Aldehydes infrared frequencies

SCHEME 9.10 Molecular motions of H2O and an aldehyde (a) vibrational motions of H2O, where arrows indicate the directions in which the atoms move, and (b) rotational motions of the whole molecule around three axes (the third axis goes through the plane of the page, pointing out of the plane), (c) Ranking of the expected infrared frequencies of the various bonds in the H2C=CH—H2C—HOO molecule. 

The infrared carbonyl stretching frequencies of n- and isobutyraldehyde in the condensed phase occur at 1727.6 and 1738.0 cm , respectively (38). The proton nmr spectra of both aldehydes are weU-known (39). 

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... 

Molecular models suggest that the aldehyde (68, X = H) can assume a coplanar conformation with the naphthyl group, whereas such a coplanar conformation is not possible for any of the other compounds. This situation is reflected in the infrared carbonyl stretching frequencies. The ap forms of these compounds absorb generally at higher frequencies than do their sp counterparts. 

The widespread use of infrared spectroscopy at that time was probably due to the observation that many chemical groups absorb in a very narrow range of frequency. Furthermore, within this frequency range, the observed frequency may be correlated to specific chemical structures. For example, aldehydes can be differentiated from ketones by the characteristic stretching frequency of the carbonyl group near 1700 cm-1, and the spectral pattern may be likened to a molecular fingerprint. ... 

A similar coupling may be observed for a, /J-unsaturated aldehydes and ketones. Table 4.1-3 shows that the s-cis or s-trans conformation of the C=C-C=0 systems is indicated by the difference of the frequencies of the C=0 and the C=C stretching vibration as well as by its relative intensity in the infrared and the Raman spectrum and the depolarization factor in the Raman spectrum (Oelichmann et al., 1982). 

Aldehydes Amides Carboxylic acid anhydrides Carboxylic acids Esters Ketones and functional groups, 56 infrared absorption frequencies, 519,... 

Different types of carbonyl groups give characteristic strong absorptions at different positions in the infrared spectrum. As a result, infrared spectroscopy is often the best method to detect and differentiate these carboxylic acid derivatives. Table 21-3 summarizes the characteristic IR absorptions of carbonyl functional groups. As in Chapter 12, we are using about 1710 cm for simple ketones, aldehydes, and acids as a standard for comparison. Appendix 2 gives a more complete table of characteristic IR frequencies. 

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