Brian C. Smith

Carboxylates are made by reacting carboxylic acids with strong bases such as inorganic hydroxides. The resultant salts are commonly used as cleansers, including in common bar soap-hence the pun in the title. Carboxylates contain two unique carbon–oxygen "bond and half" linkages that coordinate with a metal ion to give two strong infrared peaks, which make them easy to see.

	So far in our investigation of the infrared spectroscopy of the carbonyl functional group, we have looked at ketones, aldehydes, carboxylic acids, and acid anhydrides (1–4). Acid anhydrides are derivatives of carboxylic acids and are made by reacting two acid molecules and splitting off an atom of water (4). Carboxylates are another derivative of carboxylic acids, and they are made via a classic acid–base reaction of a carboxylic acid with a strong base such as a hydroxide. Such a reaction is shown in Figure 1.

	As in the synthesis of acid anhydrides, one of the driving forces for the reaction is the splitting-off of a molecule of water. Note in Figure 1 that the carboxylate functional group contains ionic bonding; there is a formal positive charge on the metal ion and a negative charge on the carboxylate. Also shown in Figure 1 is that the metal ion from the hydroxide coordinates with the two oxygens from the carboxylic acid to form an unusual bonding arrangement with two equivalent carbon–oxygen bonds. We will call this the "-CO

" group because it contains two carbon–oxygen bonds of the same bond order, similar to carbon dioxide (O=C=O).


	Figure 1: The reaction of a carboxylic acid with sodium hydroxide to give a carboxylate and water.

	Carboxylates are named after the acid and base used to make them. Thus, the reaction of acetic acid and sodium hydroxide would yield sodium acetate. A classic acid–base reaction, such as between NaOH and HCl, forms a salt and water. Carboxylates are sometimes referred to as 

 because a carboxylate is a salt formed by the reaction of a carboxylic acid with a base.

	Carboxylates are unusual in that they frequently have a polar end consisting of the carboxylate, and a nonpolar end such as an alkyl chain. This structure makes them uniquely capable of solubilizing oil and dirt into water, which is why they are commonly used as cleaning agents (explaining the bad pun in the title of this column). In fact, sodium stearate, the product of the reaction of sodium hydroxide and stearic acid, is also known as hand soap.

	The infrared spectrum of a carboxylate, zinc stearate, is shown in Figure 2. This molecule is made by reacting two equivalents of stearic acid with the dibasic zinc hydroxide, yielding a molecule with two carboxylate moieties.


	Figure 2: The infrared spectrum of zinc stearate (C

	Because carboxylates contain neither a C=O bond nor a C-O bond, they will not contain a carbonyl stretching peak around 1700 cm

 (1) (going forward assume all peak positions are in wavenumber units), or a C-O stretch between 1300 and 1000 (5). There are two intense peaks at 1538 and 1398 in Figure 2 that stick down out of the spectrum like canine teeth. These are from the asymmetric and symmetric stretches of the -CO

 group. Generally, these two peaks fall from 1650 to 1540 and 1450 to 1360, respectively. Unlike the C=O group, carboxylate stretches are insensitive as to whether saturated or unsaturated carbons are attached to them.

	The carbon–oxygen bond order in the -CO

 group is about one and half, and the infrared spectrum shown in Figure 2 will help prove this. A C=O stretch falls at about 1700 (1), and a C-O stretch typically falls at about 1200 (5). Theoretically then a C-O "bond and a half stretch" might fall at the average peak position of these two carbon–oxygen stretching vibrations. The average of 1700 and 1200 is 1450, close to where the carboxylate stretching peaks fall. This close agreement indicates that the carbon–oxygen bond order in carboxylates is approximately 1.5.

	Because carboxylates contain positive and negative charges, they are highly polar and have a large dipole moment. Recall from the first installment of this column that one of the characteristics that determine infrared peak positions is the change in dipole moment with respect to bond distance during a vibration, or dµ/d

 group stretch asymmetrically and symmetrically there is a large value of dµ/d

. This attribute is why the pair of carboxylate stretching peaks are so intense.

