Organic Acids

Type of physical science: Chemistry

Field of study: Chemical compounds

Organic acids include mainly carboxylic acids, as well as phosphonic and sulfonic acids. They are compounds that undergo neutralization with bases and reduction to the corresponding alcohol. They have a great variety of derivatives that include esters, amides, anhydrides, and acylhalides.

Overview

Organic acids are a large family of compounds that includes, primarily, carboxylic acids, as well as phosphonic and sulfonic acids. The term "acid" is also occasionally used with compounds that do not belong in any of the above categories, yet have properties characteristic of an organic acid. Such an example is picric acid, which, strictly speaking, is a phenol of significant acidic properties.

Carboxylic acids, RCOOH (R = organic radical), are the end product of chemical oxidation in organic systems. They are found in nature as free acids or as various acyl derivatives, RCOX, such as esters (X = OR) or amides (X = NH₂, NHR, or NR₂). They are characterized by the carboxyl group, -COOH.

In the International Union of Pure and Applied Chemistry (IUPAC) system, carboxylic acids are named by replacing the -e of the longest hydrocarbon chain containing -COOH with -oic acid. If substituents are present, the chain is numbered beginning at the -COOH end. For example:

Aromatic acids are named as derivatives of the parent benzoic acid, while acids in which the carboxylic group is attached to a saturated ring are named as illustrated below:

Many carboxylic acids have been known for a long time, some since antiquity, primarily because of their characteristic odors. Several common names are derived from early Latin names identifying the source of these pungent liquids. Thus, HCOOH is always called formic acid (Latin formica = "ant") and never methanoic acid. Similarly, CH₃COOH is called acetic acid (Latin acetum = "vinegar"), CH₃CH₂CH₂COOH is butyric acid (Latin butyrum = "butter") and CH₃CH₂CH₂CH₂CH₂COOH is caproic acid (Latin caper = "goat").

Substituents according to common names are located by the use of Greek letters. Thus; 4-methylpentanoic acid will be called -methyl valeric acid ("valeric" from the plant Valeriana officinallis).

Many continuous-chain carboxylic acids were first isolated from fats and are called fatty acids. Propionic acid, for example (CH₃CH₂COOH), literally means "first fatty acid" (Greek protos = "first," pion = "fat"). The higher members of the series are nonvolatile, low-melting, waxy solids. Fatty acids can be saturated (no double bonds present, as in lauric, myristic, palmitic, and stearic acids) or unsaturated (one or more double bonds present, as in oleic, linoleic, and linolenic acids).

Dicarboxylic acids, which are compounds that have two carboxyl groups, are almost always referred to by their common names. Thus, HOOCCOOH is known as oxalic (and not ethandioic) acid, and HOOC(CH₂)₃COOH as glutaric (and not pentandioic) acid.

The physical and chemical properties of carboxylic acids vary according to the nature of the chemical groups present in the molecule. Thus, the acids with lower molecular weights are pungent, corrosive, water-soluble liquids with unusually high boiling points, as a result of intermolecular hydrogen bonding. As the molecular weight increases, the contribution of the hydrocarbon (R-) group becomes more significant; as a result, the sharpness of odor diminishes, water solubility decreases, but the boiling point continues to increase (although not as drastically).

Like alcohols, carboxylic acids undergo intermolecular hydrogen bonding, and are normally found as dimers (that is, hydrogen-bonded carboxylic acid pairs). As a result, both their melting and boiling points are higher than expected, and aromatic and dicarboxylic acids are solids at room temperature. Because of their long, nonpolar hydrocarbon tail and polar carboxyl group, fatty acids "spread out" to form a monomolecular layer when in contact with water.

Carboxylic acids are weak acids in comparison with other strong acids (hydrochloric, nitric, or sulfuric), but still one of the most acidic organic families. As such, they ionize in water to produce positively charged hydrogen ions (protons) and the negative carboxylate anion.

