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25.1 Introduction to Hydrocarbons

Because the compounds of carbon are so numerous, it is convenient to organize them into families that exhibit structural similarities. The simplest class of organic compounds is the hydrocarbons, compounds composed only of carbon and hydrogen. The key structural feature of hydrocarbons and of most other organic substances is the presence of stable carbon-carbon bonds. Carbon is the only element capable of forming stable, extended chains of atoms bonded through single, double, or triple bonds.

Hydrocarbons can be divided into four general types, depending on the kinds of carbon-carbon bonds in their molecules. Figure 25.1 shows an example of each of the four types: alkanes, alkenes, alkynes, and aromatic hydrocarbons. In these hydrocarbons as well as in other organic compounds, each C atom invariably has four bonds (four single bonds, two single bonds and one double bond, or one single bond and one triple bond).

FIGURE 25.1 Names, geometrical structures, and molecular formulas for examples of each type of hydrocarbon.

Alkanes are hydrocarbons that contain only single bonds, as in ethane, C2H6. Because alkanes contain the largest possible number of hydrogen atoms per carbon atom, they are called saturated hydrocarbons. Alkenes, also known as olefins, are hydrocarbons that contain a CC double bond, as in ethylene, C2H4. Alkynes contain a CC triple bond, as in acetylene, C2H2. In aromatic hydrocarbons the carbon atoms are connected in a planar ring structure, joined by both and bonds between carbon atoms. Benzene, C6H6, is the best-known example of an aromatic hydrocarbon. Alkenes, alkynes, and aromatic hydrocarbons are called unsaturated hydrocarbons because they contain less hydrogen than an alkane having the same number of carbon atoms.

The members of these different classes of hydrocarbons exhibit different chemical behaviors, as we will see shortly. However, their physical properties are similar in many ways. Because carbon and hydrogen do not differ greatly in electronegativity (2.5 for carbon, 2.1 for hydrogen), hydrocarbon molecules are relatively nonpolar. Thus, they are almost completely insoluble in water, but they dissolve readily in other nonpolar solvents. Furthermore, their melting points and boiling points are determined by London dispersion forces. Thus, hydrocarbons tend to become less volatile with increasing molar mass. (For more information, see Section 11.2) As a result, hydrocarbons of very low molecular weight, such as C2H6, are gases at room temperature; those of moderate molecular weight, such as C6H14, are liquids; and those of higher molecular weight are solids.

25.2 Alkanes

Table 25.1 lists several of the simplest alkanes. Many of these substances are familiar because of their widespread use. Methane is a major component of natural gas and is used for home heating and in gas stoves and hot-water heaters. Propane is the major component of bottled gas used for home heating and cooking in areas where natural gas is not available. Butane is used in disposable lighters and in fuel canisters for gas camping stoves and lanterns. Alkanes with from 5 to 12 carbon atoms per molecule are found in gasoline.

 

The formulas for the alkanes given in Table 25.1 are written in a notation called condensed structural formulas. This notation reveals the way in which atoms are bonded to one another but does not require drawing in all the bonds. For example, the Lewis structure and the condensed structural formula for butane, C4H10, are

 

We will frequently use either Lewis structures or condensed structural formulas to represent organic compounds. Notice that each carbon atom in an alkane has four single bonds, whereas each hydrogen atom forms one single bond. Notice also that each succeeding compound in the series listed in Table 25.1 has an additional CH2 unit.

Structures of Alkanes

The Lewis structures and condensed structural formulas for alkanes do not tell us anything about the three-dimensional structures of these substances. As we would predict from the VSEPR model, the geometry about each carbon atom in an alkane is tetrahedral; that is, the four groups attached to each carbon are located at the vertices of a tetrahedron. (For more information, see Section 9.2) The three-dimensional structures can be represented as shown for methane in Figure 25.2. The bonding may be described as involving sp3 hybridized orbitals on the carbon. (For more information, see Section 9.5)

 

FIGURE 25.2 Representations of the three-dimensional arrangement of bonds about carbon in methane.

