Overtone substituted aromatics

The differentiation is easily made by noting the characteristic absorption bands corresponding to the aromatic ring system around 1600-1450cm-1 and 850-660cm-1, and the characteristic pattern in the overtone region around 2000-1800 cm- The origin of these absorptions is discussed in detail below their presence in this spectrum clearly shows that the structure is a mono-substituted aromatic compound. 

Weak combination and overtone bands appear in the 2000-1650 cm-1 region. The pattern of the overtone bands is not a reliable guide to the substitution pattern of the ring. Because they are weak, the overtone and combination bands are most readily observed in spectra obtained from thick samples. The spectrum of Figure 3.13 is that of a typical aromatic (benzenoid) compound. 

Aromatic compounds Monosubstituted Q- 3100-3000 1600-1500 770-730 (s) 710-690 (s) All show weak combination and overtone bands between 2000 and 16,500 cm-1 see aromatic substitution pattern chart... 

The final spectrum, figure 9.24, is that of p-nitroaniline. Although complex it is immediately recognizable as being aromatic (C—H above 3 000 cm-1, skeletal and overtone bands between 1 400 and 2 000 cm-1 and a C—H out-of-plane bending vibrations at 835 cm-1 indicating / -substitution). Asymmetric and symmetric N=0 stretching bands are very prominent ai... 

The molecule is aromatic, and there are C-H stretching bands above 3000 cm The small bands just below 3000 cm are probably overtones from the very strong bands at 1530 and 1350 cm The bands at 850 and 700 cm suggest that this is a mono-substituted benzene. The two very strong bands at 1530 and 1350 cm tell us that a NO2 group is present. The Uqnid is, therefore, nitrobenzene. 

Aromatic hydrocarbons such as benzene and substituted benzene derivatives have a few distinguishing features in the infrared. This section concludes with the spectra of benzene (Figure 14.21A, an old-style infrared spectrum) and sec-butylbenzene (2-phenylbutane, Figure 14.21B). Benzene has C-H absorptions that are clearly evident, as well as a C=C absorption at about 1600-1630 cm. There is little to indicate that this is an aromatic hydrocarbon. Note the very weak signals at about 1750-2000 cm. These are known as C-H overtone absorptions, and they usually appear only with benzene derivatives. They are very weak and may easily be missed. 

Substitution on the benzene ring reduces the symmetry of the molecule, potentially allowing for additional vibrational peaks. In the case of alkyl aromatics, there is also the addition of the alkyl absorptions. Figure 4.2 compares the near-infrared spectra of benzene and toluene. The primary differences appear to be the addition of the methyl combination bands near 4300 cm (2300 nm) and the methyl first overtones near 5800 cm (1750 nm). There is also a noticeable peak at 3836 cm (2607 nm) that could be related to the sum of a C-H stretch and one of the ring-wagging vibrations involving a ring with five adjacent protons. 

The pattern of bands in this region shown by various substituted benzene ring compounds is illustrated in Figure 5-26. It can be seen that a distinct pattern is found for each type of aromatic substitution. For example, a monosubstituted benzene derivative shows a series of four maxima beginning at about 1880 cm" The assignments of these frequencies to combination and overtone bands have been given by Kakiuti and Whiffen [ ]. Since for most compounds these are weak bands, a thicker cell or more concentrated solution is used if this region is to be examined carefully. 

The spectra of pyridine compounds have many of the features of the spectra of homonuclear aromatic compounds, such as bands due to the aromatic C-H stretching vibration, overtones in the region 2080-1750cm (4.81-5.88 am) etc., with the nitrogen atom behaving in a similar fashion to that observed for a substituted carbon atom. Therefore, the contents of the previous chapter should be noted. 

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