Benzene nucleus

The benzene nucleus is then attacked by the carbonium ion (I) (. attack by +... 

The catalyst is inactive for the hydrogenation of the (isolated) benzene nucleus and so may bo used for the hydrogenation of aromatic compounds containing aldehyde, keto, carbalkoxy or amide groups to the corresponding alcohols, amines, etc., e.g., ethyl benzoate to benzyl alcohol methyl p-toluate to p-methylbenzyl alcohol ethyl cinnamate to 3 phenyl 1-propanol. 

The most notable studies are those of Ingold, on the orienting and activating properties of substituents in the benzene nucleus, and of Dewar on the reactivities of an extensive series of polynuclear aromatic and related compounds ( 5.3.2). The former work was seminal in the foundation of the qualitative electronic theory of the relationship between structure and reactivity, and the latter is the most celebrated example of the more quantitative approaches to the same relationship ( 7.2.3). Both of the series of investigations employed the competitive method, and were not concerned with the kinetics of reaction. 

The chemistry of benzenecarboxyUc acids generally is the same as that of other carboxyUc acids, which can be converted into esters, salts, acid chlorides, and anhydrides. Each carboxyl group can react separately, so that compounds in which carboxyl groups are converted into different derivatives can be prepared. Because there are aromatic hydrogens available in most of these acids, they also undergo reactions characteristic of the benzene nucleus. Some of the anhydrides have characteristic reactions. 

The PMBs, when treated with electrophilic reagents, show much higher reaction rates than the five lower molecular weight homologues (benzene, toluene, (9-, m- and -xylene), because the benzene nucleus is highly activated by the attached methyl groups (Table 2). The PMBs have reaction rates for electrophilic substitution ranging from 7.6 times faster (sulfonylation of durene) to ca 607,000 times faster (nuclear chlorination of durene) than benzene. With rare exception, the PMBs react faster than toluene and the three isomeric dimethylbenzenes (xylenes). 

Benzene Nucleus Reactions. Ring substitutions, such as sulfonation and nitration, can be effected without destmction of the carbonyl group. Under the influence of the carbonyl group, meta substitution usually occurs. 

The replacement of a benzene nucleus by a thiophene nucleus in compounds containing other chromophores causes more or less pronounced bathochromic shifts. These effects have been noticed in the thiophene analogs of malachite green (23), in cumulenes such as... 

A table listing the benzofuroxans Imown to the authors, from the literature or otherwise, with their melting points, appears in Section X, at the end of this chapter. The present section presents a brief summary of the presently available types of substituent groups on the benzene nucleus, and of the reactions they undergo. 

A completely different approach has been taken by Hine, who has considered that the substituent and reaction center are not really distinct, both being substituents in a benzene nucleus, and has then related substituent and reaction constants. Although of considerable theoretical interest, Hine s work has little bearing on practical applications of the Hammett equation since he starts from the premise of unique, single-valued substituent constants. This premise is invalid whether we are utilizing the naive approach with three separate, well-defined sets or the more refined methods with a continuous range of para values. 

Hydrazides of vicinal acetylene-substituted derivatives of benzoic and azole carboxylic acids are important intermediate compounds because they can be used for cyclization via both a- and /3-carbon atoms of a multiple bond involving both amine and amide nitrogen atoms (Scheme 131). Besides, the hydrazides of aromatic and heteroaromatic acids are convenient substrates for testing the proposed easy formation of a five-membered ring condensed with a benzene nucleus and the six-membered one condensed with five-membered azoles. 

The next series of alcohols to be examined is that in which the members retain the benzene nucleus in a more or less substituted -condition, as distinguished from those in which the benzene nucleus has been so altered as to bring the alcohols within the series known as the terpene alcohols A certain number of these alcohols are found in nature, but some of them are prepared synthetically, and, although not yet found naturally, are exceedingly useful in the preparation of perfumes. 

Benzyl alcohol, C Hj. CHjOH, is the lowest member of the normal series of aromatic alcohols containing the benzene nucleus. It exists to a certain extent in the free state, but more often in the form of esters, principally of acetic, benzoic, and cinnamic acids, in a number of essential oils, such as those of jasmin, tuberose, cassie fiowers, and ylang-ylang. 

Sadtler concluded finally that double bonds seem to aid in bringing about reaction when close to the. CHO group, e.g., citral, cinnamic aldehyde, and that proximity of the benzene nucleus to the. CHO group, as. in the case of benzaldehyde and vanillin, was also probably a factor, while the only active ketones were those containing double bonds near to the. CO group. 

Bcazal kem m. benzene nucleus. >kohlen wasserstofife, m.ji. betuene hydrocarbons. lack, m, benzol varnish, -faate, /. faouaae... 

The basicity of the nitrogen atom can be influenced by whether the imino group is attached directly to a benzene nucleus or whether a methylene group is interposed. 