RCOOH (carboxylic acid) ←→ H⁺ (proton) + RCOO^- (carboxylate anion) The relatively large degree of ionization is attributed to the electronegative carbonyl group, which pulls the bonding electron pair between oxygen and hydrogen away from hydrogen. The presence of highly electronegative groups in the R- part of the acid has the tendency to increase the acidity of the carboxylic acid. Thus, the ionization constant, K sub a (defined as the product of the concentrations of the ionized species over the concentration of the unionized acid), or trichloroacetic acid, Cl₃CCOOH, is about ten thousand times larger than that of acetic acid, CH₃COOH.

Another characteristic reaction of carboxylic acids is their neutralization by a base to produce salts and water. Such bases include inorganic hydroxides (for example, sodium hydroxide, NaOH; and potassium hydroxide, KOH), as well as weaker bases, such as amines (RNH2) and sodium bicarbonate (NaHCO₃). Reaction with the latter releases carbon dioxide, whose frothing helps distinguish organic acids from neutral or basic organic compounds.

Carboxylic acids also react with metals to produce salts and hydrogen gas.

The salt (especially sodium or potassium) of a long-chain fatty acid is an example of soap, and is characterized by its two distinct parts: the ionic head (COO-) and the long, nonpolar hydrocarbon tail (R-). The charged head is hydrophilic (water-loving), and the nonpolar tail is hydrophobic (water-hating). In aqueous solutions, the dispersed soap molecules arrange themselves in spherical clusters called micelles, which have the hydrophobic ends aggregating to create an oily, nonpolar environment protected from water at the center of the cluster. The ionic heads are in contact with the water molecules. When water is hard--in other words, when it contains large concentrations of ions from calcium, magnesium, or ferric, compounds--soaps are useless, since the carboxylate ions will react with the positive ions to form insoluble salts, thereby losing their emulsifying properties.

The carboxylic acid functional group can be prepared from the corresponding primary alcohol (RCH₂OH) or aldehyde (RCHO) using an oxidizing agent, such as sodium (or potassium) dichromate, and an acid or potassium permanganate as the catalyst. Both the alcohol and the aldehyde should have the same carbon skeleton as the desired acid.

Another method involves the hydrolysis of a carboxylic acid derivative, such as anhydride, (RCO)₂O; an ester, RCOOR'; an amide, RCONH₂; or a nitrile, RC ≡ N. The reaction, especially when using an anhydride or ester, is catalyzed by high temperature and base (or acid) medium.

Apart from the reaction with bases, carboxylic acids also react with alcohols in acid medium to produce esters. The reaction has to be performed under anhydrous conditions to maximize yield, since one of the products is water. The reaction, also known as esterification, leads to much better yields when an acid anhydride (instead of a carboxylic acid) is reacted with an alcohol. Amides are not produced when an acid is reacted with ammonia or an amine, since this involves an acid-base reaction, which leads to a salt. When high temperatures are involved, however, the initially formed salt is dehydrated to produce the amide.

Carboxylic acids are reduced to the corresponding primary alcohols via reduction with lithium aluminum hydride, LiAlH₄. Higher yields are obtained with diborane, B₂H₆, in diglyme, which serves as solvent, but the method of preference involves the use of esters or even amides (instead of the carboxylic acid).

Sulfonic acids (RSO₃H) are at least as acidic as carboxylic acids. The most important ones are aromatic, that is, R- is a benzene or substituted benzene ring, and they are prepared by reaction of the aromatic ring with sulfur trioxide in concentrated sulfuric acid.

Phosphorus-containing organic acids include phosphonic (RPO₃H) and phosphinous (R₂POH) acids. Like sulfonic acids, they undergo all reactions carboxylic acids undergo. The reaction of nitric acid, for example, with glycerol produces nitroglycerin, which is an example of a nitrate ester.

