Benzene 2-substituted

Many substituted benzene rings undergo electrophilic aromatic substitution. Common substituents include halogens, OH, NH2, aUcyl, and many functional groups that contain a carbonyl. Each substituent either increases or decreases the electron density in the benzene ring, and this affects the course of electrophilic aromatic substitution, as we will learn in Section 18.7. 

What makes a substituent on a benzene ring electron donating or electron withdrawing The answer is inductive effects and resonance effects, both of which can add or remove electron density. 

Inductive effects stem from the electronegativity of the atoms in the substituent and the polarizability of the substituent group. 

Inductive and resonance effects were first discussed in Sections 2.5B and 2.5C, 

Considering inductive effects only, an NH2 group withdraws electron density and CH3 donates electron density. 

The mono- and di-substituted benzenes, for which the relative swelling power (C as defined in Eq. 14) have been determined thus far, are listed in Tables 2-4. In every case the volume (S) of sorbed liquid per gram of polymer increased linearly with X1/3 as noted in Figs. 9-11, and the square of the correlation coefficient (r) to the line of best fit, determined by linear regression analysis of the set of six S-data points, was in every case r2 0.99, and q13 was equal to 1.84 0.09. The corresponding C and oc (calculated therefrom Eq. 15) are also collected in Tables 2-4. These data show clearly that the value a = 2.50 for benzene is greater than that for any benzene derivative listed therein. Any substituent in place of 

C is the relative swelling power (in mL/g) of the liquid as defined in Eq. 14. a is the adsorption parameter (i.e. the number of adsorbed molecules per phenyl group of polymer at liquid saturation) as defined in Eq. 15. a/2.5 is the ratio of a,iqui(1/aphH. 

Xi is the Flory-Huggins interaction parameter x at v = 1, where v is the volume fraction of polymer in the polystyrene-liquid system. Xi was calculated from the corresponding observed C using Eq. 45 in Sect. 4.2. x at any other level of v can be calculated from Xi by means of Eq. 44. 

5 is 5pol with respect to benzene substituted liquids. 

The above results emphasize that the importance of steric hindrance relative to electronic attraction is much greater in the case of molecular association with polymers than it is in classical physico-organic chemistry regarding reactions of small molecules in solution [160]. The reason of course is that the attractive forces involved in molecular association are much weaker than the short-range forces of electronic interaction (at binding distances) involved in organic reactions. 

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], 

Meta carbon atoms (C-3.5) remain almost unaffected by all kinds of substituents. In monosubstituted benzenes they usually resonate at 129 + 1 ppm (Table 4.53). Recalling Section 3.1.3.6, electron releasing substituents (( + )-M substituents, electron donors) increase n electron densities in ortho and para positions and thereby induce a shielding relative to benzene ( 5c(t) p) 128.5 ppm). Electron withdrawing groups (( —)-M substituents, electron acceptors) decrease it electron densities in o and p positions thus, a deshielding relative to benzene is observed ( 5c o,P) 128.5 ppm). 

Electric field effect of the nitro group in nitrobenzene 

Para carbon shieldings, however, clearly follow the pattern described by the cannonical formulae. They may be correlated with the total charge densities Aq, obtained by CNDO calculations, according to eq. 4.14 [387], and with the Hammet a constants, as shown in 

Substituent increments Z, obtained from 13C shifts of numerous monosubstituted benzenes according to eq. (4.15) have been tabulated [383]. They permit prediction of benzene ring carbon shifts in multisubstituted benzenes according to eq. (4.16). These increments and their practical application will be summarized in Section 4.16. 

