O—H absorption bands

Hydroxypyrrolo[3,2-c]pyridine (13) exists substantially in the carbonyl form shown. In the IR spectrum there is no detectable O—H absorption bands at 3150 and 3250 cm-1 are assigned to two N—H frequencies. The carbonyl band is at 1648 cm-1 (s). Several 2-hydroxypyrrolopyridines have been examined and most show the presence of both tautomers but the equilibrium position varies considerably. Compound (14) has a strong... 

The shape of an absorption band can be helpful in identifying the compound responsible for an IR spectrum. For example, both O — H and N—H bonds stretch at wavenumbers above 3100 cm but the shapes of their stretches are distinctive. Notice the difference in the shape of these absorption bands in the IR spectra of 1-hexanol (Figure 13.19), pentanoic acid (Figure 13.20), and isopentylamine (Figure 13.25). An N—H absorption band ( 3300cm ) is narrower and less intense than an O — H absorption band ( 3300 cm ), and the O—H absorption band of a carboxylic acid ( 3300-2500 cm ) is broader than the O — H absorption band of an alcohol (Sections 13.9 and 13.10). Notice that two absorption bands are detectable in Figure 13.25 for the N—H stretch because there are two N—H bonds in the compound. 

For liquids, quartz cells like those used in UV/VIS can be used. Because of the weak absorption coefficients most samples need not be diluted, and a cell of large pathlength, up to some centimeters, can be used. NIR spectra of some common solvents are represented in Fig. 6.7. Tetrachloromethane is very suitable because all C—Cl vibrations occur far away from the NIR range. In contrast, water and ethanol are not suitable due to their strong O—H absorption bands. For the... 

The quantitative use of these bands is limited because of its position in the spectra the N-H stretching is very close to the strong O-H absorption band (minimal amount of water perturbs its area), while the deformation band is located in the region where many signals corresponding to organic bonds ap>p)ear. 

Ethers can easily be identified because they are the only compounds of the four that do not absorb above 3200 cm. Carboxylic acids have a very broad absorption from the O-H stretching between 2500 and 3500 cm. They are readily distinguished from alcohols and phenols where the O-H absorption bands are not as broad as those of carboxylic acids. Both... 

It is important to note that the calculation of the initial concentrations of phenol ( 10-2 mol dm-3) and acetonitrile (possibly 1 mol dm-3) were corrected for the density of the solvent at each temperature. The temperature effect on the molar absorption coefficient (e) was also considered when relating [PhOH] to the absorbance of the O-H free band. This was empirically made by measuring the absorbances (A) of a phenol solution (in the same solvent and with a concentration similar to that used in the equilibrium study) over the experimental temperature range. For each temperature, the Lambert-Beer law [312],... 

For each alkylated extract, there was an absorption at 1700 cm which was absent in the untreated extract. This absorption may be attributed to esters that form from alkylation of carboxylic acids. This interpretation is consistent with the NMR analysis described below. For each O-alkylated extract, there was an increase in the intensity of the C-H absorption bands at 2800-3000 cm consistent with the introduction of aliphatic carbon. 

The conversion of a carboxylic acid to a salt can serve as confirmation of the acid structure. This is conveniently done by the addition of a tertiary aliphatic amine, such as triethylamine, to a solution of the carboxylic acid in chloroform (no reaction occurs in CC14). The carboxylate ion thus formed shows the two characteristic carbonyl absorption bands in addition to an ammonium band in the 2700-2200 cm-1 region. The O—H stretching band, of course, disappears. The spectrum of ammonium benzoate, Figure 3.24, demonstrates most of these features. 

It has been demonstrated that 2-alkylaminotetrahydropyrans (252 R = H or alkyl, R = alkyl) exist in a tautomeric equilibrium with the hydroxyimines (253) (79KGS1317). The situation is depicted in equation (8). In the absence of solvent, the IR spectra of all the compounds studied exhibit intense bands corresponding to C=N and O—H absorptions (due to 253). There is a marked decrease of these bands in the solution spectra, since (252) is predominant. The signals observed in the NMR spectrum arise from ring and chain forms. It was shown that the equilibrium concentration of tautomers is dependent upon the substituents present. 

Infrared Spectroscopy. The following bands are seen in the ir spectrum of PPG 2970, 2940, 2880 cm-1 (C—H stretch, m) 1460,1375 cm-1 (C—H bend, m) 1100,1015 cm-1 (C—O stretch, m) of which the 2940 and 1015 band are specific. The latter are also present in copolymers of EO and PO. Absorptions due to unsaturated end groups are found at 1650 cm-1 (allyl ether) and 1672 cm-1 (1-propenyl ether). The O—H stretching band at 3470 cm-1 shows the greatest variation for different hydroxyl number polyols and has been used to estimate the hydroxyl number (169). 

Because a carboxylic acid forms very strong hydrogen bonds, the band for its O—H group is extremely broad and occurs at even lower wavenumbers, usually centered near 3000 cm-1. This band overlaps the C—H absorptions but is readily distinguished from them because it is so broad. As illustrated in Figure 13.10, the C—H absorptions can often be seen superimposed on the intense O—H absorption. 

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