	In theory, one could identify the metal ion in a carboxylate from its mid-infrared spectrum. Recall, however, that as the mass of the atoms in a functional group goes up peak position goes down (7). Most metal atoms are heavier than the hydrogen, carbon, and oxygen atoms typically found in organic molecules. Thus, the metal–oxygen stretching and bending peaks that might help us determine the metal ion in a carboxylate fall below 400, out of range of most mid-infrared Fourier transform-infrared (FT-IR) spectrometers. Now, carboxylates with different metal atoms will have different infrared spectra because they have different chemical structures (7), but the inability of most FT-IRs to see metal–oxygen peaks makes it difficult to determine the metal ion from the infrared spectrum by itself. In this case, consultation of reference spectra, a library search, or a metals analysis of the sample using some form of atomic spectroscopy will be needed.

	In the chemical structure of zinc stearate shown in Figure 2 you can see that there are two very long alkyl chains, together whose CH

 ratio is 16. Recall that the relative intensity of the methylene asymmetric stretch compared to the methyl symmetric stretch is proportional to the CH

 ratio for a molecule (7). As can be seen in Figure 2, the CH

 asymmetric stretching peak at 2916 is significantly larger than the CH

 asymmetric stretching peak at 2959, as expected.

	Also note in Figure 2 that from 1400 down to 400 there are a number of weak to medium intensity peaks. These peaks include C-C stretching bands (8). Under normal circumstances these vibrations would have small values of dµ/d

 giving difficult to see peaks. Because of the highly polar nature of the carboxylate group, however, the alkyl chain itself is somewhat polarized, which increases dµ/d

 for its vibrations and yields the plethora of peaks seen. The diagnostic group wavenumbers for carboxylates are listed in Table I.

	Carboxylates are made by reacting a carboxylic acid with a strong base such as a hydroxide. The structure of the resultant functional group contains a negatively charged -CO

 group and a positively charged metal atom, and hence is highly polar. As a result, carboxylates have a large dipole moment and give two intense peaks, assigned as the asymmetric and symmetric -CO

 stretches at 1650–1540 and 1450–1360, respectively. The positions of these two peaks indicate that the bond order of -CO

Infrared Spectral Interpretation: A Systematic Approach

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Articles

Carboxylates are made by reacting carboxylic acids with strong bases such as inorganic hydroxides. Carboxylates contain two unique carbon–oxygen “bond and half” linkages that coordinate with a metal ion to give two strong infrared peaks, which make them easy to see.

The Carbonyl Group, Part V: Carboxylates—Coming Clean

There are a number of carboxylic acid derivatives that, like their parent functional group, are easy to detect by infrared (IR) spectroscopy. Acid anhydrides are one of these. These molecules are unique in that they have two carbonyl groups in them, and the intensity and position of their IR peaks can be used to determine which of the four types of anhydride exist in a sample. Other diagnostic peaks for acid anhydrides are also discussed.

	So far in our investigation of the infrared (IR) spectroscopy of the carbonyl or C=O functional group, we have looked at ketones, aldehydes, and carboxylic acids. One thing they had in common was a single C=O bond and hence a single C=O stretching peak. There exists a functional group called acid anhydrides that contain two carbonyl groups. They are made by reacting two carboxylic acid molecules to generate the anhydride and a molecule of water as shown in Figure 1.


	Figure 1: The reaction of two carboxylic acid molecules to give an acid anhydride and water.

	The "R" moieties represent carbon atoms. The term 

 means "no water" and is appropriate because of the water molecule that is split off during this reaction. Anhydrides are named after the carboxylic acids they are made from. For example, two molecules of acetic acid react to give acetic anhydride. Note that the anhydride functional group consists of two carbonyl groups connected by a single oxygen atom. As a result of this unique structure, the IR spectra of acid anhydrides contain not one but two C=O stretching peaks, and a C-O stretch. The anhydride group is somewhat reactive, so these molecules are often used as polymeric crosslinkers or as chemical intermediates.

