Spectroscopy of Carboxylic Acid Derivatives

Draw structures of the step-growth polymers you would expect to obtain from the following reactions  

Kevlar, a nylon polymer prepared by reaction of 1,4-benzenedicarboxylic acid (terephthalic acid) with 1,4-benzenediamine (p-phenylenediamine), is so strong that it s used to make bulletproof vests. Draw the structure of a segment of Kevlar. 

All carbonyl-containing compounds have intense IR absorptions in the range 1650 to 1850 cm i. As shown in Table 21.3, the exact position of the absorption provides information about the specific kind of carbonyl group. For comparison, the IR absorptions of aldehydes, ketones, and carboxylic acids are included in the table, along with values for carboxylic acid derivatives. 

Acid chlorides are easily detected by their characteristic absorption near 1800 cm . Acid anhydrides can be identified because they show two absorptions in the carbonyl region, one at 1820 cm and another at 1760 cm . Esters are detected by their absorption at 1735 cm , a position somewhat higher than 

What kinds of functional groups might compounds have if they show the following IR absorptions  

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. 

What characteristics of the methyl benzoate spectrum rule out an aldehyde or carboxylic acid functional group giving the absorption at 1723 cm  

Give the frequencies of the C — O single-bond stretching absorptions in the IR spectra of ethyl octanoate and methyl benzoate. 

Amides Simple amides have much lower carbonyl stretching frequencies than the other carboxylic acid derivatives, absorbing around 1640 to 1680 cm (often a close doublet). This low-frequency absorption agrees with the resonance picture of the amide. 

Infrared spectra of (a) ethyl octanoate and (b) methyl benzoate. The carbonyl stretching frequency of simple esters is around 1735 cm and that of conjugated esters is around 1710-1725 cm . 

Propose structures for compounds that have the following formulas and IR absorptions  

Hydrogens on the carbon next to a carbonyl group are slightly deshielded and absorb near 2 5 in the H NMR spectrum. The exact nature of the carbonyd group can t be determined by NMR, however, because the m hydrogens of all acid derivatives ab.sorb in the same range. Pigure 21.10 shows the NMR spectrum of ethyl acetate. 

Although NMR is useful for determining the presence or absence of a carbonyl group in a molecule, the identity of the carbonyl group is difficult to determine. Aldehydes and ketones absorb near 200 8, while the carbonyl carbon atoms of various acid derivatives absorb in the range 160 to 180 8 (Table 21.4). 

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Polyamides and Polyesters Step-Growth Polymers 818 21.10 Spectroscopy of Carboxylic Acid Derivatives 822... 

Structure and Nomenclature of Acid Derivatives 982 21-3 Physical Properties of Carboxylic Acid Derivatives 988 21-4 Spectroscopy of Carboxylic Acid Derivatives 991 21-5 Interconversion of Acid Derivatives by Nucleophilic Acyl Substitution 997... 

IV. Spectroscopy of carboxylic acid derivatives and nitriles (Section 21.10). 

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