Group frequencies esters

Because of the participation of the enamine double bond in the resonance of the aromatic ring of heteroaromatic /J-enamino esters, the group frequency character of the individual bands is low. Thus the El band is essentially vc=Q. Similarly, jS-enamino nitriles do not exhibit coupled vibrations since the nitrile absorption does not participate.24,61... 

In the IR spectra, the partially saturated heterocyclic /S-enamino nitriles do not exhibit the normal group frequencies that the corresponding esters display. The comparison of the 13C-NMR spectra of nitriles 299,300,302, and 305 with those of the esters 8,71, and 72 reveal for the nitriles a greater high-field shift of 27 ppm (Table III). As a consequence, it is concluded that in the enamino nitriles the double bond is more polarized than that of the corresponding esters several canonical forms can be discussed (Scheme 85). This difference in polarization (8, 71, and 72 versus 299, 300, 302, and 305) leads to several significant differences in the chemical behavior, which are briefly discussed in the following sections. 

Figure 6.2 illustrates the infrared spectrum of poly(methyl methacrylate) (PMMA). The structure of PMMA is -[-CH2-C(CH3)(COOCH3)-] -. The infrared spectrum of PMMA results from the group frequencies of the C-C and C-H groups of the backbone chain, the C-C, C=0 and C-0 units of the ester group and the C-H units of the methyl substituent. The infrared assignments for this spectrum are listed in Table 6.1. 

The infrared spectrum of this new material (Fig. 6.15) indicates that significant changes have occurred in the structure of the material. The proposed structure involves a rather spectacular molecular rearrangement of the ds-unsaturated ketone to yield an ethyl ester containing a phenoxy-substituted double bond. Can you rationalize the data to fit this structure Suggest possible macro group frequencies that are present in the IR spectrum of the rearranged product. 

Infrared Analysis. The conversion of a branched-chain aliphatic primary alcohol to an acetate ester results in significant changes in the infrared spectrum of the molecule. These changes are similar to those observed in straight-chain systems. The two macro group frequency trains in the present example are... 

If the macro group frequencies are further constrained to include only those systems in which the chain branching involves the presence of an isopropyl group, two additional bands near 1385 and 1365 cm are required. These peaks, which are present in both the alcohol and the ester, arise from the spatially coupled and split symmetric methyl bending vibrations of the aliphatic backbone. In both isopentyl alcohol and isopentyl acetate, this pair of bands are found at identical locations, 1386 and 1367 cm . ... 

Objective Attempt an identification of the class of esters from the characteristic group frequencies in their spectra. Reference compounds similar to compound 21-A are found in the Aldrich Handbook of Spectra and can be used for comparison, while 21-B is found in both the Coblentz Society Desk Book and the Aldrich Handbook. Once a spectrum has been established, refer to the Answers to the Exercises for a detailed discussion. 

The role of specific interactions in the plasticization of PVC has been proposed from work on specific interactions of esters in solvents (eg, hydrogenated chlorocarbons) (13), work on blends of polyesters with PVC (14—19), and work on plasticized PVC itself (20—23). Modes of iateraction between the carbonyl functionaHty of the plasticizer ester or polyester were proposed, mostly on the basis of results from Fourier transform infrared spectroscopy (ftir). Shifts in the absorption frequency of the carbonyl group of the plasticizer ester to lower wave number, indicative of a reduction in polarity (ie, some iateraction between this functionaHty and the polymer) have been reported (20—22). Work performed with dibutyl phthalate (22) suggests an optimum concentration at which such iateractions are maximized. Spectral shifts are in the range 3—8 cm . Similar shifts have also been reported in blends of PVC with polyesters (14—20), again showing a concentration dependence of the shift to lower wave number of the ester carbonyl absorption frequency. 

Spectral Characteristics. The iafrared stretching frequency of the penicillin P-lactam carbonyl group normally occurs at relatively high frequencies (1770 1815 cm ) as compared to the absorptions for the secondary amide (1504-1695 cm ) and ester (1720-1780 cm ) carbonyl groups. 

Steric effects on both the amide and the acyloxyl side chain are similar. Tert-butyl and adamantyl groups on the amide side chain in 29v, 29x, 29c, and 29e (Table 2 entries 53 and 54, 63 and 65) result in lower stretch frequencies that, on average, are only 40 cm-1 higher than their precurser hydroxamic esters. Streck and coworkers have suggested that such changes in dialkyl ketones can be ascribed to destabilisation of resonance form II through steric hindrance to solvation which, in the case of tert-butyl counteracts the inductive stabilisation.127... 

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