Aromatic compounds group frequencies

Characteristic Frequencies of Aromatic Compounds (Group Frequencies of Arenes)... 

Compound CH stretching 1465 scissors 1380 scissors Aromatic ring group frequencies ... 

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

Different organic functional groups (i.e., methyl, methylene, phenyl, and the hydrogen atoms adjacent to the carbonyl carbon in aldehydes and organic add groups) absorb at different frequencies and thus can be easily identified. Similarly, different 13C environments result in different absorption characteristics. For instance, carbon atoms in aromatic compounds absorb different frequencies than do those in carbonyl groups. 

Kross and Fassel [30], on the basis of investigation of over thirty aromatic compounds, gave the following vibration frequencies of the nitro group ... 

A similar phenomenon was noticed in the case of aromatic compounds. Thus, Bellamy [24] has pointed out that in polynitro compounds when one nitro group remains coplanar whilst another is twisted out of the plane of the ring under the influence of steric hindrance, the degree of aromatic conjugation is reduced and a new higher frequency band appears. 

Frank, Horman and Scheibe [34] also found that asymmetric N02 vibrations could be shifted towards lower frequencies under the influence of conjugation in aromatic compounds. They examined their substances in potassium bromide. A number of aromatic nitro compounds in dilute solutions were recently examined by T. Urbanski and Dabrowska [35], They found that when a nitro group was placed in the para position with respect to another group, its symmetric stretching modes were of a lower frequency than in the meta position. This was most likely due to the strong conjugation of substituents with the ring ... 

Monocyclic aromatic compounds exhibit a strong characteristic band at a group frequency near 400 cm, this is the shown in Fig. 8.7. 

Fig. 8.7 The form of both components of the, group frequency mode at ca 400 cm in aromatic compounds ( , atoms at nodes the larger the symbol the greater the atomic displacement).

Infrared spectroscopy is an excellent technique for determining the structure of a polymer. For example, polyethylene and polypropylene have relatively simple spectra because they are saturated hydrocarbons. Polyesters have stretching frequencies associated with the C=0 and C—O groups in the polymer chain. Polyamides (nylon) show absorptions that are characteristic for the C=0 stretch and N—H stretch. Polystyrene has characteristic features of a monosubstituted aromatic compound (see Technique 25, Figure 25.12). You may determine the infrared spectra of the linear polyester from Experiment 46A and polystyrene from Experiment 46C in this part of the experiment. Your instructor may ask you to analyze a sample that you bring to the laboratory or one supplied to you. 

This compound also possesses a second powerful macro group frequency train that assesses the substitution pattern of the aromatic ring system (see Chapter 8, IR discussions). The para-disubstituted benzene ring macro group frequency train requires peaks in the following regions 1950,1880,1800,1730, 750, and 690 cm . ... 

The acidic product generated in the reaction is assumed to be 4-chlorobenzoic acid. This material also is a para-substituted aromatic compound thus, the spectrum of this compound (Fig. 6.17) possesses a macro frequency train (a), similar to those of the aldehyde and alcohol. In addition, this benzoic acid derivative exhibits an extended aromatic acid macro group frequency train (b).The macro frequencies are as follows ... 

A second problem of this revision is the vast literature which has grown up in the twenty years since this book was first written. At that time it was possible, at least to aspire, to include all the relevant references. To try to do so now would require a bibliography as large as the book itself. Moreover a number of specialist texts have appeared in which the whole book is devoted to the material which has to be presented here in one chapter, and these already offer the complete documentation that a specialist may need. Thus, two reviews by Katritzky and his coworkers deal exclusively with the group frequencies of heterocyclics and between them cite over 1500 references. Varsanyi has provided a comparable review of the aromatics, Adams of coordination compounds and Bentley et al. of the far infrared. There is also a wide coverage on inorganics by Nakamoto and by Lawson, whilst the supplementary Raman group frequency data has recently been very excellently covered by Dollish, Fateley and Bentley. I have therefore confined myself to those references which are most relevant and contented myself with reference to these more detailed sources where appropriate. 

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