Rotation about the carbon-carbon single bond is relatively easy, and it occurs very rapidly at room temperature. To visualize such rotation, imagine grasping the top left methyl group in Figure 25.3, which shows the structure of propane, and twisting it relative to the rest of the structure. Because motion of this sort occurs very rapidly in alkanes, long-chain alkanes are constantly undergoing motions that cause it to change its shape, something like a length of chain that is being shaken.

 

FIGURE 25.3 Three-dimensional models for propane, C3H8, showing rotations about the carbon-carbon single bonds.

Structural Isomers

The alkanes listed in Table 25.1 are called straight-chain hydrocarbons because all the carbon atoms are joined in a continuous chain. Alkanes consisting of four or more carbon atoms can also form branched chains; hydrocarbons with branched chains are called branched-chain hydrocarbons. Figure 25.4 shows the condensed formulas and space-filling models for all the possible structures of alkanes containing four and five carbon atoms. Notice that there are two ways that four carbon atoms can be joined to give C4H10: as a straight chain (left) or a branched chain (right). For alkanes with five carbon atoms, C5H12, there are three different arrangements.

 

FIGURE 25.4 Possible structures, names, and melting and boiling points of alkanes of formula C4H10 and C5H12.

Compounds with the same molecular formula but with different bonding arrangements and hence different structures are called structural isomers. The structural isomers of a given alkane differ slightly from one another in physical properties. Note the melting and boiling points of the isomers of butane and pentane, given in Figure 25.4. The number of possible structural isomers increases rapidly with the number of carbon atoms in the alkane. For example, there are 18 possible isomers having the molecular formula, C8H18, and 75 possible isomers with the molecular formula, C10H22.

Nomenclature of Alkanes

The first names given to the structural isomers shown in Figure 25.4 are the so-called common names. The straight-chain isomer is called the normal isomer, abbreviated with the prefix n-. The isomer in which one CH3 group is branched off the major chain is labeled the iso- isomer, for example, isobutane. However, as the number of isomers grows, it becomes impossible to find a suitable prefix to denote each isomer. The need for a systematic means of naming organic compounds was recognized early in the history of organic chemistry. In 1892 an organization called the International Union of Chemistry met in Geneva, Switzerland, to formulate rules for systematic naming of organic substances. Since that time the task of updating the rules for naming compounds has fallen to the International Union of Pure and Applied Chemistry (IUPAC). Chemists everywhere, regardless of their nationality or political affiliation, subscribe to a common system for naming compounds.

The IUPAC names for the isomers of butane and pentane are the ones given in parentheses for each compound in Figure 25.4. The following steps summarize the procedures used to arrive at these names and the names of other alkanes. We use a similar approach to write the names of other organic compounds.

1. Find the longest continuous chain of carbon atoms, and use the name of this chain as the base name of the compound. The longest chain may not always be written in a straight line, as seen in the following example:

 

Because this compound has a chain of six C atoms, it is named as a substituted hexane. Groups attached to the main chain are called substituents because they are substituted in place of an H atom on the main chain.

2. Number the carbon atoms in the longest chain, beginning with the end of the chain that is nearest to a substituent. In our example we number the C atoms from the upper left because that places the CH3 substituent on the second C atom of the chain; if we number from the lower right, the CH3 would be on the fifth C atom. The chain is numbered from the end that gives the lowest number for the substituent position.

3. Name and give the location of each substituent group. A substituent group that is formed by removing an H atom from an alkane is called an alkyl group. Alkyl groups are named by replacing the -ane ending of the alkane name with -yl. For example, the methyl group, CH3, is derived from methane, CH4. Likewise, the ethyl group, C2H5, is derived from ethane, C2H6. Table 25.2 lists several common alkyl groups. The name 2-methylhexane indicates the presence of a methyl, CH3, group on the second carbon atom of a hexane (six carbon) chain.

 

4. When two or more substituents are present, list them in alphabetical order. When there are two or more of the same substituent, the number of substituents of that type is indicated by a prefix: di- (two), tri- (three), tetra- (four), penta- (five), and so forth. Notice how the following example is named:

 

 

Sample Exercise 25.1

Name the following alkane:

SOLUTION To name this compound properly, you must first find the longest continuous chain of carbon atoms. This chain, extending from the upper left CH3 group to the lower right CH3 group, is five carbon atoms long:

 

The compound is thus named as a derivative of pentane. We might number the carbon atoms starting from either end. However, IUPAC rules state that the numbering should be done so that the numbers of carbons bearing side chains are as low as possible. This means that we should start numbering with the upper carbon. There is a methyl group on carbon 2, and one on carbon 3. The compound is thus called 2,3-dimethylpentane.

