CHEMISTRY THE CENTRAL SCIENCE
24 THE CHEMISTRY OF LIFE: ORGANIC AND BIOLOGICAL CHEMISTRY
24.4 ORGANIC FUNCTIONAL GROUPS
The C═C double bonds of alkenes and C≡C triple bonds of alkynes are just two of many functional groups in organic molecules. As noted earlier, these functional groups each undergo characteristic reactions, and the same is true of all other functional groups. Each kind of functional group often undergoes the same kinds of reactions in every molecule, regardless of the size and complexity of the molecule. Thus, the chemistry of an organic molecule is largely determined by the functional groups it contains.
TABLE 24.6 lists the most common functional groups. Notice that, except for C═C and C≡C, they all contain either O, N, or a halogen atom, X.
We can think of organic molecules as being composed of functional groups bonded to one or more alkyl groups. The alkyl groups, which are made of C—C and C—H single bonds, are the less reactive portions of the molecules. In describing general features of organic compounds, chemists often use the designation R to represent any alkyl group: methyl, ethyl, propyl, and so on. Alkanes, for example, which contain no functional group, are represented as R—H. Alcohols, which contain the group —OH, are represented as R—OH. If two or more different alkyl groups are present in a molecule, we designate them R, R', R”, and so forth.
TABLE 24.6 • Common Functional Groups
FIGURE 24.11 Condensed structural formulas of six important alcohols. Common names are given in blue.
Alcohols are hydrocarbon derivatives in which one or more hydrogens of a parent hydrocarbon have been replaced by the functional group —OH, called either the hydroxyl group or the alcohol group. Note in FIGURE 24.11 that the name for an alcohol ends in -ol. The simple alcohols are named by changing the last letter in the name of the corresponding alkane to -ol—for example, ethane becomes ethanol. Where necessary, the location of the OH group is designated by a numeric prefix that indicates the number of the carbon atom bearing the OH group.
The O—H bond is polar, so alcohols are much more soluble in polar solvents than are hydrocarbons. The—OH functional group can also participate in hydrogen bonding. As a result, the boiling points of alcohols are much higher than those of their parent alkanes.
FIGURE 24.12 shows several commercial products that consist entirely or in large part of an organic alcohol.
The simplest alcohol, methanol (methyl alcohol), has many industrial uses and is produced on a large scale by heating carbon monoxide and hydrogen under pressure in the presence of a metal oxide catalyst:
Because methanol has a very high octane rating as an automobile fuel, it is used as a gasoline additive and as a fuel in its own right.
Ethanol (ethyl alcohol, C2H5OH) is a product of the fermentation of carbohydrates such as sugars and starches. In the absence of air, yeast cells convert these carbohydrates into ethanol and CO2:
In the process, the yeast cells derive energy necessary for growth. This reaction is carried out under carefully controlled conditions to produce beer, wine, and other beverages in which ethanol is the active ingredient.
The simplest polyhydroxyl alcohol (an alcohol containing more than one OH group) is 1,2-ethanediol (ethylene glycol, HOCH2CH2OH), the major ingredient in automobile antifreeze. Another common polyhydroxyl alcohol is 1,2,3-propanetriol [glycerol, HOCH2CH(OH)CH2OH], a viscous liquid that dissolves readily in water and is used in cosmetics as a skin softener and in foods and candies to keep them moist.
Phenol is the simplest compound with an OH group attached to an aromatic ring. One of the most striking effects of the aromatic group is the greatly increased acidity of the OH group. Phenol is about 1 million times more acidic in water than a nonaromatic alcohol. Even so, it is not a very strong acid (Ka = 1.3 × 10–10). Phenol is used industrially to make plastics and dyes, and as a topical anesthetic in throat sprays.
FIGURE 24.12 Everyday alcohols. Many of the products we use every day— from rubbing alcohol to hair spray and antifreeze—are composed either entirely or mainly of alcohols.