Some additional derivatives containing extended 7t-systems in place of the benzene nucleus are naphthalene, anthracene (2,3-Ac) and phenanthrene (9,10-Phc). They also belong to the phthalocyanine family. For the naphthalene system two types of macrocyclcs, the 1,2-naph-thalocyanine (1,2-Nc) and the 2,3-naphthalocyanine (2,3-Nc), are known. 

Subphthalocyanines are the products of the cyclotrimerization of molecules like phthalonitriles or isoindolinediimines. The formation of structural isomers occurs if these precursors are un-symmetrically substituted at the benzene nucleus (see also p736). 

As the name suggests, LABs consist of a benzene nucleus to which an alkyl chain is bound. The alkyl chain shows a distribution of carbon chain lengths dependent on the cut of the starting material, while point of attachment to the benzene ring (phenyl isomer distribution) is governed to a large extent by the manufacturing process. 

Values found for interatomic distances and bond angles in the thirteen hydrocarbons studied are given in Table XIV. The carbon-carbon singlebond distance is found to have the constant value 1.54 = = 0.02 A., being unaffected by the presence of an adjacent double bond or benzene nucleus (provided that it does not form part of a conjugated system). The carbon-carbon double-bond distance in allene and acetylene has the value 1.34 A. This is 0.04 A. less than that formerly given by the table of covalent radii, which has accordingly been revised. The effect of the revision on the bond distance-resonance curve is discussed. 

The effect of introducing electron-donating alkyl groups into the benzene nucleus is found to be small ... 

A group with a more powerful (electron-withdrawing) inductive effect, e.g. NOa, is found to have rather more influence. Electron-withdrawal is intensified when the nitro group is in the o- or p-position, for the interaction of the unshared pair of the amino nitrogen with the delocalised it orbital system of the benzene nucleus is then enhanced. The neutral molecule is thus stabilised even further with respect to the cation, resulting in further weakening as a base. Thus the nitro-anilines are found to have related p a values ... 

The formation of diazoamino compounds, on coupling ArN2 with primary amines, does not constitute a total preparative bar to obtaining products coupled on the benzene nucleus for diazoamino compounds (33) may be rearranged to the corresponding amino-azo... 

They are thus all electron-withdrawing with respect to the benzene ring, i.e. aromatic species containing them all have a dipole with the positive end located on the benzene nucleus. Taking Y = NR3 as exemplar of the rest, we can write the a complexes for attack by an electrophile, E (e.g. N02), o-, m- and p- to the original NR3 substituent ... 

The positive charge will clearly further destabilise any of the a complexes for electrophilic substitution, as did a substituent such as NR3 on the benzene nucleus (p. 152), but the destabilisation will be much more marked than with NR3 as the charge is now on an atom of the ring itself and not merely on a substituent. 

It is to be expected that attack by nucleophiles on an unsubstituted benzene nucleus will be much more difficult than attack by electrophiles. This is so (o) because the n electron cloud of the nucleus (p. 130) is likely to repel an approaching nucleophile, and (b) because its n orbital system is much less capable of delocalising (and so stabilising) the two extra electrons in the negatively charged (72), than the positively charged Wheland intermediate (73) ... 

A particularly useful displacement reaction on ArN2 is the introduction of F into the benzene nucleus (not possible by direct reaction with F2, cf. p. 140) ... 

The addition of chlorine or bromine to benzene—one of the few overall addition reactions of a simple benzene nucleus—has also been shown to proceed via a radical pathway, i.e. it is catalysed by light and by the addition of peroxides, and is slowed or prevented by the usual inhibitors. With chlorine this presumably proceeds ... 

The rearranged radical (119) is more stable than the original one (117) not only because the former is tertiary and the latter primary, but also because (119) is stabilised by delocalisation of the unpaired electron over the n orbital system of a benzene nucleus. It is significant that only Ph migrates in (117), despite the fact that migration of Me... 

It was discovered by Michler and Meyer in 1879 and was made by the nitration of dimethylaniline. One methyl group is oxidised and at the same time the benzene nucleus is nitrated in the 2, 4, 6-positions. The usual method of preparation from dimethylaniline is to dissolve 1 part of dimethylaniline in 14 to 15 parts of sulphuric acid to this solution about 9 parts are added of a mixed acid containing 67% of nitric acid and 16% of... 

Although diphenyl ether and dibenzofuran were very stable for thermolysis at 450°C for 120 min, the rate of decomposition increased with increasing the number of benzene nucleus, that is, 2,2 -dinaphthyl ether was converted to the value of 23.3% and phenyl phenanthryl ether 64.8% at the same reaction conditions. 

The thermal decomposition of dibenzyl was not affected by the addition of phenol or p-cresol. In contrast, the decomposition of 2,2 -dinaphthyl ether increases remarkably in the presence of phenolic compounds as shown in Table III, and the effect seems to increase with increasing the electron donating property of substituent on the benzene nucleus. 

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