Aromatic compounds infrared stretching frequency

Tertiary pyrrolines (49, = 1) and piperideines (49, = 2) (if R = H and the enamine can exist in the monomeric form or if R = aryl) evidently possess an endocyclic -double bond (79,155,156). The stretching frequency of the double bond can be lowered to 1620-1635 cm by conjugation with an aromatic substituent. The double bond of an analogous compound with aliphatic substituents in position 2 may occupy either the endo or the exo position. Lukes and co-workers (157) have shown that the majority of the five-membered-ring compounds, traditionally formulated with the double bond in a position, possess the structure of 2-alkylidene derivatives (50) with an exocyclic double bond, infrared absorption at 1627 cm . Only the 1,2-dimethyl derivative (51) is actually a J -pyrroline, absorbing at 1632 cm . For comparison, l,3,3-trimethyl-2-methylene pyrrolidine (52) with an unambiguous exocyclic double bond has been prepared (54). 

Infrared radiation, electromagnetic spectrum and, 419, 422 energy of. 422 frequencies of, 422 wavelengths of, 422 Infrared spectroscopy, 422-431 acid anhydrides, 822-823 acid chlorides, 822-823 alcohols. 428, 632-633 aldehydes, 428. 730-731 alkanes, 426-427 alkenes, 427 alkynes, 427 amides. 822-823 amines, 428, 952 ammonium salts, 952-953 aromatic compound, 427-428, 534 bond stretching in, 422... 

Infrared Spectroscopy (Review) Aromatic compounds are readily identified by their infrared spectra because they show a characteristic C—C stretch around 1600 cm-1. This is a lower C—C stretching frequency than for isolated alkenes (1640 to 1680 cm-1) or conjugated dienes (1620 to 1640 cm-1) because the aromatic bond order is only about 1 The aromatic bond is therefore less stiff than a normal double bond, and it vibrates at a lower frequency. 

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. 

There are many reasons why scientists want to measure the Raman spectra of compounds. First, many bands that are weak in the infrared spectrum are among the strongest bands in the Raman spectrum. For example, the S—S and C=C stretching bands are often so weak as to be essentially unrecognizable in the IR spectrum but stick out like the proverbial sore thumb in a Raman spectmm. Second, some Raman bands are found at very characteristic frequencies. For instance, monosubstituted aromatic compounds, together with 1,3-disubstituted and 1,3,5-trisubstituted aromatics, have a very intense band at 1000 cm. This band, along with the presence or absence... 

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