Benzene, derivatives

Benzene belongs to the point group and IPA transitions from the ground state (Ai symmetry) are dipole-allowed only to states of Ei and A2u symmetry [30]. 2PA transitions from the groimd state are allowed to Aj, Ei, and E2g states. Transitions to all other states are forbidden. 

The lowest excited states of benzene are actually of B2u and Bi symmetry, and they can be reached directly by neither one-photon nor two-photon purely electronic transitions (the 0-0 band at energy Eq-o. the origin of the transition, is absent from the spectra). However, excitation into these states can be obtained through vibronic coupling (VC), if a vibrational mode of an appropriate symmetry is coupled to the electronic transition. The IPA or 2PA spectra can then show a series of narrow peaks shifted from the 0-0 band 

In substituted benzenes, the symmetry is lowered and the transitions into the states that correlate to the B2u and B u states of benzene become allowed by IPA, 2PA, or both. However, when the substituents induce only a weak perturbation on the benzene yr-electron system, the IPA or 2PA spectra of the substituted compounds often closely resemble the spectrum of the unsubstituted parent molecule. Various theoretical models have been developed in an attempt to predict the type of change in the band intensity and characteristics in the 2PA spectra of substituted benzenes and, more generally, of alternant hydrocarbons [34-36]. It was found that the effect of a perturbation is quite different for IP and 2P allowed transitions. In particular, 2P transitions to the state correlated to the benzene B2u state (Lb) are affected more by vibronic coupling than transitions to the state correlated to the benzene Biu state (La, in Platt notation [31,32]). In contrast, inductive perturbations enhance the La band more than the Lb band. The effects of vibronic coupling and inductive substituents are reversed for IP transitions into these states. Experimental 

During the 1800s, benzene was of limited commercial value, finding use mainly as a solvent. But after the invention of the internal combustion engine and the automobile, it was found that motors ran better when the fuel contained benzene. This added a new economic incentive to recover all of the benzene possible from the steel industry s coke ovens. However, just prior to World War II, the importance of benzene as a chemical intermediate started to be recognized. These dual incentives (gasoline and chemical intermediate) led to new and improved benzene processes based on petrochemistry rather than coal. 

The price of cyclohexane tracks that of benzene and is only marginally higher because the conversion of benzene to cyclohexane is readily accomplished in very high yield. 

The subsequent reaction of cyclohexane with air in the first step to adipic acid is not simple and, actually, is not well understood chemically. Only a small amount of cyclohexane present in the operation is allowed to react before the unreacted cyclohexane is recovered for recycle and the oxygen-containing products isolated for further reaction with nitric acid. Despite decades of research on this chemistry in efforts to increase yields and decrease by-product formation, substantial amounts of the starting cyclohex- 

About half of the nylon made in the world is made from the polymerization of caprolactam. Although the cyclohexanone needed to make caprolactam can be made from cyclohexane as shown above, most of it is made from phenol. 

With disubstimted benzene derivatives, it is necessary to consider the effect of each of the two substituents. For para-disubstituted benzenes, two possibilities exist. If both groups are electron releasing or if they are both electron withdrawing, they exert effects similar to those observed with monosubstituted benzenes. The group with the stronger effect determines the extent of shifting of 

If the two groups of a disubstituted benzene derivative are either ortho or meta to each other, the magnitude of the observed shift is approximately equal to the sum of the shifts caused by the individual groups. With substitution of these types, there is no opportunity for the kind of direct resonance interaction between substituent groups that is observed with para substituents. In the case of ortho substituents, the steric inability of both groups to achieve coplanarity inhibits resonance. 

For the special case of substituted benzoyl derivatives, an empirical correlation of structure with the observed position of the primary absorption band has been developed (Table 7.12). It provides a means of estimating the position of the primary band for benzoyl derivatives within about 5 mn. 