	The nature of the R-groups attached to the anhydride framework determine the type of anhydride. If the two attached carbons are saturated so is the anhydride, whereas if one or both of the anhydrides is unsaturated then the anhydride is said to be unsaturated.

	Unlike some functional groups, it seems the most common anhydrides are cyclic. Examples of the structures of two common cyclic anhydrides, succinic anhydride and phthalic anhydride, are shown in Figure 2.


	Figure 2: The structures of succinic anhydride, a saturated cyclic anhydride, and phthalic anhydride, an unsaturated cyclic anhydride.

	In addition to whether an anhydride is saturated or unsaturated, whether its substituents are in a ring or not also matters. Thus, it takes two adjectives to describe an anhydride and there are four types: noncyclic saturated, noncyclic unsaturated, cyclic saturated, and cyclic unsaturated. Succinic anhydride is a saturated cyclic anhydride, since there are methylene groups attached to the carbonyls in the anhydride functionality. On the other hand, phthalic anhydride is a cyclic unsaturated anhydride because there are aromatic carbons attached to the carbonyls in the anhydride functionality. An example of a noncyclic saturated anhydride would be acetic anhydride, where the two R-groups attached to the anhydride carbonyls are CH

 groups. Fortunately, we can distinguish between the four distinct types of anhydrides using IR spectroscopy as we will see below.

The Infrared Spectroscopy of Acid Anhydrides

	The two carbonyl groups in acid anhydrides give rise to two carbonyl stretching peaks. The vibrations involved are a symmetric C=O stretch where the two carbonyl groups stretch in phase with each other, and an asymmetric C=O stretch where the two carbonyl groups stretch out of phase with each other as shown in Figure 3.


	Figure 3: The symmetric (left) and asymmetric (right) carbonyl stretches of the acid anhydride functional group.

	The IR spectrum of a noncyclic saturated anhydride, valeric anhydride, is shown in Figure 4.


	Figure 4: The IR spectrum of valeric anhydride (C

	The double carbonyl stretching peaks for valeric anhydride fall at 1819 and 1752 cm

 (going forward assume all peak positions are in cm

) and are labeled A and B in Figure 4. The first unusual thing about this pair of peaks is that they are at higher wavenumbers than the carbonyl stretches. The second unusual thing is that the symmetric stretch is at a higher wavenumber than the asymmetric stretch we have seen so far. In general, and for reasons too lengthy to go into here (2), for functional groups that have asymmetric and symmetric stretches the asymmetric stretch is typically higher in wavenumber. In general, for noncyclic saturated anhydrides the symmetric C=O stretch falls at 1820 ± 5 whereas the asymmetric stretch falls at 1750 ± 5. For noncyclic unsaturated acid anhydrides these peaks fall at 1775 ± 5 and 1720 ± 5, respectively. Note that based on their C=O stretching peak positions and the narrowness of the range where the peaks fall, it is easy to distinguish between saturated and unsaturated noncyclic anhydrides.

	Anhydrides of course have a C-O stretch, and based on previous columns (3,4) we expect this bond to have a stretching peak between 1300 and 1000. Valeric anhydride follows this rule with its most intense peak being a C-O stretch at 1038, labeled C in Figure 4. In general, for noncyclic anhydrides this peak falls from 1060 to 1035 and is in the same range whether the anhydride is saturated or unsaturated.

	Note in Figure 4 that the high-wavenumber C=O stretch is more intense than the low-wavenumber C=O stretch. This intensity ratio is typical of all noncyclic anhydrides and can be used to distinguish the noncyclic from cyclic forms. Table I summarizes the useful group wavenumbers for noncyclic acid anhydrides. In Table I, note that for both types of noncyclic anhydride the higher-wavenumber symmetric C=O is more intense.

	A double carbonyl peak with one or both features at a relatively high wavenumber for a C=O stretch is a strong indication of an anhydride being present in a sample. If the higher-wavenumber peak is stronger it is noncyclic. Then use the positions of the two C=O stretches and Table I to determine whether the noncyclic anhydride is saturated or unsaturated.

