Substituted benzenes alkyl

Benzene and alkyl-substituted benzenes can be hydroxylated by reaction with H2O2 in the presence of an acidic catalyst. What is the structure of the reactive electrophile Propose a mechanism for the reaction. 

The Fricdel-Crafts type polyalkylation of alkyl-substituted benzenes with 3 becomes easier and faster as the number of electron-donating methyl groups on the phenyl group increases. This is consistent with the fact that the alkylation occurs in the fashion of electrophilic substitution. The tendency of starting incthylben-zenes to form reoriented products also increases in the same order from toluene to mesitylene. 

Alkyl-substituted benzenes are oxidized both on the benzene ring and on the side chain. Additionally, some dimerization occurs.36 Alkylbenzenes containing linear alkyl groups are oxidized preferentially at the side chain33 nearest the benzene ring for example, ethylbenzene oxidizes first to 1-phenyl ethanol and then to acetophenone.36... 

Reaction of rhenium atoms with alkyl-substituted arenes forms dirhenium- l-arylidene compounds (2 2) (Figure 3). The products require insertion, presumably sequential, into two carbon-hydrogen bonds of the alkyl substituent. These reactions seem highly specific and require only the presence of an alkyl-substituted benzene that possesses a CH2 or CH3 substituent. Thus, co-condensation of rhenium atoms with ethylbenzene gives two isomers (see Figure 3) in which the products arise from insertion into the carbon-hydrogen bonds of the methylene or the methyl group. The product distribution in this reaction is in accord with statistical attack at all available sp3 C-H bonds. 

Interesting conformational effects may be encountered in alkyl substituted benzenes such as the three rotamers of ortho xylene shown below ... 

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

The reaction of ethylbenzene with five equivalents of Ic under the same alkylation conditions used for toluene, gives pentakis- (25%), tetrakis- (9%), tris- (4%), and bis[2-(dichloromethylsilyl)ethyl]ethylbenzene (1%) as well as a mixture of many transalkylated products (44%). It is of interest that longer alkyl-substituted benzenes exhibited different behavior in peralkylations with Ic. The transalkylation of ethylbenzene is responsible for the significantly low yield (25%) of peralkylation product in comparison with yields obtained from the alkylation of benzene " or toluene. Peralkylation of K-propylbenzene and K-butylbenzene gives similar results to those of ethylbenzene. 

As shown by Table 7 above, the chiral titanium catalyst-MS 4A system is widely applicable to the reactions of a variety of dienophiles and dienes when a suitable alkyl substituted benzene is employed as a solvent, and synthetically important Diels-Alder adducts are prepared in high enantioselectivity by the present catalytic process. 

Many chromophores are suitable for use in the exciton chirality method. One of the features required for such a chromophore is its planarity or near-planarity. Nonplanar (inherently dissymmetric) chromophores would contribute to the CD spectra by other mechanisms. The other limiting factor is the position of the transition in the spectral region studied. For example, the 1B transition in the alkyl-substituted benzene chromophore appears near the short-wavelength recording limit around 200 nm, making its use in the exciton chirality method less attractive. Furthermore, the direction of polarization of the lB transition in alkyl-substituted benzene derivatives is not readily determined. In such cases calculation of the rotatory strength is more reliable than qualitative analysis. 

Assignments for C—H out-of-plane bending bands in the spectra of substituted benzenes appear in the chart of characteristic group absorptions (Appendix C). These assignments are usually reliable for alkyl-substituted benzenes, but caution must be observed in the interpretation of spectra when polar groups are attached directly to the ring, for example, in nitroben-zenes, aromatic acids, and esters or amides of aromatic acids. 

The X-substituted benzene (aniline, Figure 11.2) is activated toward electrophilic attack since the HOMO is raised significantly. The electrophile would be directed to the ortho, para, and ipso positions of the ring and to the X substituent itself. The ipso channel is usually nonproductive since the common heteroatom-based X substituents are not easily displaced as Lewis acids. Loss of substituent is frequently observed with tertiary alkyl-substituted benzenes. Attachment of the electrophile to the X substituent is most likely if... 

There have been only a few reports of direct hydroxylation362 by an electrophilic process (see, however, 2-26 and 4-5).363 In general, poor results are obtained, partly because the introduction of an OH group activates the ring to further attack. Quinone formation is common. However, alkyl-substituted benzenes such as mesitylene or durene can be hy-droxylated in good yield with trifluoroperacetic acid and boron trifluoride.364 In the case of mesitylene, the product is not subject to further attack ... 

The course of the reaction of methyl-substituted benzene derivatives with cesium fluoroxysulfate in acetonitrile at 35-40 C strongly depends on the structure of the molecule. Toluene (16, R1 = R2 = H) and other alkyl-substituted benzenes 16 and 17 are mainly functionalized on the side chain, while 1,3.5-trimethylbenzene (18) gives mainly ring-substituted products 24 however. 1.2.4,5-tetramethylbenzene (19) and hexamethylbenzene again give mainly or exclusively side-chain products.25... 

Friedel-Crafts alkenylation of arenes with various phenylacetylenes to yield 1,1-diarylalkenes was reported. Alkyl-substituted benzenes and naphthalene react with phenylacetylene in the presence of zeolite HSZ-360 to afford the products in good to excellent yields and selectivities 415... 

The order of reactivity of different alkyl-substituted benzenes is toluene > ethylbenzene > isopropylbenzene, which is opposite to the general reactivity expected from the C—H bond energies. This was explained in terms of an initiating electron transfer equilibrium between Co(OAc)3 and the arene as the rate-determining... 

The selectivity of chlorination reactions carried on in solution is increased markedly in the presence of benzene or alkyl-substituted benzenes because benzene and other arenes form loose complexes with chlorine atoms. This substantially cuts down chlorine-atom reactivity, thereby making the chlorine atoms behave more like bromine atoms. 

Detailed study of the mechanism of solvolysis of a number of non-K-region arene oxides like 86,90 alkyl substituted benzene oxides,91 45,47,88 and 4888 has been carried out. They present a simple and consistent picture (Fig. 3). Below pH 6 all of them show general acid catalysis, and above pH 6 the rate remains constant with an increase in pH. The pH dependence of aromatiza-tion of 45 is described in terms of two independent reactions taking place... 

Nickelsen, M.G., Cooper, W.J., Kurucz, C.N., and Waite, T.D., Removal of benzene and selected alkyl-substituted benzenes from aqueous solution utilizing continuous high-energy electron irradiation, Environ. Sci. Technol., 26, 144, 1992b. 

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