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The Basics of General, Organic, and Biological ChemistryBy David W. Ball, John W. Hill, and Rhonda J. Scott
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Opening Essay
Organic acids have been known for ages. Prehistoric people likely made acetic acid when their fermentation reactions went awry and produced vinegar instead of wine. The Sumerians (2900–1800 BCE) used vinegar as a condiment, a preservative, an antibiotic, và a detergent. Citric acid was discovered by an Islamic alchemist, Jabir Ibn Hayyan (also known as Geber), in the 8th century, & crystalline citric acid was first isolated from lemon juice in 1784 by the Swedish chemist Carl Wilhelm Scheele. Medieval scholars in Europe were aware that the crisp, tart flavor of citrus fruits is caused by citric acid. Naturalists of the 17th century knew that the sting of a red ant’s bite was due to an organic acid that the ant injected into the wound. The acetic acid of vinegar, the formic acid of red ants, & the citric acid of fruits all belong khổng lồ the same family of compounds—carboxylic acids. Soaps are salts of long-chain carboxylic acids. (For more information about soaps, see Chapter 7 "Lipids", Section 7.2 "Fats & Oils".)
Prehistoric people also knew about organic bases—by smell if not by name; amines are the organic bases produced when animal tissue decays.
The organic compounds that we consider in this chapter are organic acids and bases. We will also consider two derivatives of carboxylic acids: esters and amides. An ester is derived from a carboxylic acid & an alcohol. Fats và oils are esters, as are many important fragrances and flavors. (For more information about fats and oils, see Chapter 7 "Lipids", Section 7.2 "Fats & Oils".) An amide is derived from a carboxylic acid & either ammonia or an amine. Proteins, often called “the stuff of life,” are polyamides. (For more information about proteins, see Chapter 9 "Proteins, and Enzymes", Section 9.1 "Proteins".)
Learning Objectives
Identify the general structure for a carboxylic acid, an ester, an amine, and an amide.Identify the functional group for a carboxylic acid, an ester, an amine, & an amide.We introduced the carbonyl group (C=O)—the functional group of aldehydes và ketones—in Chapter 3 "Aldehydes, Ketones". The carbonyl group is also found in carboxylic acids, esters, and amides. However, in these compounds, the carbonyl group is only part of the functional group.
A carboxylic acid is an organic compound that has a carboxyl group. The carboxyl group is a functional group that contains a carbon–oxygen double bond and an OH group also attached khổng lồ the same carbon atom, but it has characteristic properties of its own. As with aldehydes và ketones, carboxylic acid formulas can be written khổng lồ show the carbon-to-oxygen double bond explicitly, or the carboxyl group can be written in condensed khung on one line. In general, carboxylic acids are represented by the formula RCOOH, where R is a hydrocarbon group.
Esters are represented by the formula RCOOR’, where R và R’ are hydrocarbon groups. The ester, which is organic compound derived from a carboxylic acid & an alcohol in which the OH of the acid is replaced by an OR group, looks somewhat lượt thích an ether và also somewhat lượt thích a carboxylic acid. Even so, compounds in this group react neither like carboxylic acids nor lượt thích ethers; they biến hóa a distinctive family. Unlike ethers, esters have a carbonyl group. Unlike carboxylic acids, esters have no acidic hydrogen atom; they have a hydrocarbon group in its place.
An amine is a compound derived from ammonia (NH3); it has one, two, or all three of the hydrogen atoms of NH3 replaced by an alkyl (or an aryl) group. Lượt thích NH3, amines are weak bases. The functional group of an amine is a nitrogen atom with a lone pair of electrons and with one, two, or three alkyl or aryl groups attached.
The amide functional group has a carbonyl group joined to a nitrogen atom from ammonia or an amine. The properties of the amide functional group differ from those of the simple carbonyl group, NH3, và amines.
Esters và amides are considered to be derivatives of carboxylic acids because the OH in the carboxyl group is replaced with another group. These functional groups are listed in Table 4.1 "Organic Acids, Bases, và Acid Derivatives", along with an example (identified by common & International Union of Pure & Applied Chemistry
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Note
Most familiar carboxylic acids have an even number of carbon atoms. As we shall see in Chapter 7 "Lipids", these acids—called fatty acids—are synthesized in nature by adding two carbon atoms at a time.