Applications

Carboxylic acids have been encountered in human life since antiquity. By far the most important of these is acetic acid, which is used as a reactant and as a solvent both in the laboratory and in industry. It can be made by the aerobic fermentation of a mixture of cider and honey, which yields a 4 to 10 percent solution of acetic acid, together with a series of other compounds that give vinegar its flavor. Industrially, it is synthesized by air oxidation of acetaldehyde, CH₃CHO, produced from the hydration of acetylene. Acetic acid is marketed as glacial (approximately 99.5 percent), so called because on cold days it freezes to an icelike solid (freezing point 16.7 degrees Celsius). It is also the product of the hydrolysis of several compounds in the human body, such as acetylcholine, one of the neurotransmitters in the autonomic nervous system. Once acetylcholine has worked, it has to be deactivated, and in the synapse it hydrolyzes via the enzyme choline acetyltransferase into choline and acetic acid.

Chloroacetic acid is an important acid commercially and is prepared from a reaction mixture of acetic acid, acetic anhydride, sulfuric acid, and chlorine. It is used in the manufacture of 2,4,5-T, which gained public attention as a defoliating agent in Vietnam.

Formic acid is another very useful compound. It was first obtained by the destructive distillation of ants. When an ant, bee, or wasp bites or stings, it injects formic acid, among other poisonous materials, to create the allergic reaction. Formic acid is produced industrially from carbon monoxide (CO) and sodium hydroxide (NaOH), under the influence of both heat and pressure, followed by acid hydrolysis of the crude sodium formate. It is often used in reactions as a reducing agent.

Butyric acid, one of the most foul-smelling substances, is isolated from butterfat after hydrolysis and is responsible for the odor of rancid butter. Nevertheless, its esters find great use in flavors and perfumes. For example, methyl butyrate, made from butyric acid and methanol, is used in artificial rum and fruit flavors. Pineapple oil is a solution of ethyl butyrate in alcohol. It is also a main ingredient in body odor. It is this property that gives dogs, especially bloodhounds, the ability to differentiate one person from another. Low-molecular-weight carboxylic acids are products of a person's metabolism and are always present in trace amounts on the skin. Since each person's metabolism is a little bit different, the composition of the fatty acid on the skin will be different. This enables the bloodhound to distinguish among people, since it is capable of detecting the approximate composition of the mixture of low-molecular-weight carboxylic acids on the skin.

Aromatic carboxylic acids are also significant. Benzoic acid, which is found in berries, is used in antifungal ointments. Salicyclic acid, isolated for the first time in 1860 from willow bark, is a good analgesic (pain reliever) and antipyretic (fever-reducing agent) but is sour when taken orally. Gradual modifications to decrease the acidity led first to the sodium salt, then to the phenyl ester, and finally to the present form of acetyl salicylate that the German Bayer Company assigned the trade name "aspirin" in 1899.

Saturated fatty acids are widely found in the animal and vegetable kingdoms as the triesters of glycerol. Thus, the triglyceride of lauric acid is in coconut oil, that of myristic acid in nutmeg butter and milk fat, that of palmitic acid in palm oil, and that of stearic acid in animal fat.

The triglyercides of unsaturated fatty acids are encountered only in vegetables: those of linoleic and linolenic acids in linseed oil, and that of oleic acid in olive oil.

Dicarboxylic acids also find great application in industry. A very important starting material in the synthesis of plasticizers is phthalic acid, 1,2- C₆H₄(COOH)₂, which can be synthesized by oxidation of naphthalene by air oxidation at elevated temperature, in the presence of vanadium pentoxide (V₂O₅). The derivatives are, generally, esters, such as dibutyl phthalate (DBP), dimethoxyethylphthalate (DMEP), and dioctyl phthalate (DOP). Those plasticizers appear to have low toxicity and have in many instances replaced polyvinyl chloride (PVC). Terephthalic acid, 1,4- C₆H₄(COOH)₂, another dicarboxylic acid, is used as the starting material (together with ethylene glycol) to produce Dacron. Adipic acid, HOOC(CH₂)₄COOH, is one of the starting materials (the other one being 1,6-hexanediamine) for Nylon 66, the most common polyamide. Finally, oxalic acid, HOOCCOOH, found in fruits and vegetables, is used for bleaching wood and removing rust stains.

