, lead to the formation of three isomeric bromotoluene products. In practice, however, only o- and jp-bromotoluene are formed in substantial amounts. The meta isomer is not formed. Draw the structures of the three possible carbocation intermediates (Problem 15.48), and explain why ortho and para products predominate over meta. 

Partenheimer showed (ref. 15) that when toluene was subjected to dioxygen in acetic acid no reaction occurred, even at 205 °C and 27 bar. He also showed that when a solution of cobalt(II) acetate in acetic acid at 113 °C was treated with dioxygen ca. 1 % of the cobalt was converted to the trivalent state. In the presence of a substituted toluene two reactions are possible formation of a benzyl radical via one-electron oxidation of the substrate or decarboxylation of the acetate ligand (Fig. 9). Unfortunately, at the temperatures required for a reasonable rate of ArCH3 oxidation (> 130 °C) competing decarboxylation predominates. As noted earlier, two methods have been devised to circumvent this undesirable... 

The general validity of these equations is supported by a great deal of experimental data on aromatic substitution reactions of toluene. Examples of values for some reactions obtained from these equations are given in Table 11.4. For other substituents, the treatment works well with groups that, like methyl, are not very... 

No electrophilic aromatic substitution reactions of toluene, ethylbenzene, and cumene occur with BBrj in the dark the electrophile is too weak for these reactions. The photochemical reactions followed by hydrolysis give the p-isomers of the corresponding boronic acids as the major products (delocalization band in Scheme 9) [44]. 

A complex reaction takes place when dichlorobis(triphenylphosphine)-nickel (5) is treated with excess methylmagnesium bromide in ether. Detectable amounts of benzene, toluene, and biphenyl are formed, together with mixed phosphines. Nickel appears to be necessary for the substitution reaction since triphenylphosphine alone does not react with the Grignard reagent. 

In addition to complexes of the type, trans-[Tc02(pyr)2]+ (pyr pyridine or imidazole), various species, such as Jrans-[TcO(RO)X2(pyr)2] (R CH3 or CH3CH2 X Cl or Br) were detected in alcohol. Further complicated mixed-valence species, [X2(pyr)3Tc-0-Tc(pyr)2X3] and [X(pyr)4TcO Tc(pyr)X4], appeared on long standing or heating in pyridine [44,45]. Rather peculiar features were found in the substitution reaction of trans-[Tc02(py)4] + with 4-aminopyridine (apy) in mixtures of methanol and toluene in the presence of excess pyridine ([py] — 0.14 M) [46]. Its rate was expressed as... 

Substitution reactions of Ir4(CO)12 have generally been carried out in boiling toluene and this gives the tetra-substituted derivatives46,86,219 The di- and tri-substituted derivatives have generally been obtained by reaction of tertiary phosphines, at room temperature, with the anions [Ir4(C0)nH] and [Ir8(CO)2o]2-respectively9,86) reactions which probably involve the disproportionation of these anionic species. The tri-substituted derivatives have also been obtained using the reaction —... 

Historically, the thermal transesterification of (-)-ethyl p-toluene-sulfinate 224 with n-butanol affording (+)-n-butyl p-toluenesulfinate 225 described by Phillips in 1925 (100) is the first nucleophilic substitution reaction at chiral sulfur involving a Walden-type inversion. The evidence for inversion of configuration in this reaction was based on the assumption that both (-)-esters 224 and 225 obtained from the kinetic resolution have the same configuration. 

Spurred by our desire to avoid use of expensive dipolau aprotic solvents in nucleophilic aromatic substitution reactions, we have developed two alternative phase transfer systems, which operate in non-polar solvents such as toluene, chlorobenzene, or dichlorobenzene. Poleu polymers such as PEG are Inexpensive and stable, albeit somewhat inefficient PTC agents for these reactions. N-Alkyl-N, N -Dialkylaminopyridinium salts have been identified as very efficient PTC agents, which are about 100 times more stable to nucleophiles than Bu NBr. The bis-pyridinium salts of this family of catalysts are extremely effective for phase transfer of dianions such as bis-phenolates. 

Table 8.12 Activation Parameters for Substitution Reactions of Ni°L. in Toluene at 25°C243...

Kenawy 64) immobilized ammonium and phosphonium peripheral functionalized dendritic branches on a montmorillonite supported chloromethylstyrene/methyl methacrylate copolymer (74-75). These polymer/montmorillonite-supported dendrimers were used as phase transfer catalysts (PTC) for the nucleophilic substitution reaction between -butyl bromide and thiocyanate, cyanide, and nitrite anions in a toluene or a benzene/water system. These PT catalysts could be recycled by filtration of the functionalized montmorillonite from the reaction mixture. Generally,... 

If toluene is chlorinated to benzo-trichloride, the substitution reaction has oxidized the carbon atom of the methyl group to the acid stage and no further oxidation is necessary. At that point, hydrolysis alone is sufficient ... 

The understanding of polar effects on free radical reactions arose from studies of free radical polymerization where transition state effects were empha-sized. Further studies involved diacyl peroxide reactions (equation 45), hydrogen abstraction from ring-substituted toluenes, and reactions of peresters involving transition state 38 (equation 57). ... 

Radical fragmentation of 2-nitrophenyl-azo-trityl resin was studied in the presence of various radical acceptor solvents to elucidate possible radical reaction pathways. When using benzene as solvent, only 2-nitro-bi-phenyl was formed as the product of radical substitution reaction (SNR) in 67% yield. Hydrogen-radical abstraction from the polymer backbone (e.g., from the benzylic units of polystyrene) was completely suppressed. When toluene was used as solvent, a mixture of the following products was obtained nitrobenzene, 4-methyl-2 -nitrobisphenyl, 2-methyl-2 -nitro-bisphenyl, and 3-methyl-2 -nitrobisphenyl (9 9 1 1). In the case of toluene, the nitro-aryl radicals undergo H-abstraction with radical substitution as a competing reaction pathway. These results indicate that H-abstraction... 

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