Of monosubstituted benzene

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

Nitration of monosubstituted benzenes by HNO3 in CH3NO2 or AC2O 0 or 25 -6.22... 

Bromination of monosubstituted benzenes by HOBr and HC1O4 in 50% dioxane 25 -5.78... 

Recently, a kinetic study has been made of the substitution of diazotised sulphanilic acid in the 2 position of 4-substituted phenols under first-order conditions (phenol in excess) in aqueous buffer solutions at 0 °C131a. A rough Hammett correlation existed between reaction rates and am values, with p about -3.8 however, the point for the methoxy substituent deviated by two orders of magnitude and no explanation was available for this. The unexpectedly low p-factor was attributed to the high reactivities of the aromatic substrates, so that the transition state would be nearer to the ground state than for reaction of monosubstituted benzene derivatives. 

We have discussed orientation in the case of monosubstituted benzenes entirely in terms of attack at the ortho, meta, and para positions, but attack at the position bearing the substituent (called the ipsoposition ) can also be important. Ipso attack has mostly been studied for nitration. When NOj attacks at the ipso position there are at least five possible fates for the resulting arenium ion (13). 

In these terms, Shatenshtein s partial rate factors (37) for the H—D exchange of monosubstituted benzenes in liquid ammonia are not anomalous (2m). The analysis of this rate data is included in Table XXVIII. Even though the data are not precise enough to meet the desired standard of precision of fits, the sequences > X and P/> p p - P/ are clearly evident in the results. Consequently, this data set (which is of the or type) may be taken as providing supporting evidence with the pyridinium ionization (which is of the gr type) for unique X blends for the positions o- and p- to ionization from the ring position as compared with the X blends for side-chain ionization. 

The first recorded correlation of dipole moments with substituent constants was observed by Taft (3), who reported results for alkyl cyanides, chlorides, iodides, and tertiary amines. Kross and Fassel (31) have reported the correlations of dipole moments for 4-substituted nitrobenzenes with the simple Hammett equation. Rao, Wohl, and Williams (32) have studied the correlation of dipole moments of disubstituted benzenes with eq. (1) and of monosubstituted benzenes with the equation... 

The alkylation of benzene derivatives with methyl(vinyl)dichlorosilane (3) will be described in detail. Alkylation of monosubstituted benzenes such as toluene, chlorobenzene, and biphenyl at 75-80 C for 2 h afforded the corresponding alkylated products in 50-63% yields." ... 

Allen CCR, DR Boyd, H Dalton, ND Sharma, I Brannigan, NA Kerley, GN Sheldrake, SC Taylor (1995) Enantioselective bacterial biotransformation routes to cw-diol metabolites of monosubstituted benzenes, naphthalene and benzocycloalkenes of either absolute configuration. J Chem Soc Chem Commun 117-118. 

Retention volumes of monosubstituted benzenes, benzoic acid, phenols, and anilines have been measured in RPLC [76]. Buffered acetonitrile/water and tetrahydrofuran/water eluents were used with an octadecylsilica adsorbent. From the net retention volumes, a substituent interaction effect was calculated and described with the linear free energy relationship developed by Taft. The data was interpreted in terms of hydrogen bonding between the solutes and the eluent. 

Evaluation of the only appropriate Fukui function is required for investigating an intramolecular reaction, as local softness is merely scaling of Fukui function (as shown in Equation 12.7), and does not alter the intramolecular reactivity trend. For this type, one needs to evaluate the proper Fukui functions (/+ or / ) for the different potential sites of the substrate. For example, the Fukui function values for the C and O atoms of H2CO, shown above, predicts that O atom should be the preferred site for an electrophilic attack, whereas C atom will be open to a nucleophilic attack. Atomic Fukui function for electrophilic attack (fc ) for the ring carbon atoms has been used to study the directing ability of substituents in electrophilic substitution reaction of monosubstituted benzene [23]. In some cases, it was shown that relative electrophilicity (f+/f ) or nucleophilicity (/ /f+) indices provide better intramolecular reactivity trend [23]. For example, basicity of substituted anilines could be explained successfully using relative nucleophilicity index ( / /f 1) [23]. Note however that these parameters are not able to differentiate the preferred site of protonation in benzene derivatives, determined from the absolute proton affinities [24],... 

Theory and Spectra of Monosubstituted Benzenes. VI. Styrene, Phenyl-acetylene and Phenylcycloalkanes, J. Amer. chem. Soc. 72, 5260 (1950). 