Quite a number of workers have examined specific aspect of the direct photolysis of substituted benzenes and, although the mechanisms are not fully understood, some important conclusions have been reached. Hentz and Burton examined the photolysis of toluene, ethyl benzene and mesitylene in both liquid and vapor states using a medium-pressure mercury lamp. They concluded that the gas-phase products, hydrogen, methane and ethane, were formed with a quantum yield of about 10 , while polymer formation was much more important. At 150 °C hydrogen was the most important gas-phase product except for the case of ethyl benzene in the vapour, where both methane and ethane were more important than hydrogen. Porter and Wright have shown by flash photolysis that benzyl radicals are formed in the photolysis of toluene and ethyl benzene and have observed the absorption spectrum of the benzyl radical. 

The importance of products of intermediate volatility has received attention in recent investigations, and it has been found that photoisomerization plays an important role with substituted benzenes. Van Tamelen and Pappas showed that a medium-pressure mercury lamp converted 1,2,4-tri-t-butylbenzene in ether solution to a Dewar benzene, l,2,5-tri-t-butylbicyclo[2.2.0]hexa-2,5-diene. Burg-stahler and Chien found that, in ether solution, o-di-t-butylbenzene, when photolysed with a mercury lamp, was converted entirely to a 4 1 mixture of / w-di-/-butylbenzene while either the meta or para compound was converted to the same mixture. Wilzbach and Kaplan have conducted a thorough investiga- 

A recent study by Ward on the photolysis of o-xylene in the vacuum ultraviolet (1600-2100 A) and at 2537 A has confirmed the observations of Wilzbach and Kaplan. Benzene, toluene, ethylbenzene, m- and p-xylene and o- and m-ethyltoluene were found in much the same proportions. Furthermore the product distribution was not altered substantially at the short wave lengths. The dimer o,o -dimethylbibenzyl was observed. A new product which was observed only in the vacuum ultraviolet photolysis was benzocyclobutene from isotopic studies this product was shown to arise from the loss of a single hydrogen atom from each of the methyl groups of o-xylene. 

In a study at 1470 A, McNesby et have shown that the mechanism of photolysis of cyclopropane involves the following reactions 

The most important products of decomposition are ethylene, allene, hydrogen, butenes and ethane. Ethylene is found to arise almost entirely from the primary decomposition. Photolysis of cyclo-C3H6-cyclo-C3D6 mixtures gives about 20% HD in the hydrogen, and indicates a significant contribution of an atomic hydrogen process. The formation of propene in a primary step may represent a small fraction of the total primary processes. 

When one hydrogen is replaced in an aromatic compound such as benzene, it is called mono-suhstitution. 

In most cases, the mono-suhstituted compounds are named as derivatives of benzene. 

Replacement of a hydrogen of benzene by chlorine is termed chlorination. When one or more hydrogens are replaced by an -NO2 (nitro group), it is called nitration. Reaction of benzene with sulfuric acid, a reaction known as sulfonation, leads to a sulfonic acid. Note that in each substitution reaction, a small hydrogen-containing compound is formed. 

An organic compound can be both aromatic and aliphatic that is, one or more of the hydrogens of a benzene ring can be replaced by an aliphatic group. Such compounds are always classihed as being aromatic. 

Aromatic compounds can be converted to totally aliphatic compounds by reaction with hydrogen. 

Intramolecular Friedel-Crafts acylation formed a product containing a new six-membered ring (in red), which was converted to LSD in several steps. 

All of the Friedel-Crafts reactions discussed thus far have resulted from intermolecular reaction of a benzene ring with an electrophile. Starting materials that contain both units are capable of intramolecular reaction, and this forms a new ring. For example, treatment of compound A, which contains both a benzene ring and an acid chloride, with AICI3, forms a-tetralone by an intramolecular Friedel-Crafts acylation reaction. 

Draw a stepwise mechanism for the intramolecular Friedel-Crafts acylation of compound A to form B. B can be converted in one step to the antidepressant sertraline (trade name Zoloft). 