 

Practice Exercise

Name the following alkane:

Answer: 2,4-dimethylpentane

 

Sample Exercise 25.2

Write the condensed structural formula for 3-ethyl-2-methylpentane.

SOLUTION The longest continuous chain of carbon atoms in this compound is five. We can therefore begin by writing out a string of five C atoms:

 

CCCCC

We next place a methyl group on the second carbon, and an ethyl group on the third carbon atom of the chain. Hydrogens are then added to all the other carbon atoms to make the four bonds to each carbon. Thus, the structural formula is

 

The formula can be written more concisely as CH3CH(CH3)CH(C2H5)CH2CH3.

 

Practice Exercise

Write the condensed structural formula for 2,3-dimethylhexane.

Answer:

Cycloalkanes

Alkanes can form not only branched chains, but rings or cycles as well. Alkanes with this form of structure are called cycloalkanes. Figure 25.5 illustrates a few examples of cycloalkanes. Cycloalkane structures are sometimes drawn as simple polygons in which each corner of the polygon represents a CH2 group. This method of representation is similar to that used for benzene rings. (For more information, see Section 8.7) In the case of aromatic structures each corner represents a CH group.

 

FIGURE 25.5 Condensed structural formulas for three cycloalkanes.

Carbon rings containing fewer than five carbon atoms are strained because the CCC bond angle in the smaller rings must be less than the 109.5 tetrahedral angle. The amount of strain increases as the rings get smaller. In cyclopropane, which has the shape of an equilateral triangle, the angle is only 60; this molecule is therefore much more reactive than its straight-chain analog, propane.

Reactions of Alkanes

Most alkanes are relatively unreactive. For example, at room temperature they do not react with acids, bases, or strong oxidizing agents, and they are not even attacked by boiling nitric acid. One reason for their low chemical reactivity is the strength of the CC and CH bonds.

Alkanes are not completely inert, however. One of their most commercially important reactions is combustion in air, which is the basis of their use as fuels. (For more information, see Section 3.2) For example, the complete combustion of ethane proceeds as follows:

 

In the following sections we will see two ways in which hydrocarbons can be modified to impart greater reactivity: the introduction of unsaturation into the carbon-carbon framework and the attachment of other reactive groups to the hydrocarbon backbone.

25.3 Unsaturated Hydrocarbons

The presence of one or more multiple bonds makes unsaturated hydrocarbons significantly different from alkanes both in terms of their structures and their reactivity.

Alkenes

Alkenes are unsaturated hydrocarbons that contain a CC bond. The simplest alkene is CH2CH2, called ethene or ethylene. The next member of the series is CH3CHCH2, called propene or propylene. For alkenes with four or more carbon atoms, several isomers exist for each molecular formula. For example, there are four isomers of C4H8, as shown in Figure 25.8. Notice both their structures and their names.

 

FIGURE 25.8 Structures, names, and boiling points of alkenes with molecular formula C4H8.

The names of alkenes are based on the longest continuous chain of carbon atoms that contains the double bond. The name given to the chain is obtained from the name of the corresponding alkane (Table 25.1) by changing the ending from -ane to -ene. For example, the compound on the top in Figure 25.8 has a double bond as part of a three-carbon chain; thus, the parent alkene is considered to be propene.

 

The location of the double bond along an alkene chain is indicated by a prefix number that designates the number of the carbon atom that is part of the double bond and is nearest an end of the chain. The chain is always numbered from the end that brings us to the double bond sooner and hence gives the smallest number prefix. In propene the only possible location for the double bond is between the first and second carbons; thus, a prefix indicating its location is unnecessary. For the compound on the top in Figure 25.8, numbering the carbon chain from the end closer to the double bond places a methyl group on the second carbon. Thus, the name of the isomer is 2-methylpropene. For the other compounds in Figure 25.8, the longest carbon chain contains four carbons, and there are two possible positions for the double bond, either after the first carbon (1-butene) or after the second carbon (2-butene).

