Aromatic compounds ultraviolet spectra

The ultraviolet spectra of these compounds are similar to those of trans stilbene or of 2- and 4-stilbazole. The effect on the ultraviolet spectrum of various substituents have been found to parallel in many respects the efiects produced by the corresponding group in derivatives of aromatic hydrocarbons (142). 

The simplest examples of this type of compound are enamines derived from the quinuclidine skeleton (67). The formulation of enamines of qflmuclidine in a inesomeric form would violate Bredt s rule. Actually, the ultraviolet spectrum of 2,3-benzoquinuclidine shows that there exists no interaction of aromatic ring tt electrons and the nitrogen-free electron pair (160,169). The overlap of the olefinic tt orbital and the lone pair orbital on nitrogen is precluded. 

Aromatic rings are detectable by ultraviolet spectroscopy because they contain a conjugated rr electron system. In general, aromatic compounds show a series of bands, with a fairly intense absorption near 205 nm and a less intense absorption in the 255 to 275 nm range. The presence of these bands in the ultraviolet spectrum of a molecule is a sure indication of an aromatic ring. 

Aromatic compounds. These compounds exhibit characteristic absorption in the ultraviolet-visible region of the spectrum, and although they are frequently easily recognised from their other spectroscopic properties, examination of their electronic spectra can often lead to the elucidation or confirmation of some of the detailed structural features. 

The formulation of enamines of quinuclidine in a mesomeric form would break Bredt s rule. No mesomerism occurs in 2,3-benzo-quinuclidine between the nitrogen atom and the aromatic ring thus, the compound does not exhibit the characteristic ultraviolet absorption of aromatic amines.43 Dehydroquinuclidine may only be formulated as l-azabicyclo[2.2.2]oct-2-ene the overlap of the olefinic ir-orbital and the lone pair orbital on nitrogen is precluded. Its ultraviolet spectrum exhibits merely end-absorption the compound is... 

Ultraviolet Spectroscopy The ultraviolet spectra of aromatic compounds are quite different from those of nonaromatic polyenes. For example, benzene has three absorptions in the ultraviolet region an intense band at Amax = 184 nm (e = 68,000), a moderate band at Amax = 204 nm (e = 8800), and a characteristic low-intensity band of multiple absorptions centered around 254 nm (e = 200 to 300). In the UV spectrum of benzene in Figure 16-19, the absorption at 184 nm does not appear because wavelengths shorter than 200 nm are not accessible by standard UV-visible spectrometers. 

The absorption bands in the ultraviolet and visible part of the spectrum correspond to changes in the energy of the electrons but simultaneously in the vibrational and rotational energy of the molecule. In this way a system of bands is produced in the gaseous state. In the liquid state there is nothing of the rotational fine structure to be seen, and usually little or nothing of the vibrational structure, as a result of the interaction with the molecules of the solvent. With aromatic compounds in non-polar solvents such as hexane and carbon tetrachloride the vibrational structure is, however, still clearly visible in the ultraviolet absorption spectrum. This vibrational structure is mainly determined by the vibrations of the excited state, which therefore do not occur in the infrared and Raman spectrum of the normal molecule. 

Protonation of the triaryl-1,2-diazepines (17 and 18) gave cations (45).21 In the case of 18 [R=p-(CH3)2N—C6H4] the compound is diprotonated and the effect of this on the ultraviolet spectrum implies planarity in the parent diazepine. It is also possible to monoprotonate the compound 18 [R=p-(CH3)2N—C6H4] and the deeply colored cation suggests aromatic resonance (46) for the diazepinium salt.21... 

Obtain infrared and nuclear magnetic resonance spectra following the procedures of Chapters 19 and 20. If these spectra indicate the presence of conjugated double bonds, aromatic rings, or conjugated carbonyl compounds obtain the ultraviolet spectrum following the procedures of Chapter 21. Interpret the spectra as fully as possible by reference to the sources cited at the end of the various spectroscopy chapters. 

If the solvent is to be used for ultraviolet spectroscopy, it is necessary to remove all the aromatic compounds. This may be done by shaking the hydrocarbon with a mi.xture of concentrated sulfuric and nitric acids, which will nitrate the aromatic compounds. The hydrocarbon is separated, washed with water, distilled, and passed through a column of activated alumina which will remove any residual unsaturated or nonhydrocarbon materials. The spectrum of the solvent is monitored as it passes from the column, and when significant absorption at 210 m/i is observed, the alumina is replaced. 

Aromatic compounds also show characteristic infrared and ultraviolet absorption spectra. In the mass spectrum of aromatic substances, peaks corresponding to ions such as CgH and CgHg" are often seen. A commonly observed peak occurs at m/z 91, corresponding to the stable ion CgHgCH./, These features are all helpful in assigning aromatic character. 

The ultraviolet spectrum of the parent heterocycle in water shows three main peaks, at 221.5, 282, and 302 nm. The similarity of this spectrum to that of indolizine is apparently due to the fortuitous cancellation of the effects of the introduction of successive nitrogen atoms into indolizine. UV data are available for several quaternary derivatives of this heterocycle and their V-oxides." 2,3-Dihydroimidazo[l,2-a]pyr izine (5) has only two peaks, at 258 and 394 nm, in its ultraviolet spectrum in water. Both this compound and the unsubstituted aromatic heterocycle undergo large hypsochromic shifts on protonation. This contrasts with the bathochromic shift observed on protonation of aminopyrazines and is consistent with the hypsochromic effect obtained on protonation of the analogous imino compounds. These observations confirm the conclusion that protonation occurs at the 1-position in this heterocycle to give the cation 6. 

FIGURE 7.4 Ultraviolet spectrum of benzoic acid in cyclohexane. (From Friedel, R. A., and M. Orchin, Ultraviolet Spectra of Aromatic Compounds, John Wiley and Sons, New York, 1951. Reprinted by permission.)... 

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