R 200 mp band (K or E or primary band) -Wax, tttyt Cmax 260 mp band (B or secondary band) Wax m/i f aax Solvent 

Several objections have been raised for this interpretation17 Unlike in the 260 m/x band, no definite trend has been found in the position of the 200 m/x bands in alkyl benzenes or p-alkyl substituted benzenes. In many cases, the observed wavelengths are actually found to be in the reverse order (i.e., in the inductive order) to that predicted by C-—H hyperconjugation. At this point it may be stated that the evidence for hyperconjugation from the absorption spectra of alkyl substituted benzene derivatives is not definitive. 

Substitution with unsaturated groups has a more profound effect on the absorption bands than with alkyl groups (e.g. styrene and phenylacetylene. see Table 5.2). The effect of the ethyny group is less than that of a vinyl group. Cyclopropyl group shifts the 260 m/x band to 271 m/x and also intensifies it. thus lending evidence of its unsaturation properties13. 

Substitution of benzene with polar groups containing unshared electrons (auxochromes like OH or NH2). shifts the absorption bands to longer wavelengths and also intensifies them. The spectra of phenol and aniline in heptane solution are reproduced ifl Figwes 5.1 and 5.2. The vibrational structure of the 260 m/x band is not seen in the aniline spectrum. In general, the fine structure of the 260 m/x band disappears in polar solvents when the 

Br and I and R = H or alkyl groups, and have found that the spectra bear no simple relation to basicities, ionization potentials and other parameters. Recently, Rao and co-workers19 studied the absorption spectra of several polyphenyl derivatives of the elements of groups IVb and Vb. In the case of the Vb elements, the spectra of derivatives in which the atoms possess and do not possess unshared / -electrons (i.e. derivatives of the atoms in their trivalent and pentavalent states respectively) have been studied. The unshared / -electrons on the central atoms of the triphenyl derivatives of the Vb elements interact strongly with the r-orbitals of the benzene rings and the vibrational structure of the 260 m/x band of benzene is completely absent. 

---

Cheng Y-W and Dunbar R C 1995 Radiative association kinetics of methyl-substituted benzene ions J. Rhys. Chem. 99 10 802-7... 

A point in case is provided by the bromination of various monosubstituted benzene derivatives it was realized that substituents with atoms carrying free electron pairs bonded directly to the benzene ring (OH, NH2, etc) gave 0- and p-substituted benzene derivatives. Furthermore, in all cases except of the halogen atoms the reaction rates were higher than with unsubstituted benzene. On the other hand, substituents with double bonds in conjugation with the benzene ring (NO2, CHO, etc.) decreased reaction rates and provided m-substituted benzene derivatives. 

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefimction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears coimterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds imder electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when daey are involved in chemical bonds. 

Dimethyl acetylenedicarboxylate (DMAD) (125) is a very special alkyne and undergoes interesting cyclotrimerization and co-cyclization reactions of its own using the poorly soluble polymeric palladacyclopentadiene complex (TCPC) 75 and its diazadiene stabilized complex 123 as precursors of Pd(0) catalysts, Cyclotrimerization of DMAD is catalyzed by 123[60], In addition to the hexa-substituted benzene 126, the cyclooctatetraene derivative 127 was obtained by the co-cyclization of trimethylsilylpropargyl alcohol with an excess of DMAD (125)[6l], Co-cyclization is possible with various alkenes. The naphthalene-tetracarboxylate 129 was obtained by the reaction of methoxyallene (128) with an excess of DMAD using the catalyst 123[62],... 

The name phenylene o-, m-, or p-) is retained for the radical —C5H4—. Bivalent radicals formed from substituted benzene derivatives and having the free valences at ring atoms are named as substituted phenylene radicals, with the carbon atoms having the free valences being numbered 1,2-, 1,3-, or 1,4-, as appropriate. 

Table 7.9 Electronic Absorption Bands for Representative Chromophores Table 7.10 Ultraviolet Cutoffs of Spectrograde Solvents Table 7.11 Absorption Wavelength of Dienes Table 7.12 Absorption Wavelength of Enones and Dienones Table 7.13 Solvent Correction for Ultraviolet-Visible Spectroscopy Table 7.14 Primary Bands of Substituted Benzene and Heteroaromatics Table 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives...

