Benzenes substituted, reactivity

Cesium fluoroxysulfate in acetonitrile medium at 35 C converts primary alcohols and alkyl and aryl aldehydes into acid fluorides in high yields.i" Hammett correlation analysis of the fluorination of various benzene-substituted aldehydes gives the reactivity constant31 q = —0.38. It has been shown that solvent polarity plays an extremely important role in the conversion of aldehydes into fluorides the conversion is almost quantitative in acetonitrile, but completely stopped in dichloromethane, hexane or tetrahydrofuran. The presence of ni-... 

It is agreed that attack of an electrophilic reagent is directed towards the most reactive centers of a core with high electron density. These centers are carbon atoms in the ortho- and /j ra-positions of the benzene substituted if a substituting group is an electron donor, and in the meta-positions if a substituting group is an electron acceptor. 

The parallels between Cgo solubility in alkyl derivatives of benzene and reactivity of these derivatives to the reactions of electrophilic substitution have been established. The parallels allow the Cgo dissolution to be considered as a reaction of electrophilic substitution of aromatic hydrocarbons. 

The other simple five-membered heterocycles are furan, with an oxygen atom instead of nitro- pyrrole gen, and thiophene with a sulfur atom. They also undergo electrophilic aromatic substitution very Ft ft readily, though not so readily as pyrrole. Nitrogen is the most powerful electron donor of the three, 1 J> oxygen the next, and sulfur the least. Thiophene is very similar to benzene in reactivity. N... 

Electrophilic Substitution Reactivity Much of the electrophilic reactivity of aromatics is described in great detail in a comprehensive recent book of Taylor [10]. We shall focus attention on the electrophilic substitution reactivity of annelated benzenes and try to interpret the orientational ability of fused small rings. For this purpose we consider here Wheland transition states of the electrophilic substitution reactions. It is also convenient to take the proton as a model of the electrophilic reagent. In order to delineate rehybridization and 7r-electron localization effects, let us consider a series of angularly deformed benzenes (Fig. 21), where two vicinal CH bonds bent toward each other mimick a fused small ring. Angles c of 110° and 94° simulate five and four membered... 

Radiation-Induced Oxidation of Substituted Benzenes Structure-Reactivity Relationship... 

A group is classified as activating if the ring it is attached to is more reactive than benzene, and is classified as deactivating if the ring it is attached to is less reactive than benzene. The reactivities of benzene and a substituted benzene are compared in one of the following ways. 

Relatively straightforward benzene analogue reactivity is found with thenyl halides, alcohols (conveniently preparable by reducing aldehydes) and amines, from, for example, reduction of oximes. One exception is that 2-thenyl Grignard reagents usually react to give 3-substituted derivatives, presumably via a non-aromatic intermediate. ... 

Like benzene, substituted benzenes undergo the five electrophilic aromatic substitution reactions discussed in Chapter 15 and listed in Section 16.2 halogenation, nitration, sulfonation, alkylation, and acylation. Now we need to find out whether a substituted benzene is more reactive or less reactive than benzene itself. The answer... 

This book has been divided into three areas chemical detection, biological detection, and decontamination. The subject matter in the chapters include cross-linked divinyl benzene-substituted methacrylate polymers (Chapter 2), porous silicon (Chapter 3), reactive glass surfaces (Chapter 4), polycarbosilanes (Chapter 5), non-aqueous, chemically cross-linked polybutadiene gels (Chapter 6), conducting polyaniline nanofibers (Chapter 7), organically doped polystyrene and polyvinyltoluene (Chapter 8), electroplated polymer cast resins (Chapter 9), self assembled monolayers (Chapter 10), amphiphilic functionalized norbomene polymers (Chapter 11), transition metal substituted polyoxometalates (POMs) (Chapter 12), cross-linked divinyl-benzamide phospholipids (Chapter 13), and silica and organo silyl polymers (Chapter 14). 

FIGURE 14.95 Some substituted benzenes are reactive toward nucleophiles. l-Chloro-2,4-dinitrobenzene can be swiftly transformed into the corresponding phenol by hydroxide ion in water. 

The iodine atom in iodobenzene (unlike that in the corresponding aliphatic compounds) is very resistant to the action of alkalis, potassium cyanide, silver nitrite, etc. This firm attachment of the iodine atom to the benzene ring is typical of aromatic halides generally, although in suitably substituted nitio-compounds, such as chloro-2,4-dinitrobenzene, the halogen atom does possess an increased reactivity (p. 262). 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

In nitration with nitronium salts in sulpholan, nitrobenzene was substituted in the following proportions 8% ortho, 90% meta and 2% paraf under the same conditions benzylidyne trifluoride yielded 8%, 88% and 4% of 0-, m- and p-nitro compound respectively Both of these aromatic compounds were stated to be io -10 times less reactive than benzene. "... 

Dewar and his co-workers, as mentioned above, investigated the reactivities of a number of polycyclic aromatic compounds because such compounds could provide data especially suitable for comparison with theoretical predictions ( 7.2.3). This work was extended to include some compounds related to biphenyl. The results were obtained by successively compounding pairs of results from competitive nitrations to obtain a scale of reactivities relative to that of benzene. Because the compounds studied were very reactive, the concentrations of nitric acid used were relatively small, being o-i8 mol 1 in the comparison of benzene with naphthalene, 5 x io mol 1 when naphthalene and anthanthrene were compared, and 3 x io mol 1 in the experiments with diphenylamine and carbazole. The observed partial rate factors are collected in table 5.3. Use of the competitive method in these experiments makes them of little value as sources of information about the mechanisms of the substitutions which occurred this shortcoming is important because in the experiments fuming nitric acid was used, rather than nitric acid free of nitrous acid, and with the most reactive compounds this leads to a... 

The above definition implies that the reactivity of an aromatic compound depends upon the reaction which is used to measure it, for the rate of reaction of an aromatic compound relative to that for benzene varies from reaction to reaction (table 7.1). However, whilst a compoimd s reactivity can be given no unique value, different substitution reactions do generally set aromatic compoimds in the same sequence of relative reactivities. 

Brown noticed that the reactivities of toluene relative to benzene in aromatic substitutions were proportional to the ratios in which toluene underwent p- and -substitutions. This point is illustrated in table 7.3. 

The selectivity of an electrophile, measured by the extent to which it discriminated either between benzene and toluene, or between the meta- and ara-positions in toluene, was considered to be related to its reactivity. Thus, powerful electrophiles, of which the species operating in Friedel-Crafts alkylation reactions were considered to be examples, would be less able to distinguish between compounds and positions than a weakly electrophilic reagent. The ultimate electrophilic species would be entirely insensitive to the differences between compounds and positions, and would bring about reaction in the statistical ratio of the various sites for substitution available to it. The idea has gained wide acceptance that the electrophiles operative in reactions which have low selectivity factors Sf) or reaction constants (p+), are intrinsically more reactive than the effective electrophiles in reactions which have higher values of these parameters. However, there are several aspects of this supposed relationship which merit discussion. 

Nitration in sulphuric acid is a reaction for which the nature and concentrations of the electrophile, the nitronium ion, are well established. In these solutions compounds reacting one or two orders of magnitude faster than benzene do so at the rate of encounter of the aromatic molecules and the nitronium ion ( 2.5). If there were a connection between selectivity and reactivity in electrophilic aromatic substitutions, then electrophiles such as those operating in mercuration and Friedel-Crafts alkylation should be subject to control by encounter at a lower threshold of substrate reactivity than in nitration this does not appear to occur. 

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