Carboxylic Acids

FIELDS OF STUDY: Organic Chemistry

ABSTRACT

The characteristic properties and reactions of carboxylic acids are discussed. Carboxylic acids are very useful in organic synthesis reactions and are also essential components of the biochemistry of living systems.

The Nature of the Carboxylic Acids

According to Brønsted-Lowry acid-base theory, developed separately in 1923 by chemists Johannes Nicolaus Brønsted and Martin Lowry, the term "acid" indicates that a compound is capable of giving up one or more hydrogen ions (H⁺), also sometimes known as "protons." The carboxylic acids satisfy this criterion. Whereas ionic acids dissociate completely into positive and negative ions when put into water, carboxylic acids typically do not. When acetic acid (CH₃COOH), for example, is dissolved in water, a small percentage of the acetic acid molecules are dissociated into ions. The vast majority of the material, however, exists in solution as dissolved, but undissociated molecules of acetic acid.

The carboxylic acids are characterized by the presence of the carboxyl, or carboxylic acid, functional group, often represented as −COOH attached to an R− or Ar− placeholder. In this notation, R− and Ar− indicate an alkyl group (a hydrocarbon chain with the general formula C_nH_2n+1) or an aryl group (derived from an aromatic ring), respectively. The carboxyl group comprises a carbonyl group (C=O) that is bonded to a hydroxyl group (−OH).

Since the carboxyl function is a substituent functional group, or side chain, rather than a structural component in the molecular structure, the number of compounds that can be classed as carboxylic acids is effectively infinite.

The acidity of carboxylic acids is the result of the bonding between the hydroxyl group and the carbon atom of the carbonyl group. An atomic nucleus is composed of positively charge protons and neutral neutrons and is in turn surrounded by negatively charged electrons. The numbers of protons and electrons are equal in neutral atoms, while atoms with unequal electrons and protons are known as "ions." Electrons can be thought of as existing within specific regions about the nucleus known as "orbitals"; the location of electrons within the orbitals controls how an atom forms bonds and undergoes chemical reactions. In carboxylic acids, the oxygen atom exerts an electron-withdrawing effect through the pi (π) bond (one of two bonds that make up the double bond) with the carbon atom. This effect induces the hydroxyl oxygen atom to draw in the electrons from the O–H bond and hybridize from its normal sp³ orbital configuration to the sp² and p orbital configuration. The p orbital forms an extended pi-system with the carbonyl group, and the extra electron becomes delocalized, or detached from a specific atom or covalent bond. Through delocalization, both of the oxygen atoms become equivalent, and the C–O bond lengths are equalized. The addition of a proton, or hydrogen ion, to an atom or molecule occurs naturally as the reverse step in the equilibrium process

R−COOH ⇋ R−COO⁻ + H⁺

and can revert either one of the oxygen atoms to the hydroxyl group. Over time, the proportion of carbonyl oxygen and hydroxyl oxygen becomes equal. In addition to the interaction of the atoms’ orbitals, some of the properties of carboxylic acids are due to those compounds’ ability to form hydrogen bonds.

The linear carboxylic acids are essential components of both plant and animal biochemistry. Apart from its popular use as vinegar, acetic acid is the basic material for a vast number of hydrocarbon compounds called "acetogenins," which are produced by plants. It is also the source of the acetyl group of acetyl coenzyme A, which is essential in different stages of respiration and in other biochemical processes. Higher carboxylic acids and dicarboxylic acids such as succinic acid are produced during the breakdown of glucose in cellular respiration and glycolysis. The long-chain fatty acids are the essential component that form the phospholipid bilayer of animal cells. The amino acids, a special class of carboxylic acids, are the material from which all proteins are made, as specified by the genetic code carried by the deoxyribonucleic acid (DNA) molecule.

Nomenclature of Carboxylic Acids

There are common names for many of the simpler carboxylic acids, often based on the source of the compound, which continue to be used despite the development of a standardized system of chemical nomenclature. Methanoic acid (HCOOH) is responsible for the characteristic odor of ants; the common name, formic acid, derives from the Latin word for "ant," formica. Next in the series is ethanoic, or acetic, acid (H₃CCOOH), which is responsible for the taste and smell of vinegar. The common name comes from the Latin word for "sour," as does the word "acid." Third in the series is propanoic (propionic) acid, and the fourth is butanoic (butyric) acid. Butyric acid is responsible for the taste and smell of rancid butter, and the name derives from the Latin word butyrum, meaning "butter." Pentanoic acid (C₅H₁₀O₂) is also known as "valeric acid," and hexanoic acid (C₆H₁₂O₂), is known as "caproic acid" (from the Latin word caper, meaning "goat"). Octanoic acid (C₈H₁₆O₂) and decanoic acid (C₁₀H₂₀O₂) were also identified from soured goat’s milk and are known as "caprylic acid" and "capric acid," respectively. Other carboxylic acids were isolated from various vegetable oils and have common names that reflect that heritage, including lauric acid (from laurel oil), myristic acid (nutmeg), palmitic acid (palm), stearic acid (beef fat, or stearin), and oleic, linoleic, and linolenic acids (linseed).

