Substituted benzenes, infrared spectra

Pyridine. Pyridine and its methyl substituted derivatives (picolines and lutidines) were found to polymerize electrochemically and, under certain circumstances, catalytically. This behavior was not expected because usually pyridine undergoes electrophilic substitution and addition slowly, behaving like a deactivated benzene ring. The interaction of pyridine with a Ni(100) surface did not indicate any catalytic polymerization. Adsorption of pyridine below 200 K resulted in pyridine adsorbing with the ring parallel to the surface. The infrared spectrum of pyridine adsorbed at 200 K showed no evidence of either ring vibrations or CH stretches (Figure 5). Desorption of molecular pyridine occurred at 250 K, and above 300 K pyridine underwent a... 

Although the infrared spectrum of a perchlorobenzene derivative may differ greatly from that of benzene and its substituted derivatives, some correlations have nevertheless been found, and these have proved to be most useful in structural assignments within the perchloro-organic domain. 

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. 

In para-substituted benzenes with identical substituents the 1175 cm vibration (2,5 vs. 3,6) in centro-symmetric and is infrared inactive but is Raman active, " whereas the 1013 and 1117 cm centro-antisymmetric vibrations are Raman inactive. For the other isomers in Table 8.7 the vibrations are active (but not necessarily strong) in the Raman " as well as the infrared spectrum. Two especially prominent Raman bands are the 1027 cm" band for mono and the 1033 cm band for ortho which show up at 1018-1030 cm" and 1020-1060 cm" respectively. " As was mentioned before, mono-, meta-y and 1,3,5-trisubstituted benzenes have a very strong Raman band at 990-1010 cm (see Fig. 8.11). 

The infrared spectrum reveals substitution patterns in benzene derivatives... 

Flash-photolysis studies of the hexacarbonyls Cr(CO)g, Mo(CO)g and W(CO)g are, like previous years, still abundant Nayak and Burkey have found that there are low quantum yields for Cr(CO)g substitution in fluorocarbon solvents - which provides yet more evidence that metal-fluorocarbon interactions are very weak There have been estimates made of solvent-metal bond strengths in (Solvent)M(CO)5 complexes (Solvent = Benzene [M = Mo, W] and Tetrachloromethane [M = Cr]) and the photolysis of W(CO)g in the presence of hex-l-ene has been reported Flash-photolysis studies of the photochemical reactions of silanes with Cr(CO)g have been published Using time-resolved infrared spectroscopy RIR), Turner and co-workers have captured the IR spectrum of the MLCT excited state of W(CO)5(4-cyanopyridine) which rapidly decays to W(CO)5. The excited state of W(CO)5(4-cyanopyridine) is relatively long lived, which makes the experiment possible. This reporter will be interested to see how this technique develops. 

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. 

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