Cholesterol, shown in Figure 24.11, is a biochemically important alcohol. The OH group forms only a small component of this molecule, so cholesterol is only slightly soluble in water (0.26 g per 100 mL of H2O). Cholesterol is a normal component of our bodies; when present in excessive amounts, however, it may precipitate from solution. It precipitates in the gallbladder to form crystalline lumps called gallstones. It may also precipitate against the walls of veins and arteries and thus contribute to high blood pressure and other cardiovascular problems.
Compounds in which two hydrocarbon groups are bonded to one oxygen are called ethers. Ethers can be formed from two molecules of alcohol by splitting out a molecule of water. The reaction is catalyzed by sulfuric acid, which takes up water to remove it from the system:
A reaction in which water is split out from two substances is called a condensation reaction. (Sections 12.8 and 22.8)
Both diethyl ether and the cyclic ether tetrahydrofuran are common solvents for organic reactions:
Aldehydes and Ketones
Several of the functional groups listed in Table 24.6 contain the carbonyl group, C═O. This group, together with the atoms attached to its carbon, defines several important functional groups that we consider in this section.
In aldehydes the carbonyl group has at least one hydrogen atom attached:
In ketones the carbonyl group occurs at the interior of a carbon chain and is therefore flanked by carbon atoms:
Notice that the systematic names of aldehydes contain -al and that ketone names contain -one.
Aldehydes and ketones can be prepared by controlled oxidation of alcohols. Complete oxidation results in formation of CO2 and H2O, as in the burning of methanol:
Controlled partial oxidation to form other organic substances, such as aldehydes and ketones, is carried out by using various oxidizing agents, such as air, hydrogen peroxide (H2O2), ozone (O3), and potassium dichromate (K2Cr2O7).
GIVE IT SOME THOUGHT
Write the condensed structural formula for the ketone that would result from partial oxidation of the alcohol
Many compounds found in nature contain an aldehyde or ketone functional group. Vanilla and cinnamon flavorings are naturally occurring aldehydes. Two isomers of the ketone carvone impart the characteristic flavors of spearmint leaves and caraway seeds.
Ketones are less reactive than aldehydes and are used extensively as solvents. Acetone, the most widely used ketone, is completely miscible with water, yet it dissolves a wide range of organic substances.
Carboxylic Acids and Esters
Carboxylic acids contain the carboxyl functional group, often written COOH. (Section 16.10) These weak acids are widely distributed in nature and are common in consumer products [FIGURE 24.13 (a)]. They are also important in the manufacture of polymers used to make fibers, films, and paints. FIGURE 24.14 shows the formulas of several carboxylic acids.
The common names of many carboxylic acids are based on their historical origins. Formic acid, for example, was first prepared by extraction from ants; its name is derived from the Latin word formica, “ant.”
Carboxylic acids can be produced by oxidation of alcohols in which the OH group is attached to a CH2 group. Under appropriate conditions, the aldehyde may be isolated as the first product of oxidation, as in the sequence
where (O) represents any oxidant that can provide oxygen atoms. The air oxidation of ethanol to acetic acid is responsible for causing wines to turn sour, producing vinegar.
FIGURE 24.13 Everyday carboxylic acids and esters. (a) Vinegar contains acetic acid; vitamin C is ascorbic acid; citrus fruits and tomatoes contain citric acid; and aspirin is acetylsalicylic acid (which is both an acid and an ester). (b) Many sunburn lotions contain the ester benzocaine; some nail polish removers contain ethyl acetate; vegetable oils are also esters.
Which of these substances have both a carboxylic acid functional group and an alcohol functional group?
FIGURE 24.14 Structural formulas of common carboxylic acids. The monocarboxylic acids are generally referred to by their common names, given in blue type.
Acetic acid can also be produced by the reaction of methanol with carbon monoxide in the presence of a rhodium catalyst:
This reaction involves, in effect, the insertion of a carbon monoxide molecule between the CH3 and OH groups. A reaction of this kind is called carbonylation.
