Benzenediazonium ion

Hydroxythiophene also exists mainly in ketonic forms. Electrophilic reagents react either at oxygen or at C-5. O-Methyl and O-acetyl derivatives are obtained in alkaline solution, probably through intermediacy of the anion. In acidic solution, coupling with benzenediazonium ion, a characteristic phenolic reaction, is found to take place (Scheme 72). 

Pyridine-2- and -4-diazonium ions are far less stable than benzenediazonium ions. Azolediazonium salts generally show intermediate stability provided diazotization is carried out in concentrated acid, many of the usual diazonium reactions succeed. Indeed, azolediazonium salts are often very reactive in coupling reactions. 

Benzenediazonium fluoroborate, 2-carboxy-xanthone synthesis from, 3, 838 Benzenediazonium ions phenyl azide formation from, 5, 839 Benzenediazonium salts, o-(imidazol-l-yl)-intramolecular diazo coupling, 5, 404 Benzene-1,2-disulfonimides N-substituted reactions, 6, 930 Benzene episulfide formation, 7, 577 Benzeneimine... 

Electrostatic potential map—cont d ammonia, 145 aniline, 925 anilinium ion, 925 anisole, 111 18]annulene, 535 arene, 74 azulene, 541 benzaldehyde, 565, 704 benzene, 44, 521, 565 benzenediazonium ion, 945 benzoquinone, 631 benzyl carbocation, 377 benzyne, 576 borane, 223... 

More recently Gorelik s group determined the structure of 1-phenylsulfonyl-2-pyrazoline-3-diazonium tetrafluoroborate by X-ray crystallography (Gorelik et al., 1989) and calculated (Glukhovtsev et al. 1990) the heats of the dediazoniation reaction of seven cyclic five-membered diazonium ions (including 2.13, R=H) relative to that of the benzenediazonium ion by the MNDO method (see Sec. 8.4). 

The unsubstituted diazotetrazole (2.16 ) even couples with hydrazine in a manner not observed with benzenediazonium ions to form the hexazadiene 2.17 (Scheme 2-9, Horwitz and Grakauskas, 1957). 

The general applicability of this type of synthesis of quinone diazides is nevertheless limited since, depending on the type and number of substituents in the 2-, 4-, and 6-positions of benzenediazonium ions, either hydroxy-de-diazoniation (reaction A in Scheme 2-20) or nucleophilic substitution of one of the groups in the 2-, 4-, or 6-position (reaction B) will predominate. It is difficult to predict the ratio of the two reactions in a specific case. This is exemplified by two investigations carried... 

Based on observations by Bamberger, Bucherer, and Wolff at the turn of the century, Matrka et al. (1967) described experiments which show that alkaline solutions (pH 8.5-9.2) of substituted benzenediazonium chlorides form nitrite ions and triazenes. The latter is obviously the reaction product of the amine formed in a retro-diazotization with the diazonium ion that is still present. The yield of nitrite formed was between 0.5% (benzenediazonium ion) and 50.2% (2-nitrobenzenediazonium ion). 

Table 4-1. Uncorrected bond lengths (in pm) and angles of the benzenediazonium ion.

In the literature discussing these results, the coincidence of the NN bond lengths in diazonium ions with that in dinitrogen seems always to be regarded with complete satisfaction. In the opinion of the present author this close coincidence is somewhat surprising, firstly because of the fact that in diazonium ions one of the nitrogen atoms is bonded to another atom in addition to the N(2) atom, and secondly because work on dual substituent parameter evaluations of dediazoniation rates of substituted benzenediazonium ions clearly demonstrates that the nx orbitals of the N(l) nitrogen atom overlap with the aromatic 7t-electron system (see Sec. 8.4). 

The benzenediazonium ion absorbs strongly at 259 nm (e = 5640 M -1cm-1, EtOH, Sukigahara and Kikuchi, 1967a). Substitution by electron donors in the 4-position causes a strong bathochromic shift, as seen in the results with different halogen substi-... 

Sukigara and Kikuchi interpreted these experimental data by carrying out Pariser-Parr (PP) calculations on the benzenediazonium ion. On the basis of Romming s X-ray structure of the benzenediazonium ion (1963), with fixed bond integrals for the CC and CN bonds (/ Cc and / CN), but an adjustable bond integral / NN, they were able to ascribe the 259 nm and 295 nm bands to the lAx — xAi and transi-... 