Kralj, F. and Sincic, D. Mutual solubilities of phenol, salicyaldehyde, phenol-salicyaldehyde mixture, and water with and without the presence of sodium chloride and sodium chloride plus sodium sulfate, J. Chem. Eng. Data, 25 (4) 335-338,1980. Kramer, C.R. and Henze, U. Partitioning properties of benzene derivatives. 1. Temperature dependence of the partitioning of monosubstituted benzenes and nitrobenzenes in the n-octanol/water system, Z. Phys. Chem., 271(3) 503-513,1990. Krasnoshchekova, R.Ya. and Gubergrits, M. Solubility of paraffin hydrocarbons in fresh and salt water, Neftekhlmlya, 13(6) 885-888, 1973. 

The solution electron affinities of a series of monosubstituted benzenes were measured by the ESR method (Stevenson et al. 1991b). The equilibrium constant for the electron transfer in XCgHj + XCgDj = XCgHj + XCgDj, where X = H, tert-Bu, OMe, Ph, CN, was found to be less than unity for all cases (entries 1-6 in Table 2.1). These equilibrium constants are in linear correlation to the appropriate constants o of the substituents. 

In the infrcired spectra of monosubstituted benzenes, usually two very strong peaks appear one between 690 and 710 cm" and one between 730 and 770 cm". ... 

Similarly, in the series of monosubstituted benzenes, the calculated changes in cr and TT populations at the para-carbon atom are accurately inversly related to one another, as convincingly demonstrated elsewhere [127]. The tt population shows the greater change, and the a population seems to be altered by approximately 55% in the opposite direction. The results obtained for the tt and cr populations at the mefa-carbon atom are similar, but these points show some scatter from linearity. However, most of the meta points fall close to the correlation line drawn for the para-carbon atoms. 

The 5 ring H s of monosubstituted benzenes, C H G, are not equally reactive. Introduction of E into Cf,HjG rarely gives the statistical distribution of 40% ortho, 40% meta, and 20% para disubstituted benzenes. The ring substituent(s) determine(s) (a) the orientation of E (meta or a mixture of ortho and para) and (b) the reactivity of the ring toward substitution. 

Table A3. Observed and calculated Faraday B values of the first (a) and second (b) n - n transitions of monosubstituted benzenes (10 5 pD/fcm ) 297)...

Table A4. Theoretical values of the first and second tt — tt transitions of monosubstituted benzenes (10 5 [lD2/cm )2911...

These formulae explain why the o and p carbons of monosubstituted benzenes are shielded by electron releasing substituents but deshielded by electron acceptors, while the m carbons remain almost unaffected by both kinds of substituents [73]. 

Inductive and electric field effects of the substituents may overlap the o-mesomeric effects. However, the m and p shifts of monosubstituted benzenes generally follow the pattern discussed above. 

Thus, the C —H nuclei in the para position of monosubstituted benzene derivatives relax faster than those in the ortho or meta positions (Table 3.16 [151]). The reason for this behavior lies in a preferred rotation about the molecular axis passing through the substituent X and the p-carbon. During this motion, the para C —H bond does not change its direction relative to the field B0 fluctuating local fields can only arise at the p-C nucleus by rotations of the molecule perpendicular to the preferred axis. However,... 

Carbon-13 shift values of a small selection of monosubstituted benzenes [383] are collected in Table 4.53. Signal assignments are based on conventional techniques such as proton off-resonance and gated decoupling as well as comparative measurements of specifically deuterated compounds [384],... 

Table 1 ortho para Selectivities in chlorination of monosubstituted benzenes with TBH -zeolite HNaX in acetonitrile... 

The orientation and reactivity effects of substituents discussed for the substitution of monosubstituted benzenes also hold for disubstituted benzenes, except that the directing influences now come from two groups. Qualitatively, the effects of the two substituents are additive on the reactivity. We therefore would expect 4-nitromethylbenzene to be less reactive than methylbenzene... 

Figure 2.2, in which a constants are plotted against log klk0, for the bromina-tion of monosubstituted benzenes, shows an example of the usefulness of these new parameters. As can be seen from Structures 12 and 13—which are representations of the intermediates in the ortho and para bromination of anisole—substituents electron-donating by resonance ortho or para to the entering bromine can stabilize the positive charge in the intermediate and therefore also in the transition state by through-resonance. 

Figure 2.2 Bromination of monosubstituted benzenes in acetic acid. From H. C. Brown and Y. Okamoto, J. Amer. Chem. Soc., 80, 4979 (1958). Reprinted by permission of the American Chemical Society.

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