Carboxylic acids can undergo condensation reactions with alcohols to form esters:
Esters are compounds in which the H atom of a carboxylic acid is replaced by a carbon-containing group:
Figure 24.13(b) shows some commercial products containing esters. The name of any ester consists of the name of the group contributed by the alcohol followed by the name of the group contributed by the carboxylic acid, with the -ic replaced by -ate. For example, the ester formed from ethyl alcohol, CH3CH2OH, and butyric acid, CH3(CH2)2COOH, is
Notice that the chemical formula generally has the group originating from the acid written first, which is opposite of the way the ester is named.
Esters generally have very pleasant odors and are largely responsible for the pleasant aromas of fruit. Pentyl acetate (CH3COOCH2CH2CH2CH2CH3), for example, is responsible for the odor of bananas.
An ester treated with an acid or a base in aqueous solution is hydrolyzed; that is, the molecule is split into an alcohol and a carboxylic acid or its anion:
The hydrolysis of an ester in the presence of a base is called saponification, a term that comes from the Latin word for soap, sapon. Naturally occurring esters include fats and oils, and in making soap an animal fat or a vegetable oil is boiled with a strong base. The resultant soap consists of a mixture of salts of long-chain carboxylic acids (called fatty acids), which form during the saponification reaction. (Section 13.6)
Soap has been manufactured and used for thousands of years. Directions for making soap from cassia oil were written on a Babylonian clay tablet around 2200 B.C. For a long time, soap was made by heating animal fat with wood ashes, which contain potassium carbonate (also known as potash) and made the solution basic. (Section 16.9) The modern commercial process for making soap usually uses sodium hydroxide as the base. Using potassium hydroxide produces soft or liquid soaps.
SAMPLE EXERCISE 24.6 Naming Esters and Predicting Hydrolysis Products
In a basic aqueous solution, esters react with hydroxide ion to form the salt of the carboxylic acid and the alcohol from which the ester is constituted. Name each of the following esters, and indicate the products of their reaction with aqueous base.
Analyze We are given two esters and asked to name them and to predict the products formed when they undergo hydrolysis (split into an alcohol and carboxylate ion) in basic solution.
Plan Esters are formed by the condensation reaction between an alcohol and a carboxylic acid. To name an ester, we must analyze its structure and determine the identities of the alcohol and acid from which it is formed. We can identify the alcohol by adding an OH to the alkyl group attached to the O atom of the carboxyl (COO) group. We can identify the acid by adding an H to the O atom of the carboxyl group. We have learned that the first part of an ester name indicates the alcohol portion and the second indicates the acid portion. The name conforms to how the ester undergoes hydrolysis in base, reacting with base to form an alcohol and a carboxylate anion.
(a) This ester is derived from ethanol (CH3CH2OH) and benzoic acid (C6H5COOH). Its name is therefore ethyl benzoate. The net ionic equation for reaction of ethyl benzoate with hydroxide ion is
The products are benzoate ion and ethanol.
(b) This ester is derived from phenol (C6H5OH) and butanoic acid (commonly called butyric acid) (CH3CH2CH2COOH). The residue from the phenol is called the phenyl group. The ester is therefore named phenyl butyrate. The net ionic equation for the reaction of phenyl butyrate with hydroxide ion is
The products are butyrate ion and phenol.
Write the condensed structural formula for the ester formed from propyl alcohol and propionic acid.
Amines and Amides
Amines are compounds in which one or more of the hydrogens of ammonia (NH3) are replaced by an alkyl group:
As we have seen earlier, they are the most common organic bases. (Section 16.7)
An amine with at least one H bonded to N can undergo a condensation reaction with a carboxylic acid to form an amide, which contains the carbonyl group (C═O) attached to N (Table 24.6):
We may consider the amide functional group to be derived from a carboxylic acid with an NRR' group replacing the OH of the acid, as in these examples:
The amide linkage
where R and R' are organic groups, is the key functional group in proteins, as we will see in Section 24.7.