Phosphoric acid, by itself, is used in trace amounts in soda beverages. Its wider use is in the form of salts, known as phosphates. It is of fundamental significance to plant growth, from which it is absorbed as the dihydrogen phosphate ion, H₂PO₄^-, an anion, and incorporated into DNA and RNA. It is also a constituent of compounds essential to the conversion of starches to sugars and plays a significant role in photosynthesis, mainly by involvement in energy-transfer processes.

Soaps and detergents are salts of organic acids. Soaps are the sodium or potassium salts of long-chain fatty acids, while detergents are salts of phosphonic or sulfonic acids. One of the most popular detergents is sodiumdodecylbenzene sulfonate, 4-CH₃(CH₂)₁₁C₆H₄ SO₃^-Na⁺. Detergent molecules are like soap molecules because they have an ionic or polar head and a nonpolar tail. They, too, act as emulsifying agents for oils and greases but do not form precipitates with ions present in hard water, so clothes that are washed with detergents are not covered with scum that is formed by the precipitates of the soap with the cations in hard water.

Context

Organic acids, and especially carboxylic acids, play a great role in human life. Apart from the fact that many of the acids and derivatives are found in nature, they also play an important role in animal and plant metabolism. Acetic acid, for example, is extremely important in many biosynthetic pathways, such as fatty acid synthesis and the mevalonic acid synthesis, and is the end product of fermentation. It is estimated that in mid-1970's, more than 2 billion pounds of acetic acid were used annually in the United States alone.

The late twentieth century human has been completely dependent on organic acids.

Fatty acids find a wide applicability in the manufacture of soaps and detergents and in the thickening of lubricating grease. Carboxylic acids are also used in compounding natural and synthetic rubber, in buffing bricks and abrasives, and in the modification of plastics' rigidity.

They are important in the manufacture of popular products that range from crayons and inks to carbon paper and phonograph records. As solvents, they are often used in carrying out reactions that require high-temperature conditions. They are also significant as the precursors of important compounds such as aromatic hydrocarbons, polyesters, polyamides, and other plasticizers.

Finally, carboxylic acids are very important in the synthesis of pharmaceuticals such as pain relievers. The first successful synthetic pain relievers were derivatives of salicylic acid, which, in the present derivative form of aspirin, has an estimated consumption of twenty billion tablets annually. It was Edward Stone, an English clergyman, who first reported, in 1763, that a willow bark extract (almost a century later found to contain salicylic acid) had fever-reducing properties.

Phosphates as fertilizers in agriculture--in the form of bone, bird droppings, and fish remains--have found application since ancient times. Their role was first recognized in the beginning of the nineteenth century, and a large number of the deposits created by ancient fish and animals in ores have been depleted, or will be exhausted by the end of the twentieth century.

Organic phosphates also play important roles in biological systems such as DNA. It is believed that the phosphate ion can serve as a binding site by which the coenzymes attach to the enzymes.

Finally, some carboxylic acids have been of great importance to plant growth and humans. Plants synthesize five types of regulating hormones, three of which are carboxylic acids: indole-3-acetic acid (auxin), abscisic acid (ABA), and gibberellic acid. Auxin, for example, promotes cell enlargement of shoots but not roots, in a process called auxin activity. Synthetic compounds that have the same activity, for example, 2,4-dichlorophenoxyacetic acid, are used for this purpose as herbicides. Prostaglandins, compounds that stimulate contraction of smooth muscle such as that found in the uterus, are also carboxylic acids and are biosynthetically formed from smaller-size carboxylic acids.

