CHAPTER IX

ORGANIC ACIDS, ACID SALTS, AND ESTERS

Organic acids, either in free form, or partially neutralized with calcium, potassium, or sodium, forming acid salts, or combined with various alcohols in the form of esters, are widely distributed in plants. They occur in largest proportions in the fleshy tissues of fruits and vegetables, where they are largely responsible for the flavors which make these products attractive as food for men and animals. But organic acids and their salts are also found in the sap of all plants, and undoubtedly play an important and definite part in the vital processes of metabolism and growth.

CHEMICAL CONSTITUTION

All organic acids contain one (or more) of the characteristic O // acid group, --COOH, or --C, known as "carboxyl." This \ OH group is monovalent, and in the simplest organic acid, formic acid (H_{2}CO_{2}), it is attached to a single hydrogen atom, thus, H·COOH. In all other monobasic acids, it is attached to some other monovalent group, usually an alkyl radical, i.e., a radical derived from an alcohol and containing only carbon and hydrogen (as methyl, CH_{3}, ethyl, C_{2}H_{5}, butyl, C_{4}H_{9}, acryl, C_{2}H_{3}, etc.). Hence, the general formula for all monobasic organic acids is R·COOH, the R representing any monovalent radical. In the simplest dibasic acid, oxalic (H_{2}C_{2}O_{4}), two carboxyl groups are united to each other, thus, HOOC·COOH; but in the higher members of the series, the two characteristic acid groups are united through one or more --CH_{2}-- groups, or their oxy-derivatives (as HOOC·CH_{2}·COOH, malonic acid; HOOC·CH_{2}·CH_{2}·CH_{2}·COOH, glutaric acid; HOOC·CHOH·CH_{2}·COOH, malic acid, etc.). Polybasic acids, containing three or more carboxyl groups, linked together through one or more alkyl carbon atoms, are also possible, and a few typical ones (as

COOH | HOOC·CH_{2}·COH·CH_{2}·COOH, citric acid)

are found in fruits and other plant tissues.

The H atom of the COOH group may be replaced by metals, in exactly the same way as it is replaceable in inorganic acids, producing either neutral or acid salts, depending upon whether all or only a part of the acid H atoms are replaced by the basic element.

Thus, with sulfuric acid:

OH ONa / / SO_{2} (H_{2}SO_{4}) + NaOH = SO_{2} (NaH_{2}SO_{4}) + H_{2}O \ \ OH OH

Sulfuric Acid Acid sodium sulfate

OH ONa / / or, SO_{2} (H_{2}SO_{4}) + 2NaOH = SO_{2} (Na_{2}SO_{4}) + 2H_{2}O \ \ OH ONa

Sulfuric Acid Neutral sodium sulfate

Similarly, with oxalic acid:

COOH COOK | (H_{2}C_{2}O_{4}) + KOH = | + H_{2}O COOH COOH

Oxalic acid Acid potassium oxalate

or, COOH COOK | (H_{2}C_{2}O_{4}) + 2KOH = | + 2H_{2}O COOH COOK

Oxalic acid Neutral potassium oxalate

Similarly, the acid H atom of either an organic or an inorganic acid may be replaced by the alkyl group of an alcohol, producing "ethereal salts," or "esters."

Thus, with nitric acid;

NO_{2}OH(HNO_{3})+C_{2}H_{5}OH = NO_{2}OC_{2}H_5(C_{2}H_{5}NO_{3})+H_{2}O

Nitric acid Ethyl alcohol Ethyl nitrate

And, with acetic acid;

CH_{3}·COOH(H_{4}C_{2}O_{2})+C_{2}H_{5}OH = CH_{3}·COOC_{2}H_{5}+H_{2}O

Acetic acid Ethyl acetate

With dibasic or polybasic acids, either one or more of the carboxyl H atoms may be replaced with an alcohol radical, so that both acid and neutral esters of all such acids are possible. Examples of all of these different types of derivatives of organic acids are frequently found in plant tissues.

The occurrence, properties, and functions of a particular type of glycerol, and other esters of organic acids, which are known as fats and waxes, are not taken into consideration in the following discussions, but reserved for a subsequent chapter dealing specially with them.

SOME COMMON ORGANIC ACIDS

Free organic acids, or their mineral salts or volatile esters, sometimes occur as separate and characteristic individual compounds in particular species of plants, or fruits; but much more commonly, two, three, or even more acids or their derivatives, are associated together.

=Formic acid=, H·COOH (H_{2}CO_{2}), occurs in free form and in considerable proportions in the leaves of several species of nettle, where it is responsible for the unpleasant effects of the "sting." It may be detected in small amounts in the vegetative parts of many, if not all, plants, especially during periods of rapid growth, and is probably one of the intermediate products in the photosynthesis of carbohydrates (see