If a substance contains two or more double bonds, each is located by a numerical prefix. The ending of the name is altered to identify the number of double bonds: diene (two), triene (three), and so forth. For example, CH2CHCH2CHCH2 is 1,4-pentadiene.

Notice that the two isomers on the bottom in Figure 25.8 differ in the relative locations of their terminal methyl groups. These two compounds are examples of geometrical isomers, compounds that have the same molecular formula and the same groups bonded to one another but differ in the spatial arrangement of these groups. (For more information, see Section 24.4) In the cis isomer the two methyl groups are on the same side of the double bond, whereas in the trans isomer they are on opposite sides. Geometrical isomers possess distinct physical properties and often differ significantly in their chemical behavior.

Geometrical isomerism in alkenes arises because, unlike the CC bond, the CC bond is resistant to twisting. Recall that the double bond between two carbon atoms consists of a and a bond. (For more information, see Section 9.6) Figure 25.9 shows a cis alkene. The carbon-carbon bond axis and the bonds to the hydrogen atoms and to the alkyl groups (designated R), are all in a plane. The p orbitals that overlap sideways to form the bond are perpendicular to the molecular plane. As Figure 25.9 shows, rotation around the carbon-carbon double bond requires the bond to be broken, a process that requires considerable energy (about 250 kJ/mol). The rotation about a double bond is a key process in the chemistry of vision.

 

FIGURE 25.9 Schematic illustration of rotation about a carbon-carbon double bond in an alkene. The overlap of the orbitals that form the bond is lost in the rotation. For this reason, rotation about carbon-carbon double bonds does not occur readily.

 

Sample Exercise 25.3

Name the following compound:

 

SOLUTION Because this compound possesses a double bond, it is an alkene. The longest continuous chain of carbons that contains the double bond is seven in length. The parent compound is therefore considered a heptene. The double bond begins at carbon 2 (numbering from the end closest to the double bond); thus the parent hydrocarbon chain is named 2-heptene. Continuing the numbering along the chain, a methyl group is bound at carbon atom 4. Thus the compound is 4-methyl-2-heptene. Finally we note that the geometrical configuration at the double bond is cis; that is, the alkyl groups are bonded to the double bond on the same side. For this reason, the full name is 4-methyl-cis-2-heptene.

 

Practice Exercise

Draw the structural formula for the compound trans-1,3-hexadiene. Answer:

 

Alkynes

Alkynes are unsaturated hydrocarbons containing one or more CC bonds. The simplest alkyne is acetylene, C2H2, a highly reactive molecule. When acetylene is burned in a stream of oxygen in an oxyacetylene torch, the flame reaches a very high temperature, about 3200 K. The oxyacetylene torch is widely used in welding, which requires high temperatures. Alkynes in general are highly reactive molecules. Because of their higher reactivity, they are not as widely distributed in nature as alkenes; however, they are important intermediates in many industrial processes.

Alkynes are named by identifying the longest continuous chain in the molecule containing the triple bond and modifying the ending of the name as listed in Table 25.1 from -ane to -yne, as shown in Sample Exercise 25.4.

 

Sample Exercise 25.4

Name the following compounds:

 

SOLUTION In (a) the longest chain of carbon atoms is six. There are no side chains. The triple bond begins at carbon 2 (remember, we always arrange the numbering so that the smallest possible number is assigned to the carbon containing the multiple bond). Thus, the name is 2-hexyne.

In (b) the longest continuous chain of carbon atoms is seven; but because this chain does not contain the triple bond we do not count it as derived from heptane. The longest chain containing the triple bond is six, and so this compound is named as a derivative of hexyne, 3-propyl-1-hexyne.

 

Practice Exercise

Draw the condensed structural formula for 4-methyl-2-pentyne. Answer:

 

Addition Reactions of Alkenes and Alkynes

The presence of carbon-carbon double or triple bonds in hydrocarbons markedly increases their chemical reactivity. The most characteristic reactions of alkenes and alkynes are addition reactions, in which a reactant is added to the two atoms that form the multiple bond. A simple example is the addition of a halogen such as Br2 to ethene:

 

[25.1]

The pair of electrons that form the bond in ethylene is uncoupled and is used to form two new bonds to the two bromine atoms. The bond between the carbon atoms is retained.

