Structure of Arenediazonium Salts

As explained in the preceding section, we will discuss the structure of aromatic diazonium salts on the basis of evidence from X-ray investigations. We will supplement those results with data obtained by other physical methods, in particular NMR and IR spectroscopy. Earlier experience with the more stable arenediazonium salts enabled those scientists who first obtained alkanediazonium ions in solution to characterize them by NMR spectroscopy (see Zollinger, 1995, Sec. 2.1). 

This summary of X-ray investigations of aromatic diazonium salts will concentrate on those results that are essential to understanding the structure and reactivity of these compounds. Most important are, first of all  

The first crystal structure investigation was reported by Romming (1959, 1963) for benzenediazonium chloride. An unpublished low temperature study ( —160 °C) by Romming is mentioned by Sorriso (1978). Romming and Tjornhom (1968) investigated the structure in the presence of one equivalent of CH3COOH. More 

Reference Anion Romming (1959, 1963) ci- Romming (1978) Cl Andresen and Romming (1962) Br3- Cygler et al. (1982) bf4- 

The experimental data are clearly consistent with the (modernized) formula of Blomstrand (4.1a), remembering of course, that in his time the concepts of bond angles and ionic bonding of the diazonio group were still unknown in organic chemistry. We will discuss the data in Table 4-1 in relation to the mesomeric structures 4.1a and 4.1c (X = H). 

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As an alternative to electrochemical or radiolytic initiation, homolytic dediazoniation reaction products can be obtained photolytically. The organic chemistry of such photolyses of arenediazonium salts will be discussed with regard to mechanisms, products, and applications in Section 10.13. In the present section photochemical investigations are only considered from the standpoint that the photolytic generation of aryldiazenyl radicals became the most effective method for investigating the mechanisms of all types of homolytic dediazoniations —thermal and photolytic —in particular for elucidating the structure and the dissociation of the diazenyl radicals. 

The reason for the stability of arenediazonium salts, relative to their alkane counterparts, is resonance and the high energy of the aryl cations formed by loss of nitrogen. One of the electron pairs making up the aromatic rr system can be delocalized into the functional group, which results in charge-separated resonance structures containing a double bond between... 

The fundamental understanding of the diazonio group in arenediazonium salts, and of its reactivity, electronic structure, and influence on the reactivity of other substituents attached to the arenediazonium system depends mainly on the application of quantitative structure-reactivity relationships to kinetic and equilibrium measurements. These were made with a series of 3- and 4-substituted benzenediazonium salts on the basis of the Hammett equation (Scheme 7-1). We need to discuss the mechanism of addition of a nucleophile to the P-nitrogen atom of an arenediazonium ion, and to answer the question, raised several times in Chapters 5 and 6, why the ratio of (Z)- to ( -additions is so different — from almost 100 1 to 1 100 — depending on the type of nucleophile involved and on the reaction conditions. However, before we do that in Section 7.4, it is necessary to give a short general review of the Hammett equation and to discuss the substituent constants of the diazonio group. 

Barbituric acid can be considered as a cyclized malonic acid diamide (malonyl-urea). It is therefore a cyclic diketone that may be classified, in the sense of the compounds discussed in Section 12.6, as a coupling component with a methylene group activated by two carbonyl groups in the a- and a -positions. The reaction with arenediazonium salts was studied by Nesynov and Besprozvannaya (1971). These authors obtained coupling products (in good yield) that they considered to be arylhydrazones. Coupling with 4-(phenylazo)benzenediazonium chloride was studied by Chandra and Thosh (1991). The lH NMR spectra of these compounds are consistent with the arylhydrazone structure 12.68. 

Although the exact mechanism for iodobenzene formation from the diazonium cation is unclear, the phenyl cation is not an impossible intermediate, it is still considered to be the best candidate for the intermediate in phenol formation from arenediazonium salts and hot water (Section 22-4). In addition, it is known that diazonium cations exchange their bonded N2 with gaseous N2 (detected in isotope studies), direct evidence that reversible dissociation of N2 to leave behind a phenyl cation can take place. This system is an example of how a seemingly simple structure can exhibit a lot of complexity in its behavior. 

Arenediazonium salt A derivative of the resonance-stabilized benzenediazonium cation, C6H5N2 +. Aromatic compound (1) Benzene or a derivative—represented by a structure with three double bonds in... 

Arenediazonium salts 130 have also been applied to a Heck reaction of MBH adducts to give a-(substituted-aryl)-p-keto esters 131 in the presence of palladium catalyst. The use of aerobic conditions, short reaction times and the tolerance towards many structurally diverse reactants offer several advantages over previous reports (Scheme 3.51). ... 

The Heck arylation with arenediazonium salts was studied by Eberlin and coworkers through ESI-MS tandem mass spectrometry experiments. The group detected and structurally characterized the main cationic intermediates of this catalytic cycle direcfly from solution to the gas phase [52]. They proposed a detailed catalytic cycle of... 

Bis(arenediazonium) salts of general structure (12) are reported to crosslink poly(vinyl alcohol) by formation of ether linkages," as shown in equation (13), where the polyol is represented by POH. As in the case of bisazido compounds, crosslinking requires that both diazonium groups react, each with a different polymer. 

Another group of stable diazonium salts are the so-called diazonium metal double salts the zinc double chlorides are particularly important. The term is misleading, as they are not associations between two salts but, in the case of Zn, are formed from two arenediazonium ions with the complex anion ZnCL 2-. This fact became obvious from crystal structure studies38-41. The reason for their increased stability relative to ArN2+Cl- salts is that ZnCLt2- complex ions are less nucleophilic than free chloride ions. 

Diazonium Salts as Electrophiles Diazo Coupling Arenediazonium ions act as weak electrophiles in electrophilic aromatic substitutions. The products have the structure Ar—N=N—Ar, containing the —N=N— azo linkage. For this reason, the products are called azo compounds, and the reaction is called diazo coupling. Because they are weak electrophiles, diazonium salts react only with strongly activated rings (such as derivatives of aniline and phenol). 

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