Table 7.53 Carbon-13 Chemical Shifts in Substituted Benzenes 7.104...

TABLE 7.14 Primary Bands of Substituted Benzene and Heteroaromatics In methanol. 

TABLE 7.15 Wavelength Calculation of the Principal Band of Substituted Benzene Derivatives In ethanol. 

The recurring para-substituted benzene rings and sulfur atoms form a symmetrical rigid backbone. 

Upon treatment with suitable cobalt complexes, methylbutynol cyclizes to a 1,2,4-substituted benzene. Nickel complexes give the 1,3,5-isomer (196), sometimes accompanied by linear polymer (25) or a mixture of tetrasubstituted cyclooctatetraenes (26). 

Neither benzenepentacarboxylic acid nor mellitic acid are manufactured commercially, but synthetic mellitic acid can be purchased as a laboratory chemical (99). Both can be synthesized by oxidizing the corresponding methylbenzenes or other substituted benzenes, and both are present in trace amounts after oxidation of coal or coal-like substances. 

Reactions of acetylene and iron carbonyls can yield benzene derivatives, quinones, cyclopentadienes, and a variety of heterocycHc compounds. The cyclization reaction is useful for preparing substituted benzenes. The reaction of / fZ-butylacetylene in the presence of Co2(CO)g as the catalyst yields l,2,4-tri-/ f2 butylbenzene (142). The reaction of Fe(CO) and diphenylacetylene yields no less than seven different species. A cyclobutadiene derivative [31811 -56-0] is the most important (143—145). 

When the pyrazole ring bears two adjacent functional substituents, it reacts like an o-substituted benzene. For example, 4,5-diaminopyrazoles behave similarly to... 

We will return to the aromatic stabilization of benzene in more detail in Chapter 9, but substituted benzenes provide excellent examples of how proper use of the resonance concept can be valuable in predicting reactivity. Many substituents can be readily classified... 

Fig. 4.3. Resonance, field, and inductive components of substituent effects in substituted benzenes.

Other matters that are important include the ability of the electrophile to select among the alternative positions on a substituted aromatic ring. The relative reactivity of different substituted benzenes toward various electrophiles has also been important in developing a firm understanding of electrophilic aromatic substitution. The next section considers some of the structure-reactivity relationships that have proven to be informative. 

Table 10.1. Energy Changes for Isodesmic Proton-Transfer Reactions of Substituted Benzenes"...

Fig. 10.3. Orbital coefficients for HOMO and next highest n orbital for some substituted benzenes. (From CNDO/2 ealculations. Ortho and meta eoefficients have been averaged in the case of the unsymmetrical methoxy and formyl substituents. Orbital energies are given in atomic units.)...

Fig. 10.4. Total 7i-electron density for some substituted benzenes. [From STO-3G calculations as reported by W. J. Hehre, L. Radom, and J. A. Pople, J. Am. Chem. Soc. 94 1496 (1972).]...

Table 10.3. Isomer Proportions in the Nitration of Some Substituted Benzenes ...

Addition of nucleophiles sueh as ammonia or alcohols or their conjugate bases to benzynes takes place very rapidly. These nueleophilie additions are believed to involve eapture of the nueleophile, followed by protonation to give the substituted benzene. ... 

The table below gives first-order rate constants for reaction of substituted benzenes with w-nitrobenzenesulfonyl peroxide. From these data, calculate the overall relative reactivity and partial rate factors. Does this reaction fit the pattern of an electrophilic aromatic substitution If so, does the active electrophile exhibit low, moderate, or high substrate and position selectivity ... 

Irradiation of solutions of alkenes in benzene or substituted benzenes gives primarily 1 1 adducts in which the alkene bridges meta positions of the aromatic ring. ... 

---