Addition of H2 to an alkene converts it to an alkane:

 

[25.2]

The reaction between an alkene and H2, referred to as hydrogenation, does not occur readily under ordinary temperature and pressure conditions. One reason for the lack of reactivity of H2 toward alkenes is the high bond enthalpy of the H2 bond. To promote the reaction, it is necessary to use a catalyst that assists in rupturing the HH bond. The most widely used catalysts are finely divided metals on which H2 is adsorbed. (For more information, see Section 14.6)

Hydrogen halides and water can also add to the double bond of alkenes, as illustrated by the following reactions of ethene:

 

[25.3]

 

[25.4]

The addition of water is catalyzed by a strong acid such as H2SO4.

The addition reactions of alkynes resemble those of alkenes, as shown in the following examples:

 

   

[25.5]



 

[25.6]

 

Sample Exercise 25.5

Predict the product of the hydrogenation of 3-methyl-1-pentene.

SOLUTION The name of the starting compound tells us that we have a chain of five carbon atoms with a double bond at one end (position 1) and a methyl group on the third carbon from that end (position 3):

 

The addition of H2 across the double bond leads to the following alkane:

 

The longest chain in this alkane has five carbon atoms; its name is therefore 3-methylpentane.

 

Practice Exercise

Addition of HCl to an alkene leads to the formation of 2-chloropropane. What is the alkene? Answer: propene

Aromatic Hydrocarbons

Aromatic hydrocarbons are members of a large and important class of hydrocarbons. The simplest member of the series is benzene (see Figure 25.1), with molecular formula C6H6. As we have already noted, benzene is a planar, highly symmetrical molecule. The structure for benzene suggests a high degree of unsaturation. You might therefore expect benzene to resemble the unsaturated hydrocarbons and to be highly reactive. In fact, however, benzene is not at all similar to alkenes or alkynes in chemical behavior. The great stability of benzene and the other aromatic hydrocarbons as compared with alkenes and alkynes is due to stabilization of the electrons through delocalization in the orbitals. (For more information, see Section 9.5)

Each aromatic ring system is given a common name as shown in Figure 25.10. The aromatic rings are represented by hexagons with a circle inscribed inside to denote aromatic character. Each corner represents a carbon atom. Each carbon is bound to three other atoms--either three carbons or two carbons and a hydrogen. The hydrogen atoms are not shown.

 

FIGURE 25.10 Structures and names of several aromatic compounds.

Although aromatic hydrocarbons are unsaturated, they do not readily undergo addition reactions. The delocalized bonding causes aromatic compounds to behave quite differently from alkenes and alkynes. For example, benzene does not add Cl2 or Br2 to its double bonds under ordinary conditions. In contrast, aromatic hydrocarbons undergo substitution reactions relatively easily. In a substitution reaction, one atom of a molecule is removed and replaced (substituted) by another atom or group of atoms. For example, when benzene is warmed in a mixture of nitric and sulfuric acids, hydrogen is replaced by the nitro group, NO2:

 

More vigorous treatment results in substitution of a second nitro group into the molecule:

 

[25.8]

There are three possible isomers of benzene with two nitro groups attached. These three isomers are named ortho-, meta-, and para-dinitrobenzene:

 

Only the meta isomer is formed in the reaction of nitric acid with nitrobenzene.

Another example of a substitution reaction is the bromination of benzene, which is carried out using FeBr3 as a catalyst:

 

[25.9]

In a similar reaction, called the Friedel-Crafts reaction, alkyl groups can be substituted onto an aromatic ring by reaction of an alkyl halide with an aromatic compound in the presence of AlCl3 as a catalyst:

 

[25.10]

25.4 Functional Groups; Alcohols and Ethers

The reactivity of organic compounds can be attributed to particular atoms or groups of atoms within the molecules. A site of reactivity in an organic molecule is called a functional group because it controls how the molecule behaves or functions. As we have seen, the presence of CC double bonds or CC triple bonds in a hydrocarbon markedly increases its reactivity. Furthermore, these functional groups each undergo characteristic reactions. Each distinct kind of functional group 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 25.4 lists the most common functional groups and gives examples of each. Notice that in addition to CC double bonds or CC triple bonds, there are also many functional groups that contain elements other than just C and H. Many of the functional groups contain other nonmetals such as O and N.

