Toluene protonation

A number of other features apparent in the toluene proton spectrum are worthy of note at this stage. Each absorption is accompanied by a number of small satellite peaks equally spaced on either side of the main absorptions. These may be spinning side-bands or 13C satellites (p. 342). The spinning side-bands are caused by inhomogeneities in the magnetic field and in the sample tube. They... 

The NMR spectrum is a curve showing the variation in magnetisation as a function of frequency, the latter being expressed as a relative value (ppm) in relation to a reference frequency. For a given isotope, at least for 1/2 spin nuclei (by far the most commonly encountered), the surface of the signal is proportional to the number of nuclei resonating at this frequency. For example, in the toluene proton spectrum, the surface of the signal CH3 will be... 

Figure 4.3 An illustration of the impact of humidity on the detection of acetone (protonated signal at m/z 59) and toluene (protonated signal at m/z 93). Panel (a) shows an approximately linear decrease in [HsO ] with humidity and a corresponding linear increase in [HjO (H2O)]. Panel (b) shows the corresponding effects on the protonated acetone and toluene signals and panel (c) shows the response when the humidity effect was included via the factor X. A value oIXm = 0.5, which reflects the relative transmission of H3O (H2O) toHiO, was appropriate for acetone since it reacts with both and HsO (H2O) at roughly the same rate. On...

Carbanions stabilized by phosphorus and acyl substituents have also been frequently used in sophisticated cyclization reactions under mild reaction conditions. Perhaps the most spectacular case is the formation of an ylide from the >S-lactam given below using polymeric Hflnig base (diisopropylaminomethylated polystyrene) for removal of protons. The phosphorus ylide in hot toluene then underwent an intramolecular Wlttig reaction with an acetyl-thio group to yield the extremely acid-sensitive penicillin analogue (a penem I. Ernest, 1979). 

The cation—radical intermediate loses a proton to become, in this case, a benzyl radical. The relative rate of attack (via electron transfer) on an aromatic aldehyde with respect to a corresponding methylarene is a function of the ionization potentials (8.8 eV for toluene, 9.5 eV for benzaldehyde) it is much... 

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. 

The low temperature limitation of homogeneous catalysis has been overcome with heterogeneous catalysts such as modified Ziegler-Natta (28) sohd-supported protonic acids (29,30) and metal oxides (31). Temperatures as high as 80°C in toluene can be employed to yield, for example, crystalline... 

In some processes the reactant bases are too weak to be protonated significantly except in the presence of very strong acids such as fuming sulfuric acid or a mixture of concentrated sulfuric and nitric acids, ie, mixed acid. Nitration of toluene, for example, requires such solutions two Hquid phases are present in the reactor. 

Bromination has been shown not to exhibit a primary kinetic isotope effect in the case of benzene, bromobenzene, toluene, or methoxybenzene. There are several examples of substrates which do show significant isotope effects, including substituted anisoles, JV,iV-dimethylanilines, and 1,3,5-trialkylbenzenes. The observation of isotope effects in highly substituted systems seems to be the result of steric factors that can operate in two ways. There may be resistance to the bromine taking up a position coplanar with adjacent substituents in the aromatization step. This would favor return of the ff-complex to reactants. In addition, the steric bulk of several substituents may hinder solvent or other base from assisting in the proton removal. Either factor would allow deprotonation to become rate-controlling. 

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

Figure 13.18 The 1H NMR spectrum of toluene, showing the accidental overlap of the five nonequivaient aromatic ring protons.

The Lewis acid of choice for most of the cyclization reactions is ethylaluminum dichloride, because of its exceptional properties it is a mild Lewis acid, and, as an organometallic compound, can serve as a proton sponge , and thereby inhibit competitive protodesilyla-tion37. The desired precursors reacted smoothly with 1.1 equivalents of ethylaluminum dichloride in toluene or dichloromethane at low temperature to generate diastereoselectively the desired spiro[4.5]decanones38. 

With the exception of Lin s results all studies on the catalysis by strong protonic adds (mainly H2SO4, benzene-, toluene- and naphthalenesulfonic adds) fit an overall second-order reaction1 7-13-702 219 254- 308,313-315,319,321,325,327)... 

The acidity of benzylic protons of aromatics complexed to transition-metal groups was first disclosed by Trakanosky and Card with (indane)Cr(CO)3 [61]. Other cases are known with Cr(CO)3 [62], Mn(CO)3 [63], FeCp+ [64, 65], and Fe(arene)2+ [31, 66] but none reported the isolation of deprotonated methyl-substituted complexes. We found that deprotonation of the toluene complex gives an unstable red complex which could be characterized by 13C NMR ( Ch2 = 4.86 ppm vs TMS in CD5CD3) and alkylated by CH3I [58] Eq. (13) ... 

As a measure of the stereoregularity, an index EQ-H% was defined as the precent of the equatorial acetal protons to the total acetal protons. Figure 2 illustrates the temperature dependence of EQ-H% s of the polymer obtained in toluene (A), methylene chloride (B), and 1-nitropropane (C). No significant difference is observed at... 

Diazenyl radicals have also been detected in related systems. The rapid rearrangement of 1,3,5-triarylpentazadienes [equation (47)] involves intermediate triazenyl-diazenyl radical pairs, as indicated by the appearance in emission of the n.m.r. transitions of the -methyl protons of the starting material when Ar = Ar =j -CHg.C6H4 (Hol-laender and Neumann, 1970). The weak emission of benzene which accompanies a much more intense emission due to toluene when the 1,3-diaryltetrazene 6 decomposes in acetone at 50° has been interpreted... 

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