Spectroscopy of Carboxylic Acids

Carboxylic acids show two characteristic IR absorptions that make the I -COOH group easily identifiable. The 0-H bond of the carboxyl group gives rise to a very broad absorption over the range 2500-3300 cm S and the C=0 bond shows an absorption between 1710 cm and 1760 cm The exact position of C=0 absorption depends both on the structure of the molecule and on whether the acid is free (monomeric) or hydrogen-bonded (dimeric). Free carboxyl groups absorb at 1760 cm but the more commonly encountered dimeric carboxyl groups absorb in a broad band centered around 1710 cm  

Problem 20.12 Cyclopentanecarboxylic acid and 4-hydroxycyclohexanone have the same formu (CcHioOj), and both contain an -OH and a C=0 group. How could you disting between them by IB spectroscopy  

Carboxylic acid groups can be detected by both and C NMR spectroscopy. Carboxyl carbon atoms absorb in the range 165-185 d in the C NMR spectrum, with aromatic and a,0-unsaturated acids near the upfield end of the range (-165 6) and saturated aliphatic acids near the downfield end (—185 5). The acidic -COOH proton normally absorbs as a singlet near 12 8 in the H NMR spectrum. As with alcohols (Section 17.12), the -COOH proton can be replaced by deuterium when D-jO is added to the sample tube, causing the absorption to disappear from the NMR spectrum. 

How could you distinguish between cyclopentanecarboxylic acid and 4-hydroxy-cyclohcxanone by H and NMR spectroscopy (See Problem 20.12.) 

Vitamin C, or ascorbic acid, is surely the best known of all vitamins. It was the first vitamin to be discovered (1928), the first to be structurally characterized (1933), and the first to be synthesized in the laboratory (1933). Over 80 million pounds of vitamin C are now synthesized worldwide each year, more than the total amount of all other vitamins combined. In addition to its use as a vitamin supplement, vitamin C is used as a food preservative, a flour improver in bakeries, and an animal food additive. 

The most obvious feature in the infrared spectrum of a carboxylic acid is the intense carbonyl stretching absorptioa In a saturated acid, this vibration occurs around 1710 cm , often broadened by hydrogen bonding involving the carbonyl group. In conjugated acids, the carbonyl stretching frequency is lowered to about 1690 cm . 

Many blood-clotting proteins are carboxylated at specific locations to produce a modified protein that binds calcium, which is essential for the clotting process. In the absence of this carboxylation, blood clotting will not take place. 

Carboxylic acid protons are the most deshielded protons we have encountered, absorbing between SIO and S13. Depending on the solvent and the concentration, this acid proton peak may be sharp or broad, but it is always unsplit because of proton exchange. 

The carbon NMR chemical shifts of carboxylic acids resemble those of ketones and aldehydes. The carbonyl carbon atom absorbs around 180 ppm, and the -carbon atom absorbs around 30 to 40 ppm. The chemical shifts of the carbon atoms in hexa-noic acid are the following  

Saturated carboxylic acids have a weak transition that absorbs around 200 to 

---

Spectroscopy of Carboxylic Acids and Nitriles 770 Focus On. .. Vitamin C 772... 

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

Carboxylic acids and their derivatives are also an important part of Organic 11. We spend quite a few pages looking at the structure, nomenclature, synthesis, reactions, and spectroscopy of carboxylic acids. While on this topic in Chapter 12, we use a lot of acid-base chemistry, most of which you were exposed to in your introductory chemistry course. (For a quick review, look over a copy of Chemistry For Dummies or Chemistry Essentials For Dummies, both written by John T. Moore and published by Wiley.)... 

You can use the unique spectroscopy of carboxylic acids and derivatives, described in the following list, to help you identify those compounds. 

The earliest microwave work on hydrogen bonded complexes was the low resolution absorption spectroscopy of carboxylic acid dimers by Costain and Srivastava 10). 

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). 

Kubicki, J.D. et al., Attenuated total reflectance Fourier-transform infrared spectroscopy of carboxylic acids adsorbed onto mineral surfaces, Geochim. Cosmochim. Acta, 63, 2709, 1999. 

---