	The IR spectrum of an unsaturated cyclic anhydride, maleic anhydride, is shown in Figure 5. This anhydride is unsaturated because there is a C=C bond containing unsaturated carbons in the ring attached directly to the carbonyl carbons of the two C=O groups.

	The symmetric carbonyl stretching peak for maleic anhydride is at 1853, while the asymmetric stretch is at 1780. For unsaturated cyclic anhydrides these two peaks typically fall from 1860 to 1840 and 1780 to 1760, respectively. For saturated cyclic anhydrides, such as succinic anhydride whose structure is seen in Figure 2, these peaks fall at 1870–1845 and 1800–1775.

	Note in Figure 5 that the high-wavenumber symmetric C=O stretching peak is weaker than the lower-wavenumber asymmetric C=O stretching peak. This pattern is opposite that for noncyclic anhydrides as seen above. Thus, the peak intensity ratio of the two anhydride C=O stretching peaks can be used to determine whether an anhydride is noncyclic or cyclic. For noncyclic anhydrides the higher-wavenumber symmetric C=O stretch is more intense, while for cyclic anhydrides the lower-wavenumber asymmetric stretch is more intense. Again, a double carbonyl stretch with one or both peaks at a relatively high wavenumber is a strong indication of an anhydride. If the lower-wavenumber peak is more intense it is cyclic. Then use the position of the two C=O stretches and Table II to determine whether the anhydride is cyclic saturated or cyclic unsaturated.

	Like noncyclic anhydrides, cyclic anhydrides have a C-O bond and hence a C-O stretch. It falls anywhere from 1300 to 1000. In the spectrum of maleic anhydride in Figure 5 this peak falls at 1058 and is labeled C. Additionally, cyclic anhydrides have a strong C-C stretching peak from 960 to 880, in Figure 5 this peak falls at 894.


	Figure 5: The IR spectrum of maleic anhydride (C

	Table II lists the important group wavenumbers for cyclic acid anhydrides. Note that it states in Table II that for both types of cyclic anhydride the lower-wavenumber C=O stretch is more intense.

	In summary there are four kinds of acid anhydride: saturated noncyclic, unsaturated noncyclic, saturated cyclic, and unsaturated cyclic. A double carbonyl stretch, with one or more of the peaks at relatively high wavenumber for a C=O stretch is a strong indication of an acid anhydride. Next, look at the intensity ratio of the two carbonyl stretching peaks. If the higher-wavenumber peak is more intense the anhydride is noncyclic. If the lower-wavenumber peak is more intense the anhydride is cyclic. Once this is known, then use the peak positions of these peaks and either Table I (noncyclic) or Table II (cyclic) to determine whether the anhydride is saturated or unsaturated. In this way, you can distinguish the four types of acid anhydrides from each other using IR spectroscopy.

	Acid anhydrides are made by reacting two carboxylic acid molecules with each other. One of the products is water, hence the name anhydride. Acid anhydrides come in four varieties: saturated noncyclic, unsaturated noncyclic, saturated cyclic, and unsaturated cyclic. Their most significant spectral feature is a double carbonyl stretch because of the symmetric and asymmetric stretching of the two carbonyl groups, one of which sometimes occurs above 1800. For noncyclic anhydrides the higher-wavenumber C=O stretching peak is more intense. For cyclic anhydrides the lower-wavenumber C=O stretching peak is more intense. The positions of these two peaks can then be used to determine whether the anhydride is saturated or unsaturated.

Infrared Spectral Interpretation: A Systematic Approach 

Acid anhydrides are unique in that they have two carbonyl groups in them. The intensity and position of their IR peaks can be used to determine which of the four types of anhydride exist in a sample.

Brian C. Smith

Articles

The Carbonyl Group, Part V: Carboxylates—Coming Clean

The C=O Bond, Part IV: Acid Anhydrides