2- Nitroaniline solvatochromic probes

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

In 1994, a review on the further development and improvement of the n scale was given by Laurence, Abboud et al. [227], They redetermined n values for a total of 229 solvents, this time using only two (instead of seven) solvatochromic nitroaromatics as indicator compounds, i.e. 4-nitroanisole and A,A-dimethylamino-4-nitroaniline, for good reasons see later and reference [227] for a more detailed discussion. A thermodynamic analysis of the n scale [and the t(30) scale] has been reported by Matyushov et al. [228]. Using six novel diaza merocyanine dyes of the type R-N=N-R (R = N-methylpyridinium-4-yl or A-methylbenzothiazolium-2-yl, and R = 2,6-disubstituted 4-phenolates or 2-naphtholate) instead of nitroaromatics as positively solvatochromic probe compounds, an analogous n azo scale was developed by Buncel et al., which correlates reasonable well with the n scale, but has some advantages for a detailed discussion, see references [333], Another n scale, based solely on naphthalene, anthracene, and y9-carotene, was constructed by Abe [338], n values are mixed solvent parameters, measuring the solvent dipolarity and polarizability. The differences in the various n scales are caused by the different mixture of dipolarity and polarizability measured by the respective indicator. The n scale of Abe is practically independent of the solvent dipolarity, whereas Kamlet-Taft s n and Buncel s n azo reflect different contributions of both solvent dipolarity and polarizability. 

Af-Butyl-4-nitroaniline (30) was also shown42 to be able to donate a hydrogen bond to HBA solvents, hence could be used as a solvatochromic probe, but has not been explored further in this respect. Drago29 reported the wavenumbers of the lowest energy absorption bands of three /V-alkyl-substituted nitroanilines 27, /V-ethyl-4-nitroanilinc (31) and N-ethyl-3-nitroaniline (32). Their spectral shifts relative to that in dioxane are shown in Figure 2 as a function of / Kt and a fair correlation is seen. 

The Lewis basicity or HB A ability of solvents is described by the Kamlet-Taft parameter, measured by means of the couples of solvatochromic probes 4-nitrophenol compared to 4-nitroanisole or 4-nitroaniline compared to 4-nitro-N, N-diethyaniline, the second probe of the couple serving to eliminate the effects of the solvent polarity and polarizability on its first probe. The resulting values of from the compilation of Jessop et al. [410], supplemented with data from Spange etal. [416] are shown inTable 6.16. An alternative measure of this property that has been applied by Schade et al. [413] to RTILs is Catalan s SB parameter, using the solvatochromic probe N.N-dimethyl-4-aminobenzodifuranone, and the resulting values are shown in Table 6.16. The SB values are linear with the values SB = 0.126 +1.056/. The basicity decreases in a series of RTILs with a common anion phosphonium > ammonium > pyrrolidinium > pyridinium > imidazolium, but more moderately than the acidity increases in the opposite direction shown above as shown by Spange et al. [416]. 

Solvatochrotnic shifts of the five probe indicators, 2-nitroaniline, 4-nitroaniline, 4-nitroanisole, 4-nitrophenol, and N,N-dimethyl-4-nitroaniline, have been used to specify the solvatochromic solubility parameters k and beta of the five pure fluids CO2,... 

The solvatochromic effects on UV/visible spectra of certain solutes are so large, that they can conveniently be employed as probes for certain solvating properties of the solvents. Those that have enjoyed widespread application in this capacity are discussed in Chapter 4. They include 2,6-diphenyl -4-(2,4,6-triphenyl-l-pyridino)-phenoxide, 4-methoxynitrobenzene, 4-(dimethylamino)-nitrobenzene, for the estimation of the polarity of solvents, acetylacetonato-N,N,N, N -tetramethylethylenediamino-copper(II) perchlorate, 4-nitrophenol, and 4-nitroaniline, for the estimation of the electron pair donicity of solvents, 4-carboxymethyl-l-ethylpyridinium iodide, 4-cyano-l-ethylpyridinium iodide, and bis-c/.v-1, lO-phenanthrolinodicyano-iron(II) for the estimation of the hydrogen bond donation abilities of solvents (Marcus 1993). 

A solvatochromic scale, based on the ultraviolet-visible, rather than the infrared, spectral band of suitable probes is that based on the Kamlet -Taft P parameter. This is again an averaged quantity, for which the wavenumber shifts of several protic indicators relative to structurally similar but aprotic homomorphs are used (Kamlet et al. 1983 Kamlet and Taft 1976). It is assumed that the nonspecific effect of a solvent on the protic probe is the same as that on the aprotic one, and that it can be expressed in terms of the n parameter for the solvent, so that the donicity of the solvent, if it is a Lewis base, causes the difference between the responses of the two probes towards the solvent. The probes originally employed were 4-nitrophenol (vs 4-nitroanisole) and 4-nitroaniline (vs 4-nitroN,N-diethylaniline), but once a n scale is known, the need for the specific aprotic homomorph values no longer exists, since the general expression ... 

Many other scales of electron-pair donation abilities have been proposed over the years, which are in general in good correlation with DN e g., the heat of complexation of the solvent molecules with boron trifluoride in dichloromethane (Maria and Gal 1985), and P e.g., SB, the solvatochromism of 5-nitroindoline compared with l-methyl-5-nitroindoline in neat solvents (Catalan el al. 1996) scales. The latter, the SB scale, has the advantage that the N-H acid function of the 5-nitroindoline probe has only a single hydrogen atom, contrary to the nitroanilines used for the P scale, that have two. It was devised quite recently for... 

Figure 19. Solvatochromic parameters of five supercritical fluids from measurements of Maiwald compared with literature data for some selected liquid solvents according to Kamlet, Abboud, and Taft. Filled symbols refer to the probe molecules 4-nitroaniline / N,N-dimethyl-4-nitroaniline (NH-hydrogen bonding), open symbols to 4-nitrophenol / 4-nitroanisole probes (OH-hydrogen bonding) adapted from [75].

Abraham, in collaboration with Taft, Kamlet, and Abboud, has made an interesting attempt to characterize the nature of a solvent in terms of its H-bond donor ability (HBD or), H-bond acceptor ability (HBA jS), and its specific dipolarity/polarizabiUty (jt ). He has used these parameters in multiparameter relationships to analyze medium effects on a variety of chemical processes. Briefly, the HBD propensities (a) were obtained from the enhanced sol-vatochromism of the t(30) probe relative to 4-nitroanisole, the HBA propensities (j8) from the enhanced solvatochromism of 4-nitroaniline relative to N,N-diethyl-4-nitroaniline in HBA solvents, and the rt values from solvent effects on the n n transition of nitro-substituted aromatic compounds. For a more detailed discussion of these parameters and their apphcations, the reader is referred to the original literature. ... 

Similarly, UV spectroscopic scales have been defined from solvatochromic shifts upon hydrogen bonding of the electronic transitions of probes such as 4-nitroaniline and 4-nitrophenol. Among these scales, it is essential to make a distinction between solute and solvent scales. A solute scale is obtained when the HBD probe is smdied in dilute solutions of a series of bases in the same given solvent, whereas a solvent scale comes from measurements on binary systems of the HBD probe dissolved in the pure bases. 

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