Researchers have observed that the primary and secondary bands in the spectra of polynuclear aromatic hydrocarbons shift to longer wavelength. In fact, even the second primary band, which appears at 184 nm for benzene, is shifted to a wavelength within the range of most UV spectrophotometers, This band lies at 220 nm in the spectrum of naphthalene. As the extent of eonjugation increases, the magnitude of the bathochromic shift also increases. 

The ultraviolet spectra of the polynuclear aromatic hydrocarbons possess charaeteristie shapes and fine structure. In the study of spectra of substituted polynuclear aromatic derivatives, it is common practice to compare them with the spectrum of the unsubstituted hydrocarbon. The nature of the chromophore can be identified on the basis of similarity of peak shapes and fine structure. This technique involves the use of model compounds. Section 7.15 will discuss it further. 

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The chlorinated and nitrated derivatives of aromatic hydrocarbons, primarily of benzene are used for the prevention of fungal infections from the soil by seed or soil treatment. Their action in the soil is enhanced by their medium volatility. 

The active substance quintozene 2,3,4,5,6-pentachloro-l-nitrobenzene (PCNB, 

Quintozene has a selective action. It prevents the growth of mycelia in Rhizoctonia solani, even at low concentrations, while even larger quantities have almost no effect on the species Pythium and Fusarium (Barnes and 2 rkel, 1961). This selective action is ascribed to the fact that the sensitive fungal species have considerable chitin in their cell walls, while Pythium, for example, is practically chitin-free (Maoris and Georgopoulos, 1969). These authors also found that the cellular membrane of Neurospora crassa treated with quintozene contained 

Reductive dechlorination proceeds by the action of light, but splitting of the NOj group may also occur. Under natural conditions, however, little if any degradation is caused by sunlight (Crosby and Hamadmad, 1971). 

An active substance effective against Fusarium spp. is tetrachoronitrobenzene, of whose three possible isomers 2,3,5,6-tetrachloronitrobenzene (tecnazen, 7) has proved most efficient. 

Insertion of a ttvo-carbon unit into a five-membered metallacycle may afford formation of a six-membered carbocycle (Eq. 60), such as a benzene derivative via a formal [2+2+2] aromatization of three alkynes. Similarly, insertion of a C-X unit such as a nitrile into a five-membered metallacycle may afford formation of a six-membered heterocycle, such as a pyridine derivative (Eq. 60). In recent years, Takahashi laboratory and other laboratories have developed a number of synthetically useful methods for six-membered cyclic compounds by taking advantage of five-membered metallacycles of zirconocenes and ti-tanocenes [1-5]. 

Reaction of zirconacyclopentadienes with carbenes affords zirconacy-clopentene-cyclopropane fused ring intermediates, which further react with CO to generate 1,2,3,5-tetrasubstituted benzenes via a novel skeletal rearrangement (Eq. 62) [30]. 

Naphthalene derivatives could be prepared by the reactions of zirconain-denes with allyl halides in the presence of ZnX2 (X=Br or Cl) and a catalytic amount of Pd(PPh3)4 (Eq. 63) [71]. 

5-Tetraalkyl styrenes were obtained when zirconacyclopentadienes were treated with l,4-dihalo-2-butyne in the presence of CuCl, representing the first example of construction of styrene derivatives from three molecules of alkynes (Eq. 64) [72]. 

Buta-2,3-diene-l-yl benezene derivatives were obtained when zirconacyclopentadienes reacted with two propargyl halides in the presence of CuCl (Eq.65) [73]. 

Martin, Aromatic Hydroxyketones Preparation and Physical Properties, 

COCH CH Preparation by bromination of 5-fluQro-2-hydroxy-II 2 3 propiophenone [6351] according to the method 

Also obtained by bromination of 5-nitrorespropiophenone in acetic acid with 

XOCH2CH3 - Preparation by bromination of various o-hydroxy-propiophenones. 

