Spectroscopic Features of Carboxylic Acid Derivatives


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Molecule list: Aliphatic Compounds 3-Phenylpropanoyl Chloride Butanoic Anhydride Ethyl Acetate Acetamide Sodium Butanoate Hexanenitrile Aronmatic Compounds p-Toluyl Chloride Benzoic Anhydride Ethyl Benzoate m-Toluamide Sodium Benzoate m-Toluonitrile	IR Spectroscopy Load the IR Spectrum Caboxylic acid derivatives all (with the exception of nitriles) show a C-O stretch as the strongest band in the spectrum, and the frequency can often be used to identify which derivative we deal with. Load the molecules to the left and sequentially identify the C=O peak for each of the aliphatic derivatives. The characteristic regions and behaviors (in cm^-1) are: Acid chlorides: 1790-1815 Acid Anhydrides: 1740-1790 & 1800-1850 Esters: 1735-1750 Amides: 1650-1690 Acid salts (carboxylates): <1600 Anhydrides usually show two bands (symmetric and asymmetric combinations) which most other groups show a single band. The position of the band reflects the electronics of the C=O bond as laid out in the following resonance:

	The more important the C=O double bond form, the higher the frequency (acid chlorides, anhydrides). The more important the C-O single bond form, the lower the frequency (amides, carboxylate salts). Likewise, ring strain raises the frequency because of a parallel effect: bending the C-C-C angle in raises the bond order a bit and raises the C=O frequency. Conjugation (particularly to an aromatic ring) will lower the C=O frequency; load the aromatic examples and find the appropriate bands. Compare these frequencies to those for the corresponding aliphatic example. Other parts of the molecule will have the extected behavior: amide N-H's come at the same place as amine N-H stretches; esters have C-O single bond stretches (again, these are difficult to identify). Nitriles, of course, show the C-N stretch at 2250-2150 cm^-1.
¹H NMR Spectroscopy Load the ¹H NMR Since most derivatives (other than maybe amides) lack protons directly on the functional group, the ¹H NMR is used more indirectly. The "pieces" look like other groups: the group connected to the carbonyl looks much like the corresponding carboxylic acid (lacking the COOH proton); an ester will liik much like an ether; an amide will look much like an amine. Assignment of the structure is largely a process of "putting together all the pieces." Click on any signal to highlight the proton responsible for it.
¹³C NMR Spectroscopy Load the ¹³C NMR spectrum The carboxyl carbon appears at a characteristic shift between 160-185 ppm just like carboxylic acids do. (Nitriles are the one exception; these sp-hybridized carbons come at 100-120 ppm.) It is often weak because of nOe effects on other carbons in a proton-decoupled spectrum. The rest of the molecule gives rise to the same kinds of peaks in the carbon spectrum that are seen in the ¹H NMR.