25.4 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 C C bond. The simplest alkene is CH2 CH2, called ethene (IUPAC) or ethylene. Ethylene is a plant hormone. It plays important roles in seed germination and fruit ripening. The next member of the series is CH3 CH CH2, 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 left 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-numbered 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 left 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, CH2 CH CH2 CH CH2 is 1,4-pentadiene.

Notice that the two isomers on the right in Figure 25.8 differ in the relative locations of their terminal methyl groups. These two compounds are examples of geometric 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. 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. Geometric isomers possess distinct physical properties and often differ significantly in their chemical behavior.

Geometric isomerism in alkenes arises because, unlike the C C bond, the C C bond is resistant to twisting. Recall that the double bond between two carbon atoms consists of a and a bond. 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). While rotation about a double bond doesn't occur easily, 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 p 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

Draw all the isomers of pentene, C5H10.

SOLUTION Because the compound is named pentene, we know that the longest chain of carbon atoms in the molecule contains five carbons. Thus, all the carbon atoms are in the chain; all the structures we are looking for are straight-chain ones. The possible structures are:

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You should convince yourself that cis- or trans-3-pentene is identical with cis- or trans-2-pentene, respectively.

PRACTICE EXERCISE

How many isomers are there of hexene, C6H12? Answer: 5

Alkynes

Alkynes are unsaturated hydrocarbons containing one or more C C 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, alkynes 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:

(a)
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(b)

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SOLUTION (a) 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. A methyl group is bound at carbon atom 4. Thus the compound is 4-methyl-2-heptene. The geometrical configuration at the double bond is cis; that is, the alkyl groups are bonded to the double bond on the same side. Thus, the full name is 4-methyl-cis-2-heptene.

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:

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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 ethylene:

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[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:

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[25.2]

The reaction between an alkene and H2, referred to as hydrogenation, does not occur readily under ordinary conditions of temperature and pressure. 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 H H bond. The most widely used catalysts are finely divided metals on which H2 is adsorbed.

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

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[25.3]

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[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:

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[25.5]

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[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):

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

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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

Mechanism of Addition Reactions

As our understanding of chemistry has grown, chemists have been able to advance from simply cataloguing reactions known to occur to explaining how they occur. An explanation of how a reaction proceeds is called a mechanism.

In Equation 25.3 we looked at the addition of HBr to an alkene. This reaction is thought to proceed in two steps. In the first step, which is the rate-determining one, the HBr molecule transfers a proton to one of the two alkene carbons. For example, in the reaction of 2-butene with HBr, the first step proceeds as follows:

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[25.7]

The pair of electrons that formed the bond between the carbon atoms in the alkane is used to form the new C H bond.

The second step, involving the addition of Br to the charged carbon center, is faster:

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[25.8]

In this reaction the bromide ion, Br, donates a pair of electrons to the positively charged carbon, forming the new C Br bond.

Since the first, rate-determining step in the reaction involves both the alkene and acid, the rate law for the reaction is second order, first order each in alkene and HBr:

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[25.9]

The energy profile for the reaction is shown in Figure 25.10. The first energy maximum represents the transition state in the first step shown above. The second maximum represents the transition state for the second step. Notice that there is an energy minimum between the first and second steps of the reaction. This enery minimum corresponds to the energies of the intermediate species,

Figure 25.10 Energy profile for addition of HBr to 2-butene, CH3CH CHCH3.

To show electron movement in reactions like these, chemists often use curved arrows. For the reaction above, the shifts in electron positions are shown as follows:

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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.

Each aromatic ring system is given a common name as shown in Figure 25.11. 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 25.11 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:

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[25.10]

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

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[25.11]

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

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Mainly 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:

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[25.12]

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:

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[25.13]

Once an alkyl chain has been attached to the aromatic ring, various reactions can be employed to establish functional groups on the alkyl chain.