Chapter 18
Carboxylic Acid
Structure
Of the organic compounds that show appreciable acidity, by far the most important are the carboxylic acids. These compounds contain the carboxyl group
Nomenclature
The aliphatic carboxylic acids have been known for a long time, and as a result have common names that refer to their sources rather than to their chemical structures. The common names of the more important acids are shown in Table 18.1. Formic acid, for example, adds the sting to the bite of an ant (Latin: Formica, ant); butyric acid gives rancid butter its typical smell.
Generally, the parent acid is taken as the one of longest carbon chain, although some compounds are named as derivatives of acetic acid.
Aromatic acids, ArCOOH, are usually named as derivatives of the parent acid, benzole acid, C6H5COOH. The methylbenzoic acids are given the special name of toluic acids.
The IUPAC names follow the usual pattern. The longest chain carrying the carboxyl group is considered the parent structure, and is named by replacing the -e of the corresponding alkane with -oic acid. For example:
- The position of a substituent is indicated as usual by a number. We should notice
- C-C C-COOH Used in WPAC names
- that the carboxyl carbon is always considered as C-l, and hence C-2 corresponds to a of the common names, C-3 to j8, and so on. (Caution: Do not mix Greek letters with IUPAC names, or Arabic numerals with common names.)
Physical properties
- We can see from that as a class the carboxylic acids are even higher boiling than alcohols. For example, propionic acid (b.p. 141) boils more than twenty degrees higher than the alcohol of comparable molecular weight, w-butyl alcohol (b.p. 118). These very high boiling points are due to the fact that a pair of carboxylic acid molecules are held together not by one but by two hydrogen bonds:
- The odors of the lower aliphatic acids progress from the sharp, irritating odors df formic and acetic acids to the distinctly unpleasant odors of butyric valeric, and caproic acids; the higher acids have little odor because of their low volatility.
Salts of carboxylic acids
- Although much weaker than the strong mineral acids (sulfuric, hydrochloric, nitric), the carboxylic acids are tremendously more acidic than the very weak organic acids (alcohols, acetylene) we have so far studied; they are much stronger acids than water. Aqueous hydroxides therefore readily convert carboxylic acids into their salts; aqueous mineral acids readily convert the salts back into the carboxylic acids. Since we can do little with carboxylic acids without encountering. this conversion to and from their salts, it is worthwhile for us to examine the properties of these salt.
- Salts of carboxylic acid like all salts are crystalline non-volatile solids made up of positive and negative ions; their properties are what we would expect of such structures. The strong electrostatic forces holding the ions in the crystal lattice can be overcome only by heating to a high temperature, or by a very polar solvent. The temperature required for melting is so high that before it can be reached carbon-carbon bonds break and the molecule decomposes, generally in the neighborhood of 300-400. A decomposition point is seldom useful for the identification of a compound, since it usually reflects the rate of heating rather than the identity of the compound.
- The alkali metal salts of carboxylic acids (sodium, potassium, ammonium) are soluble in water but insoluble in non-polar solvents; most of the heavy metal salts (iron, silver, copper, etc.) are insoluble in water.
- Thus, we see that, except for the acids of four carbons or less, which are soluble both in water and in organic solvents, carboxylic acids and their alkali metal salts show exactly opposite solubility behavior. Because of the ready interconversion of acids and their salts, this difference in solubility behavior may be used in two important ways: for identification and for separation.
- A water-insoluble organic compound that dissolves in cold dilute aqueous sodium hydroxide must be either a carboxylic acid or one of the few other kinds of organic compounds more acidic than water; that it is indeed a carboxylic acid can then be shown in other.
- We can separate a carboxylic acid from non-acidic compounds by taking advantage of its solubility and their insolubility in aqueous base; once the separation has been accomplished, we can regenerate the acid by acidification of the aqueous solution. If we are dealing with solids, we simply stir the mixture with aqueous base and then filter the solution from insoluble, non-acidic materials; addition of mineral acid to the filtrate precipitates the carboxylic acid, which can be collected on a filter. If we are dealing with liquids, we shake the mixture with aqueous base in a separatory funnel and separate the aqueous layer from the insoluble organic layer; addition of acid to the aqueous layer again liberates the carboxylic acid, which can then be separated from the water. For completeness of separation and ease of handling, we often add a water-insoluble solvent like ether to the acidified mixture. The carboxylic acid is extracted from the water by the ether, in which it is more soluble; the volatile ether is readily removed by distillation from the comparatively high-boiling acid.
Industrial sour
Acetic acid, by far the most important of all carboxylic acids, is prepared by air oxidation of acetaldehyde, which is readily available from the hydration of acetylene.
- Large amounts of acetic acid are also produced as the dilute aqueous solution known as vinegar. Here, too, the acetic acid is prepared by air oxidation; the compound that is oxidized is ethyl alcohol, and the catalysts are bacterial (Acetobacter) enzymes.
- The most important sources of aliphatic carboxylic acids are the animal and vegetable fats. From fats there can be obtained, in purity of over 90%, straight-chain carboxylic acids of even carbon number ranging from six to eighteen carbon atoms. These acids can be converted into the corresponding alcohols (Sec. 18.18), which can then be used, in the ways we have already studied (Sec. 16.10), to make a great number of other compounds containing long, straightchain units.
- The most important of the aromatic carboxylic acids, benzoyl acid and the phthalic acids, are prepared on an industrial scale by a reaction we have already encountered: oxidation of alkylbenzenes (Sec. 12.10). The toluene and xylenes required are readily available from coal tar and, by catalytic reforming of aliphatic hydrocarbons (Sec. 12.4), from petroleum; another precursor of phthalic acid (the 0/7/70 isomer) is the aromatic hydrocarbon naphthalene, also found in coal tar. Cheap oxidizing agents like chlorine or even air (in the presence of catalysts) is used.
Preparation
The straight-chain aliphatic acids up to C6 , and those of even carbon number up to Cjg, are commercially available, as are the simple aromatic acids. Other carboxylic acids can be prepared by the methods outlined below.
All the methods listed are important; our choice is governed by the availability of starting materials.
Oxidation is the most direct and is generally used when possible, some lower aliphatic acids being made from the available alcohols and substituted aromatic acids from substituted toluene.
The Grignard synthesis and the nitrile synthesis have the special advantage of increasing the length of a carbon chain, and thus extending the range of available materials. In the aliphatic series both Grignard reagents and nitriles are prepared from halides, which in turn are usually prepared from alcohols. The syntheses thus amount to the preparation of acids from alcohols containing one less carbon atom.
Grignard synthesis
The Grignard synthesis of a carboxylic acid is carried out by bubbling gaseous CO2 into the ether solution of the Grignard reagent, or by pouring the Grignard reagent on crushed Dry Ice (solid CO2 ); in the latter method Dry Ice serves not only as reagent but also as cooling agent.
The Grignard reagent adds to the carbon-oxygen double bond just as in the reaction with aldehydes and ketones. The product is the magnesium salt of the carboxylic acid, from \vhich the free acid is liberated by treatment with mineral acid.
The Grignard reagent can be prepared from primary, secondary, tertiary, or aromatic halides; the method is limited only by the presence of other reactive groups in the molecule (Sec. 15.15). The following syntheses illustrate the application of this method:
Nitrile synthesis
Aliphatic nitriles are prepared by treatment of alkyl halides with sodium cyanide in a solvenj that will dissolve both reactants; in dimethyl sulfoxide, reaction occurs rapidly and exothermically at room temperature. The resulting nitrile is then hydrolyzed to the acid by boiling aqueous alkali or acid.
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