A new period in theoretical work on arenediazonium ions began with Vincent and Radom s paper in 1978. This was the first ab initio study of the methane- and benzenediazonium ions, and was carried out with a minimal (STO-3G) basis set, subject only to some (specified) symmetry constraints and a fixed CH bond length (108.3 pm). The optimized structure of the benzenediazonium ion is given in Figure 4-2. 

Fig. 4-2. Optimized STO-3G structure for the benzenediazonium ion. I Bond lengths (pm) and angles. Non-independent angles are shown in parentheses.

Fig. 4-3. Important orbital interactions in the benzenediazonium ion. After Vincent and Radom (1978).

Following Vincent and Radom s comprehensive ab initio study (1978), Escudero et al. (1985) also performed ab initio calculations at the STO-3G level for seven 4-substituted benzenediazonium ions. For the unsubstituted benzenediazonium ion, the optimized geometries are the same as in Figure 4-1 (I). For charge density calculations another population analysis method was used (Escudero and Yanez, 1982), and this gave results different from those of Vincent and Radom. However,... 

INDO calculations were applied to five 4-substituted benzenediazonium ions by KovaFchuk et al. (1988). It is known, however, that this relatively old method is of limited accuracy for the determination of geometrical parameters for such compounds. 

Many studies of such acid-base equilibria of substituted benzenediazonium ions have been reported since the first investigation by Wittwer and Zollinger (1954), namely by Lewis and Suhr (1958 a), Littler (1963), Ritchie and Wright (1971 a, 1971 b), Machackova and Sterba (1972 a), Beranek et al. (1973), Jahelka et al. (1973 a), Vir-tanen and Kuokkanen (1977), BagaPs group (Luchkevich et al., 1986 and references therein), and in DMSO by Petrov et al. (1973). 

Luchkevich et al. (1986, Table 6) demonstrated that for the three isomeric nitro-benzenediazonium ions and their (Z)-diazohxydroxides the acidity constants can be determined by ultraviolet spectrophotometry, by potentiometry, from the kinetics of reaction with hydroxide ions, from the (Z) (E) isomerization kinetics, and from the kinetics of azo coupling reactions. These independent methods gave surprisingly consistent results. ... 

Shortly afterwards and independently, the group of Sterba in Pardubice (Czechoslovakia) published a series of papers on the kinetics of acid-base and isomerization equilibria of substituted benzenediazonium ions. Initially they used classical methods for rate measurements (Machackova and Sterba, 1972 a, 1972 b Jahelka et al., 1973a), then later stopped-flow techniques (Jahelka et al., 1973b). 

Table 5-1. Constants Kx and K2 of the acid-base equilibria ArNJ (Z)ArN2OH (Z)ArN20 for substituted benzenediazonium ions (20 °C, / = 0.25).

All the substituents in Table 5-1 are acidifying substituents. One can draw the conclusion that for the unsubstituted benzenediazonium ion and for derivatives with basifying substituents K2 is more than 105 times greater than Kx. 

The rapid formation of the (Z)-diazoate is followed by the slower (Z/J )-isomeri-zation of the diazoate (see Scheme 5-14, reaction 5). Some representative examples are given in Table 5-2. Both reactions are first-order with regard to the diazonium ion, and the first reaction is also first-order in [OH-], i.e., second-order overall. So as to make the rate constants k and k5 directly comparable, we calculated half-lives for reactions with [ArNj ]0 = 0.01 m carried out at pH = 9.00 and 25 °C. The isomerization rate of the unsubstituted benzenediazonium ion cannot be measured at room temperature due to the predominance of decomposition (homolytic dediazoniations) even at low temperature. Nevertheless, it can be concluded that the half-lives for (Z/ )-isomerizations are at least five powers of ten greater than those for the formation of the (Z)-diazohydroxide (reaction 1) for unsubstituted and most substituted benzenediazonium ions (see bottom row of Table 5-2). Only for diazonium ions with strong -M type substituents (e.g., N02, CN) in the 2- or 4-position is the ratio r1/2 (5)/t1/2 (1) in the range 6 x 104 to 250 x 104 (Table 5-2). 

Table 5-2. Selected rate constants and half-livese) for some reactions of substituted benzenediazonium ions with buffer solutions (pH 9.00) at 25 °C (rate constants from Virtanen and Kuok-kanen, 1977 half-lives calculated by the present author).

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