Also obtained by Friedel-Crafts acylation of 2,4-dibiomophenol with propionic anhydride in nitrobenzene in the presence of aluminium chloride at 120° [6370]. Also refer to [6363,6372,6373]. 

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Crum Brown s rule A guide to substitution in benzene derivatives. This rule states that a substance C Hj A yields the meia disubstituied product if the compound HA can be oxidized directly to HOA otherwise a mixture of the o-and p-compounds will be obtained. Not universally applicable.. Sec Hammick and Illingworth s rules. 

Fischer-Hepp rearrangement The nitros-amines of aromatic secondary amines when treated with hydrochloric acid give nuclear substituted nitrosoamines. Among the benzene derivatives, if the para position is free the -NO group displaces the hydrogen atom there in naphthalene derivatives it enters the 1-position ... 

OH groups are in the para or 1,4 position to each other. This use of the prefix is confined to disubstituted benzene derivatives in such cases as para-hydrogen and paraldehyde the prefix has no uniform structural significance and is always written in full. 

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. 

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. 

The course of aromatic substitution has been placed on a more scientific basis by the following rules of Hammick and Illingworth (jfour. Chem. Soc., 930. 2358), If a monosubstituted benzene derivative has the formula CgHsXY, where X is the atom joined to the benzene ring and Y is an atom or group of atoms attached to X, then —... 

A brief account of aromatic substitution may be usefully given here as it will assist the student in predicting the orientation of disubstituted benzene derivatives produced in the different substitution reactions. For the nitration of nitrobenzene the substance must be heated with a mixture of fuming nitric acid and concentrated sulphuric acid the product is largely ni-dinitrobenzene (about 90 per cent.), accompanied by a little o-dinitrobenzene (about 5 per cent.) which is eliminated in the recrystallisation process. On the other hand phenol can be easily nitrated with dilute nitric acid to yield a mixture of ortho and para nitrophenols. It may be said, therefore, that orientation is meta with the... 

CHjO), + 3CH,OH + 3HC1 —> 3CH3OCH2CI + 3H,0 Monoalkyl benzene derivatives yield para chloromethjd compounds, frequently accompanied by small amounts of the ortho isomeride. The reaction is similar in some respects to that of Friedel and Crafts. Chloromethylation is of great value in synthetic work as the —CH,C1 group can be converted into other groups such as —CH,OH, —CHO, —CH,OR, —CH,CN, —CH,CH(COOC.,Hs)2 and —CH,. 

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

TABLE 9.3 The nitration of benzene derivatives containing positively charged substituents ... 

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyclobutene derivatives tend to open to give extremely reactive dienes, namely ortho-c]uin(xlimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G, Aalbersberg, 1975 R.P. Thummel, 1980). 

This reaction sequence is much less prone to difficulties with isomerizations since the pyridine-like carbons of dipyrromethenes do not add protons. Yields are often low, however, since the intermediates do not survive the high temperatures. The more reactive, faster but less reliable system is certainly provided by the dipyrromethanes, in which the reactivity of the pyrrole units is comparable to activated benzene derivatives such as phenol or aniline. The situation is comparable with that found in peptide synthesis where the slow azide method gives cleaner products than the fast DCC-promoted condensations (see p. 234). 

Reaction that can be carried out by the oxidative coupling of radicals may also be initiated by irradiation with UV light. This procedure is especially useful if the educt contains oleflnic double bonds since they are vulnerable to the oxidants used in the usual phenol coupling reactions. Photochemically excited benzene derivatives may even attack ester carbon atoms which is generally not observed with phenol radicals (I. Ninoraiya, 1973 N.C. Yang, 1966). 

Benzene Derivatives with a Nitrogen Containing Side-Chain... 

Butyne trimerizes in the presence of aluminum chloride to give hexamethyl Dewar-benzene (W. Schafer, 1967). Its irradiation leads not only to aromatization but also to hexa-methylprismane (D.M. Lemal, 1966). Highly substituted prlsmanes may also be obtained from the corresponding benzene derivatives by irradiation with 254 nm light. The rather stable prismane itself was synthesized via another hydrocarbon, namely benzvalene, a labile molecule (T. J. Katz, 1971, 1972). 