The systematic International Union of Pure and Applied Chemistry (IUPAC) names of linear carboxylic acids follow the basic hydrocarbon series, beginning with methanoic and progressing through ethanoic, propanoic, and butanoic. The series continues with the numeric names pentanoic, hexanoic, heptanoic, and so on. The name is generated by identifying the longest carbon atom chain in the molecule and adding the suffix -oic acid. The carboxyl carbon atom is assigned the first position in the chain. For example, a seven-carbon chain with a C≡C triple bond at the third carbon atom in the chain would thus be named hept-3-ynoic acid. When different side chains are present on a hydrocarbon molecule, they are assigned by the priority of their size to attain the lowest position numbers and are named alphabetically. If a methyl group was on the fifth carbon atom in the seven-carbon chain, the compound would be named systematically as 5-methylhept-3-ynoic acid. In an alkene (an unsaturated hydrocarbon with one or more C=C bonds) and alkene-derived compounds, the relative positions of side chains are indicated in the formal names using the identifiers cis- for side chains on the same side of the double bond and trans- when the side chains are on opposite sides of the double bond.

A different convention is often used to identify the position of substituents on the chain, using the letters of the Greek alphabet as place markers relative to the carboxyl carbon atom. The adjacent position is assigned as α- (alpha), the next as β- (beta), then γ- (gamma), and so on. The omega fatty acids take their name from this practice, as ω (omega) is the last letter of the Greek alphabet. An omega-3 fatty acid is one that has a C=C bond beginning at the third carbon atom from the ω end of the molecule.

Aryl carboxylic acids are typically named as derivatives of benzoic acid or the corresponding IUPAC name of the base aryl group. Compounds bearing two carboxyl functions are called "dicarboxylic acids." One of the simplest of these is oxalic acid, found in rhubarb and other plants. Tricarboxylic acids are also known, though they are much less common. When it must be named as a substituent of another compound that is not named as a carboxylic acid, the carboxyl functional group is identified as the hydroxycarbonyl substituent.

Formation of Carboxylic Acids

Carboxylic acids are typically formed by oxidation of the corresponding alcohol—that is, the alcohol molecule loses one or more electrons. Ethanol, for example, oxidizes to form first acetaldehyde and then acetic acid. Oxidation of the aldehyde (−RCOH) to the carboxylic acid often requires only exposure to air. It is not unusual for bottles of the reagent benzaldehyde, for example, to slowly become bottles of benzoic acid over time, once they have been opened. The process is enhanced and controlled by the use of specific oxidizing agents, eliminating the wait time associated with air oxidation. Other methods of producing acetic acid in bulk quantities include catalytic oxidation of hydrocarbons and the reaction of methanol and carbon monoxide catalyzed by rhodium and iodine. Benzoic acid is typically produced from toluene obtained from the catalytic reforming of petroleum. Similarly, phthalic acid is produced from xylene or naphthalene. Essentially all other carboxylic acids, except those that can be obtained in quantity by hydrolysis of natural fats and oils, are synthesized by elaborating these basic structures through various synthesis reactions.

Reactions of Carboxylic Acids

Carboxylic acids form salts and can be titrated or neutralized by a basic compound. When combined, an acid and its corresponding salt dissolved in water form a buffer solution that will maintain a constant pH—a numerical value that represents the acidity or basicity of a solution—even after the addition of limited quantities of an acid or a base. This is a very important feature in biological systems and in laboratory methods.

Carboxylic acids are converted easily to the corresponding acid chloride by reaction with thionyl chloride (SOCl₂), phosphorus trichloride (PCl₃), or pentachloride (PCl₅). The acid chloride readily reacts with various nucleophiles (electron-rich chemical species) to produce the corresponding derivatives. Reaction with ammonia (NH₃), primary amines (RNH₂), or secondary amines (R₂NH) produces the corresponding amides (RCONH₂). Reaction with an alkoxide or aryloxide anion (RO⁻ or ArO⁻) produces the corresponding ester (RCOOR'). Acid chlorides can also be used to elaborate an aryl compound in the Friedel-Crafts acylation reaction, which results in the addition of substituents to an aromatic ring. Aryl carboxylic acids, such as benzoic acid, also undergo Friedel-Crafts alkylation and acylation reactions, although less actively than other compounds.

PRINCIPAL TERMS

• Brønsted-Lowry acid-base theory: definitions for acids and bases developed separately in 1923 by Danish chemist Johannes Nicolaus Brønsted and English chemist Martin Lowry; defines an acid as any compound that can release a hydrogen ion and a base as any compound that can accept a hydrogen ion.
• carbonyl group: a functional group consisting of a carbon atom double bonded to an oxygen atom.
• functional group: a specific group of atoms with a characteristic structure and corresponding chemical behavior within a molecule.
• hydrogen bond: a weak type of chemical bond formed by the attraction of a hydrogen atom to an electronegative atom—an atom with a strong tendency to attract electrons—in the same or another molecule.
• hydroxyl group: a primary functional group consisting of an oxygen atom covalently bonded to a single hydrogen atom.

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