 

We can think of organic molecules as being composed of functional groups that are bonded to one or more alkyl groups. The alkyl groups, which are made of CC and CH single bonds, are the unreactive portions of the organic molecules. In describing general features of organic compounds, we can use the designation R to represent any alkyl group: methyl, ethyl, propyl, and so on. For example, alkanes, which contain no functional group, are represented as RH. Alcohols, which contain the OH, or alcohol functional group, are represented as ROH. If two or more different alkyl groups are present in a molecule, we will designate them as R, R', R'', and so forth. In this section we examine the structure and chemical properties of two functional groups, alcohols and ethers. In the next section we consider some additional functional groups that contain CO bonds.

Alcohols (ROH)

Alcohols are hydrocarbon derivatives in which one or more hydrogens of a parent hydrocarbon have been replaced by a hydroxyl or alcohol functional group, OH. Figure 25.11 shows the structural formulas and names of several alcohols. Note that the accepted 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 an appropriate numeral prefix that indicates the number of the carbon atom bearing the OH group, as shown in the examples in Figure 25.11.

 

FIGURE 25.11 Structural formulas of several important alcohols. Their common names are given in parentheses.

Because the OH bond is polar, alcohols are much more soluble in polar solvents such as water than are hydrocarbons. The OH functional group can participate in hydrogen bonding. As a result, the boiling points of alcohols are much higher than those of their parent alkanes.

Figure 25.12 shows several familiar commercial products that consist entirely or in large part of an organic alcohol. Let's consider how some of the more important alcohols are formed and used.

The simplest alcohol, methanol, has many important industrial uses and is produced on a large scale. Carbon monoxide and hydrogen are heated together under pressure in the presence of a metal oxide catalyst:

 

[25.11]

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, C2H5OH, is a product of the fermentation of carbohydrates such as sugar and starch. In the absence of air, yeast converts carbohydrates into a mixture of ethanol and CO2, as shown in Equation 25.12. In the process, yeast derives energy necessary for growth:

 

[25.12]

This reaction is carried out under carefully controlled conditions to produce beer, wine, and other beverages in which ethanol is the active ingredient.

Many polyhydroxyl alcohols (those containing more than one OH group) are known. The simplest of these is 1,2-ethanediol (ethylene glycol), HOCH2CH2OH. This substance is the major ingredient in automobile antifreeze. Another common polyhydroxyl alcohol is 1,2,3-propanetriol (glycerol), HOCH2CH(OH)CH2OH. It is a viscous liquid that dissolves readily in water and is widely used as a skin softener in cosmetic preparations. It is also used in foods and candies to keep them moist.

Phenol is the simplest example of a 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 typical nonaromatic alcohol such as ethanol. Even so, it is not a very strong acid (Ka = 1.3 10-10). Phenol is used industrially in the making of several kinds of plastics and in the preparation of dyes. It is also used as a topical anesthetic in many sore throat sprays.

Cholesterol, shown in Figure 25.11, is a biochemically important alcohol. Notice that the OH group forms only a small component of this rather large molecule. As a result, cholesterol is not very soluble in water (0.26 g per 100 mL of H2O). Cholesterol is a normal component of our bodies; however, when present in excessive amounts, 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. The amount of cholesterol in our blood is determined not only by how much cholesterol we eat but also by total dietary intake. There is evidence that excessive caloric intake leads the body to synthesize excessive cholesterol.

Ethers (ROR')

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 thus a dehydration process; it is catalyzed by sulfuric acid, which takes up water to remove it from the system:

 

[25.13]

A reaction in which water is split out from two substances is called a condensation reaction. (For more information, see Sections 12.2 and 22.8)

Ethers are used as solvents; both diethyl ether and the cyclic ether tetrahydrofuran are common solvents for organic reactions.

 

25.5 Compounds with a Carbonyl Group

If you take a look at the functional groups listed in Table 25.4, you will see that several of them contain a CO double bond. This particular group of atoms is called a carbonyl group. The carbonyl group, together with the atoms that are attached to the carbon of the carbonyl group, defines several important functional groups that we consider in this section.

