Benzene, reactivity index

There are principally two different approaches of correlating experimental rate data of electrophilic substitution with reactivity indices (1) Correlating the index with the rate data of a given reaction, e.g. bromination. For example, a satisfying correlation of Dewar reactivity numbers with the log of rate constants of the bromination of benzene, naphthalene (1- and 2-position), biphenyl (4-position), phenanthrene (9-position), and anthracene (9-position) has been observed [55]. In correlations of this type the reactivity index corresponds to the reactivity constant in the Hammett equation while the slope of the linear correlation corresponds to the reaction constant (see also Sect. 3) (2) correlating the index with experimental a values. 

What Zhou and Parr found was that the smaller the activation hardness is, the faster is the reaction." Thus is a reactivity index. Table 3.18 shows the results for the amounts of ortho-, para- and weZ -substitution in the nitration of substituted benzenes. Similar good results were found for the site selectivity in a large number of condensed-ring hydrocarbons and heterocyclic molecules. 

Langenaeker W, Demel K, Geerlings P. Quantum-chemical study of the Fukui function as a reactivity index. Part 2. Electrophilic substitution on mono-substituted benzenes. J Mol Struct Theochem 1991 234 329-342. 

The HOMO—LUMO gap, a qualitative reactivity index,was obtained from Hiickel calculations of C6o. ° The HOMO-LUMO gap of Ceo (0.757/3) is much smaller than that of benzene (HOMO—LUMO = 2/3). A related index is the absolute chemical hardness which is defined by the expres-... 

Primarily, to calculate the reactivity index at the ground state of both the closed systems methane (CH4) and benzene (CgHg) along with their chlorine-substituted compounds are chosen because methane and benzene is the doorway of the understanding of the properties of larger aliphatic and aromatic compound, respectively. A systematic substitution of hydrogen for both the moieties (methane and benzene) by chlorine was performed to get all the chlorine-substituted products until CCI4 and CeCl, respectively. At first, those molecules are optimized with DPT at their neutral state and as well as cationic and anionic forms. The reactivity index values and the relative reactivity indices for individual centers of the series of chlorine-substituted methane and benzene-related compounds were computed. These molecules with the optimized structure were then subjected to the weak electric field to get the response function, followed by Cl calculation with... 

Reactivity Index and Polarizability Calculation for Benzene (C6H5) Series... 

A modification of the Hammett approach, suggested by Brown, called the selectivity relationship is based on the principle that reactivity of a species varies inversely with selectivity. Table 11.3 " shows how electrophiles can be arranged in order of selectivity as measured by two indexes (1) their selectivity in attacking toluene rather than benzene, and (2) their selectivity between the meta and para positions in toluene. As the table shows, an electrophile more selective in one respect is also more selective in the other. In many cases, electrophiles known to be more... 

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],... 

MO studies of aromatic nitration cast doubt on the existence of jt-complexes and electron-transfer complexes in liquid-phase nitrations.14 The enthalpy of protonation of aromatic substrates provides a very good index of substrate reactivity to nitration. Coulomb interaction between electrophile and substituent can be a special factor influencing regioselectivity. A detailed DFT study of the reaction of toluene with the nitronium ion has been reported.15 Calculated IR spectra for the Wheland intermediates suggest a classical SE2 mechanism. MO calculations of cationic localization energies for the interaction of monosubstituted benzenes with the nitronium ion correlate with observed product yields.16... 

Properties Liquid. D 0.77 (25C), refr index 1.4057 (25C), bp 125C, flash p 77F (25C). Soluble in acetone, benzene, ethyl ether, heptane, perchloroethy-lene reactive with methanol and water. 

Classification Mixed petroleum hydrocarbons Definition Mixed isomer contg. 85% nonane and 15% trimethylbenzene Properties Colorless clear liq. petrol, odor misc. with abs. alcohol, benzene, ether, chloroform, CCI CS2, most org. soivs. insol. in water m.w. 150 sp.gr. 0.78 f.p. -70 C b.p. 157-196 C flash pt. (CC) 40 C ref. index 1.43 (20 C) KB value 32-37 Toxicology ACGIH TLV/TWA100 ppm LD50 (oral, rat) > 5 g/kg, (skin, rabbit) > 3 g/kg LCLo (inh., cat, 2.5 h) 10 g/m mildly toxic by inh. (irritation, CNS effects, headache, dizziness, confusion, fatigue) human eye irritant probably a mild to mod. skin irritation TSCA listed Precaution Combustible flamm. exposed to heat or flame explosive in vapor form when exposed to heat or flame reactive with oxidizing materials... 

FIGURE 4.5 Heuristic representation of the concept of absolute aromaticity (for the benzene pattern) as the stabilization difference of a given index of reactivity between atoms-in-molecule and molecular orbitals bonding configurations (Putz, 2010b). 

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