Principal terms

ESTERIFICATION: a chemical reaction in which an alcohol reacts with an acid to produce an ester

FATTY ACID: a long-chain carboxylic acid produced by the hydrolysis of animal fats or vegetable oils

HYDROGEN BOND: the force of attraction between the partial positive charge on a hydrogen bonded by a covalent bond to oxygen, nitrogen, fluorine, or sulfur, and the partial negative charge on a nearby oxygen, nitrogen, fluorine, or sulfur

HYDROLYSIS REACTION: reaction that involves the splitting of a covalent bond by water

HYDROPHILIC GROUP: a structural unit in a molecule that can attract water molecules, for example, -OH, -NH2, -NH3+, or -COO–

HYDROPHOBIC GROUP: a structural unit that lacks favorable interactions with water, for example alkyl groups

NEUTRALIZATION REACTION: the reaction of an acid and a base to produce a salt and water

Bibliography

Fessenden, Ralph J., and Joan S. Fessenden. ORGANIC CHEMISTRY. 4th ed. Pacific Grove, Calif.: Brooks/Cole, 1990. A text for the undergraduate organic chemistry course that discusses carboxylic acids (chapter 14) with mechanistic details.

Hill, John W. CHEMISTRY FOR CHANGING TIMES. 4th ed. New York: Macmillan, 1984. A textbook written for the layperson, with excellent sections on the "Odorous World of Organic Acids" and "Pain Relievers: Structure and Properties." The latter discusses salicyclic acid and its derivatives.

Holum, John R. ELEMENTS OF GENERAL AND BIOLOGICAL CHEMISTRY. 6th ed. New York: Wiley, 1983. An introductory text for health sciences students, with a discussion on carboxylic acids and their derivatives (chapter 12). It also covers several applications under "special topics," such as "Some Important Carboxylic Acids and Salts" (12.1) and "Soap and Detergents" (14.2).

Loudon, G. Marc. ORGANIC CHEMISTRY. Reading, Mass.: Addison-Wesley, 1984. An undergraduate text in organic chemistry that discusses, mechanistically, the different organic families. Chapter 23 covers carboxylic acids.

McMurry, John. FUNDAMENTALS OF ORGANIC CHEMISTRY. Pacific Grove, Calif.: Brooks/Cole, 1990. An introductory text of organic chemistry for the health science student. Chapter 10 discusses carboxylic acids and derivatives.

Matta, Michael S., and A. C. Wilbraham. GENERAL, ORGANIC, AND BIOLOGICAL CHEMISTRY. 2d ed. Menlo Park, Calif.: Benjamin/Cummings, 1986. An excellent introductory textbook, with several sections titled "Closer Look" in which applications of phenomena and laws in everyday events are highlighted. Chapter 14 covers acids and their derivatives. Section 14-1 discusses hard water and water softening.

Solomons, T. W. Graham. FUNDAMENTALS OF ORGANIC CHEMISTRY. 3d ed. New York: Wiley, 1990. Another introductory text for organic chemistry, with a chapter covering carboxylic acids and derivatives.

Stacy, Gardner W., and C. C. Wamser. ORGANIC CHEMISTRY: A BACKGROUND FOR THE LIFE SCIENCES. Dubuque, Iowa: Kendall/Hunt, 1985. A simple and easy-to-read text of basic organic chemistry for the health science courses. Chapter 15 covers carboxylic acids and related substances.

Weininger, Stephen J., and F. R. Stermitz. ORGANIC CHEMISTRY. Orlando, Fla.: Academic Press, 1984. An organic chemistry text, with a section (chapter 16) on carboxylic acids, acid halides and anhydrides, and boxes discussing prostaglandin and essential fatty acids (Box 16-1) and carboxylic acids that affect plant growth (Box 16-2).

The chain is numbered beginning at the -COOH end

Acids in which carboxylic group is attached to a saturated ring

4-methylpentanoic acid will be called -methyl valeric acid

Acids and Bases

Soaps and Detergents