Aldehydes and Ketones

In aldehydes the carbonyl group has at least one hydrogen atom attached, as in the following examples:

 

In ketones the carbonyl group occurs at the interior of a carbon chain and is therefore flanked by carbon atoms:

 

Aldehydes and ketones can be prepared by carefully controlled oxidation of alcohols. It is fairly easy to oxidize 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).

Ketones are less reactive than aldehydes and are used extensively as solvents. Acetone, which boils at 56C, is the most widely used ketone. The carbonyl functional group imparts polarity to the solvent. Acetone is completely miscible with water, yet it dissolves a wide range of organic substances. Ethyl methyl ketone, CH3COCH2CH3, which boils at 80C, is also used industrially as a solvent.

Carboxylic Acids

We first discussed carboxylic acids in Section 16.10. Carboxylic acids contain the carboxyl functional group, which is often written as COOH. These weak acids are widely distributed in nature and are commonly used in consumer products [Figure 25.13(a)]. They are also important in the manufacture of polymers used to make fibers, films, and paints. Figure 25.14 shows the structural formulas of several carboxylic acids. Notice that oxalic acid and citric acid contain two and three carboxyl groups, respectively. The names of many carboxylic acids are based on their historical origins. For example, formic acid was first prepared by extraction from ants; its name is derived from the Latin word formica, meaning "ant."

 

FIGURE 25.14 Structural formulas of several common carboxylic acids.

Carboxylic acids can be produced by oxidation of alcohols in which the OH group is attached to a CH2 group, such as ethanol or propanol. Under appropriate conditions, the corresponding aldehyde may be isolated as the first product of oxidation. These transformations are shown for ethanol in the following equations, in which (O) represents an oxidant that can provide oxygen atoms (such as K2Cr2O7):

 

[25.14]

 

[25.15]

The air-oxidation of ethanol to acetic acid is responsible for causing wines to turn sour, producing vinegar.

Acetic acid can also be produced by the reaction of methanol with carbon monoxide in the presence of a rhodium catalyst:

 

[25.16]

Notice that 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.

Esters

Carboxylic acids can undergo condensation reactions with alcohols to form esters:

 

As seen in this example, esters are compounds in which the H atom of a carboxylic acid is replaced by a hydrocarbon group:

 

Figure 25.13(b) shows some common esters. Esters are named by using first the group from which the alcohol is derived and then the group from which the acid is derived as shown in the following Sample Exercise.

 

Sample Exercise 25.6

Name the following esters:

SOLUTION In (a) the ester is derived from ethanol and benzoic acid. Its name is therefore ethyl benzoate. In (b) the ester is derived from phenol and butyric acid. The residue from the phenol, C6H5, is called the phenyl group. The ester is therefore named phenyl butyrate.

 

Practice Exercise

Draw the structural formula for the compound propyl propionate. Answer:

 

Esters generally have very pleasant odors. They are largely responsible for the pleasant aromas of fruit. For example, one of the esters responsible for the odor of bananas is pentyl acetate, CH3COOCH2CH2CH2CH2CH3.

When esters are treated with an acid or a base in aqueous solution, they are hydrolyzed; that is, the molecule is split into its alcohol and acid components:

 

[25.18]

In this example the hydrolysis was carried out in basic medium. The products of the reaction are the sodium salt of the carboxylic acid and the alcohol.

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. In the soap-making process, an animal fat or a vegetable oil is boiled with a strong base, usually NaOH. The resultant soap consists of a mixture of sodium salts of long-chain carboxylic acids (called fatty acids) which form during the saponification reaction (Figure 25.15).

Amines and Amides

Amines are organic bases. (For more information, see Section 16.7) They have the general formula R3N, where R may be H or a hydrocarbon group, as in the following examples:

 

Amines containing a hydrogen bonded to nitrogen can undergo condensation reactions with carboxylic acids to form amides:

 

[25.19]

We may consider the amide functional group to be derived from a carboxylic acid with an NR2 group replacing the OH of the acid, as in these additional examples:

 

 

The amide linkage,

 

where R and R' are organic groups, is the key functional group in the structures of proteins, as we will see shortly.