Carbonyl compounds stretch 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. 

Increased single bond character in a carbonyl group is associated with a decreased carbon—oxygen stretching frequency Among the three compounds benzaldehyde 2 4 6 trimethoxybenzaldehyde and 2 4 6 trinitrobenzaldehyde which one will have the lowest frequency carbonyl absorption" Which one will have the highest" ... 

Hydrogen bonding to a carbonyl group causes a shift to lower frequency of 40 to 60 cm k Acids, amides, enolized /3-keto carbonyl systems, and o-hydroxyphenol and o-aminophenyl carbonyl compounds show this effect. All carbonyl compounds tend to give slightly lower values for the carbonyl stretching frequency in the solid state compared with the value for dilute solutions. 

IR spectroscopy has been particularly helpful in detecting the presence of keto tautomers of the hydroxy heterocycles discussed in Section 3.01.6. Some typical frequencies for such compounds are indicated in Figure 4. Here again the doublets observed for some of the carbonyl stretching frequencies have been ascribed to Fermi resonance. 

The role of IR spectroscopy in the early penicillin structure studies has been described (B-49MI51103) and the results of more recent work have been summarized (B-72MI51101). The most noteworthy aspect of a penicillin IR spectrum is the stretching frequency of the /3-lactam carbonyl, which comes at approximately 1780 cm" This is in contrast to a linear tertiary amide which absorbs at approximately 1650 cm and a /3-lactam which is not fused to another ring (e.g. benzyldethiopenicillin), which absorbs at approximately 1740 cm (the exact absorption frequency will, of course, depend upon the specific compound and technique of spectrum determination). The /3-lactam carbonyl absorptions of penicillin sulfoxides and sulfones occur at approximately 1805 and 1810 cm respectively. The high absorption frequency of the penicillin /3-lactam carbonyl is interpreted in terms of the increased double bond character of that bond as a consequence of decreased amide resonance, as discussed in the X-ray crystallographic section. Other aspects of the penicillin IR spectrum, e.g. the side chain amide absorptions at approximately 1680 and 1510 cm and the carboxylate absorption at approximately 1610 cm are as expected. 

This exercise will investigate various carbonyl compounds. Examine the frequencies for the systems pictured below and determine the frequencies associated with carbonyl stretch in each case. In addition, locate the characteristic peak produced b) the single hydrogen attached to the carbonyl for the applicable systems. (We looked a this mode in formaldehyde in Example 4.1.)... 

Examine each of the vibrational motions for acetone, and identify the motion corresponding to the CO stretch. What is its frequency Are there any other vibrations which have very similar frequencies Does your result have implications for the use of the CO stretching frequency as a diagnostic for carbonyl compounds Elaborate. Does the CO stretching frequency involve significant motion of any atoms other than the two which make up the carbonyl group Rationalize your observation. 

Adduct formation by IrCl(CO)(PPh3)2 and similar compounds results in a shift in the IR carbonyl stretching frequency (Table 2.8). 

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. 

---