Benzene chromophore, auxochromes

Figure 4.10 shows the UV absorption spectra of a solution of procaine in 0.1 M HCl and O.IM NaOH. In procaine, the benzene chromophore has been extended by addition of a C = O group and under acidic conditions, as in Figure 4.10, the molecule has an absorption at 279 nm with an A (1%, 1 cm) value of 100. In addition to the extended chromophore, the molecule also contains an auxochrome in the form of an amino group, which under basic conditions has a lone pair of electrons that can interact with the chromophore producing a bathochromic shift. Under acidic conditions the amine group is protonated and does not function as an auxochrome but when the proton is removed from this group under basic conditions a bathochromic shift is produced and an absorption with A, max at 270 nm with an A (1%, 1 cm) value of 1000 appears. 

The data of Table 22-3 show the effect on the benzene chromophore of this type of substituent —the substituent often being called an auxochrome.2 This term means that, although the substituent itself is not responsible for the absorption band, it shifts the absorption of the chromophoric group, in this case the benzene ring, toward longer wavelengths. The auxochromic groups usually increase the intensity of the absorption also. 

Effect of Auxochromic Substituents on Electronic Absorption by the Benzene Chromophore... 

The chromophore of phenylephrine is not extended but its structure includes a phenolic hydroxyl group. The phenolic group functions as an auxochrome under both acidic and alkaline conditions. Under acidic conditions it has two lone pairs of electrons, which can interact with the benzene ring and under basic conditions it has three. Figure 4.11 shows the bathochromic and hyperchromic shift in the spectrum of phenylephrine, which occurs when 0.1 M NaOH is used as a solvent instead of 0.1 M HCl. Under acidic conditions the X max is at 273 and has an A (1 %, 1 cm) value of 110 and under alkaline conditions the X max is a 292 nm and has an A (1%, 1 cm) value of 182. 

Examples of auxochromes include the —NH2, —OH and —SH groups. These functional groups possess lone pairs of non-bonded electrons that can interact with the n electrons of the chromophore and allow light of longer wavelength to be absorbed. A good example of this effect is to compare the X,max values of benzene and aniline (also called phenylamine or aminobenzene), shown in Figure 7.6. 

An auxochrome is a substituent that when attached to a chromophore, alters the - max and the intensity of the absorption, usually increasing both OH and NH2 groups are auxochromes. The lone-pair electrons on oxygen and nitrogen are available for interaction with the tt electron cloud of the benzene ring, and such an interaction... 

Thus the dye alizarin (red) contains two benzene rings, two carbonyl groups (chromophores) and two hydroxyl groups (auxochromes) ... 

Auxochromes Groups such as —OH, —NH2, and the halogens, which aU possess n electrons, are not chromophores themselves. But they can cause normal chromo-phoric absorptions to occur at longer wavelengths and with an increase in the value of molar absorptivity. These groups are called auxochromes. Auxochromes are responsible for the electronic transitions n-<- a and n —> 71. Table 17.4 shows the effect of auxochromes on the absorption of benzene. 

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