Infrared spectra, of substituted benzenes

Sarma, T.V.K., Sastry, C.V.R., and Santhamma, C., The photoacoustic spectra of substituted benzenes in the near-infrared region, Spectrochim. Acta, 43A, 1059-1065, 1987. 

Fig 8.7 The 900-700 cm-i region in the infrared spectra of some benzene ring derivatives. The X substitutes are indicated on each spectrum. In the disubstituted compounds both X substituents are the same. 

Fig. 13. Infrared spectra of methyl-substituted benzenes on Ni/Si02 (A) toluene (B) o-xylene (C) /n-xylene (D) p-xylene. [(A), (C), and (D) from Ref. 358 (B) reprinted from Ref. 242, Kinet. Kami (Kinet. Catal. Transl.) 8, D. M. Shopov and A. N. Palazov, p. 862 (p. 732 Transl.). Copyright 1967 with kind permission of Elsevier Science-NL, Sara Burgerhart-straat 25, 1055 KV Amsterdam, The Netherlands.]...

The ether extract of cane molasses yields an acidic substance with the characteristic odor of raw sugar.128 The steam distillation of molasses is stated to yield a rum oil. 129 Fractionation of cane final molasses on fuller s earth clay produces a concentrate with a strong molasses odor.70 The infrared spectra of the volatile portion of this concentrate indicated the absence of hydroxyl and carbonyl and the presence of a substituted benzene structure, of paraffinic methylene and methyl groups, of an acetate group, and of the > C=C < and —C=C— linkages. The presence of a sulfur function is probable. Further chromatography indicated complexity in this volatile concentrate.180... 

Schuur, J. and Gasteiger, J. (1997), Infrared Spectra Simulation of Substituted Benzene Derivatives on the Basis of a 3D Structure Representation. Anal.Chem., 69,2398-2405. 

With regard to the delocalized electrical effect of the NMea" " group, as it is isoelectronic with the tertiary butyl group which is well known to be a delocalized effect electron donor (thus, a/i, a/f, (Tr, and delocalized electron effect donor group. In support of this contention are studies of infrared spectra of para- and wcta-substituted benzenes (55,56,62), pA measurements (57,58) and nmr results (59-61), and SCF-MO calculations indicating that NH3+ is an electron donor by the delocalized effect (61) which supports the argument that NMes is. 

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. 

A study of the infrared spectra of a series of substituted-benzene chromium tricarbonyls has been carried out in order to compare them with the spectra of the free parent hydrocarbons (132). The property that in the infrared spectra of tt complexes of aromatic hydrocarbons the vch s show a decrease in intensity in the complex, was confirmed and the shifts of some bands were discussed. 

Some chemical structures exhibit typical distances that occur independently of secondary features, which mainly affect the intensity distribution. In particular, aromatic systems contain at least a distance pattern of ortho-, meta-, and para-carbon atoms in the aromatic ring. A monocyclic aromatic system shows additionally a typical frequency distribution. Consequently, Cartesian RDF descriptors for benzene, toluene, and xylene isomers show a typical pattern for the three C-C distances of ortho-, meta-, and para-position (1.4, 2.4, and 2.8 A, respectively) within a benzene ring. This pattern is unique and indicates a benzene ring. Additional patterns occur for the substituted derivatives (3.8 and 4.3 A) that are also typical for phenyl systems. The increasing distance of the methyl groups in meta- and para-Xylene is indicated by a peak shift at 5.1 and 5.8 A, respectively. These types of patterns are primarily used in rule bases for the modeling of structures explained in detail in the application for structure prediction with infrared spectra. 

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. 

Examples of these infrared bands are seen in Fig. 8.7. The first two rows of spectra illustrate benzene rings with alkane substituents. For these, the adjacent hydrogen wag bands tend to increase in frequency somewhat as the a carbon of the substituent is more highly substituted. We can note absorptions for toluene at 728 cm ethylbenzene at 745 cm isopropylbenzene at 759 cm and /ert-butylbenzene at 763 cm ... 

---