Benzoic acid and naphthoic acid are formed by the oxidative carbonylation by use of Pd(OAc)2 in AcOH. t-Bu02H and allyl chloride are used as reoxidants. Addition of phenanthroline gives a favorable effect[360], Furan and thiophene are also carbonylated selectively at the 2-position[361,362]. fndole-3-carboxylic acid is prepared by the carboxylation of 1-acetylindole using Pd(OAc)2 and peroxodisulfate (Na2S208)[362aj. Benzoic acid derivatives are obtained by the reaction of benzene derivatives with sodium palladium mal-onate in refluxing AcOH[363]. 

The intermolecular insertion of acrylate[278], A formal [2 + 2+2] cycloaddition takes place by the reaction of 2-iodonitroben-zene with the 1,6-enyne 402. The neopentylpalladium intermediate 403 undergoes 6-endo-lrig cyclization on to the aromatic ring to give 404[279],... 

The benzene derivative 409 is synthesized by the Pd-catalyzed reaction of the haloenyne 407 with alkynes. The intramolecular insertion of the internal alkyne, followed by the intermolecular coupling of the terminal alkyne using Pd(OAc)2, Ph3P, and Cul, affords the dienyne system 408, which cyclizes to the aromatic ring 409[281]. A similar cyclization of 410 with the terminal alkyne 411 to form benzene derivatives 412 and 413 without using Cul is explained by the successive intermolecular and intramolecuar insertions of the two triple bonds and the double bond[282]. The angularly bisannulated benzene derivative 415 is formed in one step by a totally intramolecular version of polycycli-zation of bromoenediyne 414[283,284],... 

Piperazinothiazoies (2) were obtained by such a replacement reaction, Cu powder being used as catalyst (25. 26). 2-Piperidinothiazoles are obtained in a similar way (Scheme 2) (27). This catalytic reaction has been postulated in the case of benzene derivatives as a nucleophilic substitution on the copper-complexed halide in which the halogen possesses a positive character by coordination (29). For heterocyclic compounds the coordination probably occurs on the ring nitrogen. 

The two substituted carbons are connected by a double bond in one structure but by a single bond in the other Because no such cases of isomerism m benzene derivatives were known and none could be found Kekule suggested that two isomeric structures could exist but mterconverted too rapidly to be separated... 

The prefix ortho signifies a 1 2 disubstituted benzene ring meta signifies 1 3 disubstitu tion and para signifies 1 4 disubstitution The prefixes o m and p can be used when a substance is named as a benzene derivative or when a specific base name (such as ace tophenone) is used For example... 

In these examples the base name of the benzene derivative determines the carbon at which numbering begins anisole has its methoxy group at C 1 toluene its methyl group... 

Partial rate factors may be used to estimate product distributions in disubstituted benzene derivatives The reactivity of a particular position in o bromotoluene for example is given by the product of the partial rate factors for the corresponding position in toluene and bromobenzene On the basis of the partial rate factor data given here for Fnedel-Crafts acylation predict the major product of the reaction of o bromotoluene with acetyl chlonde and aluminum chloride... 

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. 

When side chains of two or more different kinds are attached to a cyclic component, only the senior side chain is named by the conjunctive method. The remaining side chains are named as prefixes. Likewise, when there is a choice of cyclic component, the senior is chosen. Benzene derivatives may be named by the conjunctive method only when two or more identical side chains are present. Trivial names for oxo carboxylic acids may be used for the acyclic component. If the cyclic and acyclic components are joined by a double bond, the locants of this bond are placed as superscripts to a Greek capital delta that is inserted between the two names. The locant for the cyclic component precedes that for the acyclic component, e.g., indene-A - -acetic acid. 

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

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